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Reproduction and individual development of organisms
Introduction
Reproduction, or the ability to reproduce itself, is one of the main properties of all living organisms - from bacteria to mammals and flowering plants. Thanks to it, the existence of each species is ensured, continuity is maintained between parent individuals and their offspring. The forms of reproduction of organisms are diverse and will be discussed below.
All forms of reproduction are based on cell division, which proceeds quite similarly in plants and animals. Since the complex processes associated with sexual reproduction arose on the basis of cell division, we will first of all consider the process leading to the formation of two cells from one cell.
1. mitotic cell division
Interphase and various methods of cell division. There are two methods of division: I) the most common, complete division - mitosis (indirect division) and 2) amitosis (direct division). During mitotic division, the cytoplasm is restructured, the nuclear envelope is destroyed, and chromosomes are identified. In the life of a cell, there is a period of mitosis itself and an interval between divisions, which is called interphase. However, the period of interphase (non-dividing cells) in its essence can be different. In some cases, during interphase, the cell functions and simultaneously prepares for the next division. In other cases, cells enter interphase, function, but no longer prepare for division. As part of a complex multicellular organism, there are numerous groups of cells that have lost the ability to divide. These include, for example, nerve cells. Cell preparation for mitosis occurs in interphase. In order to imagine the main features of this process, remember the structure of the cell nucleus.
The basic structural unit of the nucleus is the chromosomes, which are made up of DNA and protein. In the nuclei of living nondividing cells, as a rule, individual chromosomes are indistinguishable, but most of the chromatin, which is found on stained preparations in the form of thin filaments or grains of various sizes, corresponds to the chromosomes. In some cells, individual chromosomes are also clearly visible in the interphase nucleus, for example, in rapidly dividing cells of a developing fertilized egg and in the nuclei of some protozoa. At different periods of life, chromosome cells undergo cyclical changes that can be traced from one division to another.
Chromosomes during mitosis are elongated dense bodies, along the length of which two strands can be distinguished - chromatids containing DNA, which are the result of chromosome doubling. Each chromosome has a primary constriction, or centromere. This narrowed part of the chromosome can be located either in the middle or closer to one of the ends, but for each particular chromosome its place is strictly constant. During mitosis, the chromosomes and chromatids are tightly coiled helical filaments (a spiralized or condensed state). In the interphase nucleus, the chromosomes are strongly elongated, i.e., despiralized, due to which they become difficult to distinguish. Consequently, the cycle of chromosome changes consists in spiralization, when they shorten, thicken and become clearly distinguishable, and despiralization, when they are strongly elongated, intertwined, and then it becomes impossible to distinguish each separately. Spiralization and despiralization are associated with the activity of DNA, since it functions only in a despiralized state. The release of information, the formation of RNA on DNA in a spiralized state, that is, during mitosis, stops.
The fact that chromosomes are present in the nucleus of a non-dividing cell is also proved by the constancy of the amount of DNA, the number of chromosomes, and the preservation of their individuality from division to division.
Preparation of the cell for mitosis. During interphase, a number of processes occur that enable mitosis. Let's name the most important of them: 1) centrioles are doubled, 2) chromosomes are doubled, i.e. the amount of DNA and chromosomal proteins, 3) proteins are synthesized from which the achromatin spindle is built, 4) energy is accumulated in the form of ATP, which is consumed during division, 5) cell growth ends.
Of paramount importance in preparing a cell for mitosis is the synthesis of DNA and duplication of chromosomes.
The doubling of chromosomes is associated primarily with the synthesis of DNA and the simultaneous synthesis of chromosome proteins. The doubling process lasts 6-10 hours and occupies the middle part of the interphase. Chromosome duplication proceeds in such a way that each old single strand of DNA builds a second one for itself. This process is strictly ordered and, starting at several points, spreads along the entire chromosome.
Mitosis. Phases of mitosis
Mitosis is a universal method of cell division in plants and animals, the main essence of which is the exact distribution of duplicated chromosomes between both formed daughter cells. The preparation of a cell for division, as we can see, occupies a significant part of the interphase, and mitosis begins only when the preparation in the nucleus and cytoplasm is completely completed. The whole process is divided into four phases. During the first of them - prophase - centrioles divide and begin to diverge in opposite directions. Around them, achromatin filaments are formed from the cytoplasm, which, together with centrioles, form an achromatin spindle. When the divergence of the centrioles ends, the whole cell is polar, both centrioles are located at opposite poles, and the middle plane can be called the equator. The filaments of the achromatin spindle converge at the centrioles and are widely distributed at the equator, resembling a spindle in shape. Simultaneously with the formation of a spindle in the cytoplasm, the nucleus begins to swell, and a ball of thickened threads - chromosomes - is clearly distinguished in it. During prophase, chromosomes spiralize, shortening and thickening. Prophase ends with the dissolution of the nuclear envelope, and the chromosomes are found to be lying in the cytoplasm. At this time, it can be seen that all chromosomes are already double.
Then comes the second phase - metaphase. Chromosomes, randomly arranged at first, begin to move towards the equator. All of them are usually located in the same plane at an equal distance from the centrioles. At this time, part of the spindle threads is attached to the chromosomes, while the other part of them still stretches continuously from one centriole to another - these are the supporting threads. Pulling, or chromosomal, threads are attached to centromeres (primary constrictions of chromosomes), but it must be remembered that both chromosomes and centromeres are already double. Pulling threads from the poles are attached to those chromosomes that are closer to them. There is a short pause. This is the central part of mitosis, after which the third phase begins - anaphase.
During anaphase, the pulling filaments of the spindle begin to contract, stretching the chromosomes to different poles. In this case, the chromosomes behave passively, they, bending like a hairpin, move forward by centromeres, for which they are pulled by a spindle thread. At the beginning of anaphase, the viscosity of the cytoplasm decreases, which contributes to the rapid movement of chromosomes.
Consequently, the threads of the spindle ensure the exact divergence of chromosomes (doubling even in interphase) to different poles of the cell.
Mitosis ends with the last stage, telophase. Chromosomes, approaching the poles, are closely intertwined with each other. At the same time, their stretching (despiralization) begins, and it becomes impossible to distinguish between individual chromosomes. Gradually, the nuclear envelope is formed from the cytoplasm, the nucleus swells, the nucleolus appears, and the previous structure of the interphase nucleus is restored.
At the end of anaphase or at the beginning of telophase, the division of the cytoplasm begins. In animal cells, a constriction appears on the outside in the form of a ring, which, deepening, divides the cell into two smaller ones. In plants, the cytoplasmic membrane arises in the middle of the cell and spreads to the periphery, dividing the cell in half. After the formation of the plasma membrane, a cellulose membrane appears in plant cells. Consequently, both the nucleus and the cytoplasm take an active part in cell division. The nucleus contains unique cell structures - chromosomes, and the achromatin spindle, formed from the cytoplasm, distributes them correctly and equally between both daughter cells.
Length of mitosis and interphase
Mitosis is a relatively short period in the life of a cell; interphase lasts much longer, as can be seen from the table.
In rapidly reproducing cells, mitosis can last only a few minutes. Consequently, the duration of mitosis varies from several minutes to 2-3 hours. Interphase lasts from 8-10 hours to several days.
The speed at which the individual phases of mitosis proceed is also different:
2. The constancy of the number and individuality of chromosomes
Biology grade 11. Topic: Rreproduction and individual development of organisms
The purpose of this test is to test whether the student is able to:
¾ characterize the main forms of reproduction of organisms and their biological significance;
¾ describe the features of the structure and formation of germ cells;
¾ note the features of the fertilization process in various organisms and its biological significance;
¾ indicate the features of the embryonic development of organisms;
¾ distinguish between stages of embryonic development of chordates;
¾ characterize the postembryonic development of animals and its types.
Option 1
unfulfilled.
–A. Sporulation is characteristic only for prokaryotes.
–B. Vegetative propagation is inherent only in plants.
–V. All animals reproduce only sexually.
+G. Parthenogenesis is the development of an organism from an unfertilized egg.
–A. The zygote is male sex cell.
+B. The eggs are large compared to the sperm.
–V. Sex hormones are present in all organisms.
–G. All animals are segregated.
–A. Blastomeres are formed during gastrulation.
–B. Cleavage occurs after gastrulation.
–V. All stages of ontogenesis are the same in animals and plants.
+G. Most multicellular animals develop three germ layers.
4. Which of the following organs and tissues are formed from the endoderm? Mark which of the following four answers are correct and which are incorrect.
+A. The epithelium of the pancreas.
–B. Pleura.
+V. Chord.
–G. Pericardium.
–A.
+B. Direct development occurs due to embryonization.
–V. All mammals have true live birth.
–G. Regeneration is typical only for invertebrates.
6. Note which of the following four statements about life cycles are correct and which are incorrect.
–A. All animals have complex life cycles.
–B. All algae have simple life cycles.
–V. The life cycle of mosses is dominated by the sporophyte.
–G. In all coelenterates, the polyp stage alternates with the medusa stage.
7. Mark which of the following four statements regarding the structure of germ cells are correct and which are incorrect.
+A. The size of the egg is much larger than the size of the sperm.
–B. Gametes contain a diploid set of chromosomes.
–V. The protein coat of the ovules of birds serves as the only reserve of nutrients for the embryo.
+G. The acrosome of the spermatozoon contains enzymes that break down the membrane of the egg.
–A. As a result of double fertilization, plants develop a nutritious diploid endosperm.
+V. Internal fertilization is characteristic of reptiles.
–G. In self-pollination, pollen grains land on the stigma of another plant's pistil.
+A. The morula is a collection of blastomeres.
–B. Endoderm is the outer layer of cells of the gastrula.
–V. During invagination, a primary mouth is formed, which opens into the blastula cavity.
–G. Chordates are among the protostomes.
10. Note which of the following four statements regarding embryonic induction are correct and which are incorrect.
+A. The rudiments of some organs develop as a result of interaction with the rudiments of others.
+B. It is believed that the cells of the organizer sites secrete biologically active substances that affect the cells of other sites.
–V. Induction occurs only with direct cell contact.
–G. Endoderm cells take part only in the development of the digestive tract.
11. Note which of the following four statements about regeneration are correct and which are incorrect.
+A. Regeneration is the process by which the body restores lost or damaged parts.
–B. Regeneration occurs only with mechanical disorders of tissues and organs.
+V. Regeneration processes underlie vegetative propagation.
–G. During physiological regeneration, structures lost as a result of accidental damage are restored.
12. Note which of the following four statements regarding the life cycle of higher plants are correct and which are incorrect.
+A. Gametophyte - haploid sexual generation.
+G. In the process of plant evolution, the gametophyte is reduced.
Option 2
As you write down your responses to the test questions, circle the letters that correspond to the statements you think are correct and cross out the letters that correspond to the statements that you think are incorrect. For example, if you think statements A and C are correct and statements B and D are wrong, write down . If at least one letter out of 4 is not marked, the task is considered unfulfilled.
1. Choose the correct statement regarding the forms of reproduction.
–A. Parthenogenesis is inherent in all organisms.
+B. Budding is a method of vegetative reproduction.
–V. Sporulation is characteristic only for fungi.
–G. Reproduction by fragmentation is inherent only in unicellular organisms.
2. Choose the correct statement regarding the sexual reproduction of organisms.
–A. Gametes have a diploid set of chromosomes.
–B. The eggs are smaller than the sperm.
+V. Meiosis leads to the formation of a haploid set of chromosomes.
–G. All higher plants are dioecious.
3. Choose the correct statement regarding ontogeny.
–A. The yolk of the egg performs only a protective function.
+B. Blastomeres are formed during crushing.
–V. Gastrulation occurs only in mammalian embryogenesis.
–G. In most multicellular animals, only one germ layer is formed.
4. Which of the following tissues are formed from the mesoderm? Mark which of the following four answers are correct and which are incorrect.
–A. Skin glands.
+B. The beginnings of the skeleton.
–V. nervous tissue.
+G. The beginnings of the circulatory system.
5. Mark which of the following four statements regarding the postembryonic development of animals are correct and which are incorrect.
–A. All insects develop with complete metamorphosis.
–B. During reparative regeneration, worn out parts of the body are renewed.
+V. Some sharks have a real live birth.
+G. The larvae of many animals leading an attached lifestyle are able to actively disperse.
–A. All plants have simple life cycles.
+B. In angiosperms, the asexual generation dominates.
–V. Mammals develop with metamorphosis.
–G. Animals have no alternation of generations.
–A. Sporulation is a method of vegetative reproduction.
–B. Vegetative propagation is characteristic only for plants.
–V. Parthenogenesis is the process of development of several embryos from one fertilized egg.
–G. Angiosperms reproduce by spores.
–A. Ovogenesis is the formation of spermatozoa.
+B. During the reproduction stage, the primary gametes divide mitotically.
–V. During the growth stage, haploid spermatocytes develop into mature gametes.
+G. At the formation stage, the size of the sperm decreases.
9. Mark which of the following four statements concerning the stages of ontogeny of organisms are correct and which are incorrect.
+A. Ontogeny is the individual development of an individual from its inception to death.
–B. The ontogenesis stages of higher plants are identical to the stages of animal ontogeny.
+V. In unicellular organisms, ontogeny coincides with the cell cycle.
–G. The science of gerontology studies the period of reaching puberty.
10. Mark which of the following four statements regarding histogenesis are correct and which are incorrect.
–A. Plant tissues develop from derivatives of different types of germ layers.
–B. Stem cells are not capable of differentiation.
+V. Erythrocytes are formed from the stem cells of the hematopoietic organs.
–G. Semi-stem cells lack the ability to divide.
11. Mark which of the following four statements regarding the postembryonic development of animals are correct and which are incorrect.
–A. This period lasts from birth to puberty.
–B. With direct postembryonic development, profound changes in the structure of the organism occur, as a result of which the larva turns into an adult.
–V. Spiders are characterized by indirect development.
+G. Metamorphosis is characteristic of amphibians.
12. Note which of the following four life cycle statements are correct and which are incorrect.
+A. Life cycle - the period between the same phases of development of two or more subsequent generations.
–B. The continuity of the life cycle is provided by somatic cells.
–V. Generative cells in the process of differentiation give rise to organs and tissues.
+G. Throughout the life cycle, alternation of generations is usually observed.
Option 3
As you write down your responses to the test questions, circle the letters that correspond to the statements you think are correct and cross out the letters that correspond to the statements that you think are incorrect. For example, if you think statements A and C are correct and statements B and D are wrong, write down . If at least one letter out of 4 is not marked, the task is considered unfulfilled.
1. Choose the correct statement regarding the forms of reproduction.
–A. Budding is unique to plants.
+B. Zoospores have flagella.
–V. Asexual reproduction is known only in protozoa.
–G. Vegetative reproduction is absent in animals.
2. Choose the correct statement regarding the sexual reproduction of organisms.
–A. The spermatozoon lacks a nucleus.
–B. Sex cells are called somatic.
+V. The supply of nutrients for the embryo is placed in the egg.
–G. Spermatozoa of all organisms have flagella.
3. Choose the correct statement regarding ontogeny.
–A. Embryonic development occurs only inside the mother's body.
–B. The embryonic period is unique to animals.
+V. Gastrulation is the process of formation of a two-layer embryo.
–G. In the process of crushing, the mesoderm is formed.
4. What organs are formed during neurulation? Mark which of the following four answers are correct and which are incorrect.
–A. Liver.
–B. Heart.
+V. Chord.
+G. Neural tube.
5. Mark which of the following four statements regarding the postembryonic development of animals are correct and which are incorrect.
–A. All animals develop with metamorphosis.
+B. With an increase in the level of organization of organisms, the ability to regenerate decreases.
–V. Embryoization increases the susceptibility of animals to the influence of external factors.
–G. Direct development has nothing to do with embryonization.
6. Note which of the following four statements about life cycles are correct and which are incorrect.
–A. All animals have simple life cycles.
–B. Reptiles develop with metamorphosis.
–V. All arthropods have an alternation of generations.
-G. In ferns, the sporophyte dominates.
7. Mark which of the following four statements regarding the forms of reproduction of organisms are correct and which are incorrect.
+A. Multiple fission is characteristic of malarial plasmodium.
–B. Conjugation is a method of sexual reproduction in plants.
+V. In hermaphroditic organisms, both male and female gametes are formed.
–G. Most hermaphroditic organisms are characterized by self-fertilization.
8. Mark which of the following four statements regarding gametogenesis are correct and which are incorrect.
–A. In the process of oogenesis at the stage of maturation, four identical gametes are formed.
+B. The egg cell is larger than the sperm due to the supply of nutrients.
+V. Spermatozoa at the stage of formation acquire flagella.
–G. During gametogenesis, mitosis occurs at the stage of maturation.
9. Mark which of the following four statements about embryogenesis are correct and which are incorrect.
+A. Cleavage - sequential mitotic division of the zygote.
+B. The blastula consists of a single layer of blastomeres.
–V. Blastomeres increase in size during interphase.
–G. Complete crushing occurs in the zygote, which contains a large amount of nutrients.
–A. At the neurula stage, the notochord is located above the neural tube.
–B. The epidermis of the skin is formed from the endoderm.
–V. The liver develops from the ectoderm.
–G. An intestine is laid over the chord.
11. Mark which of the following four statements regarding the indirect postembryonic development of animals are correct and which are incorrect.
–A. At this stage, the body weight of the larvae decreases.
–B. One of the ways of indirect development is live birth.
+V. Larvae of sedentary animals can actively disperse, which contributes to the expansion of the range of the species.
+G. The larvae easily penetrate the host organism.
12. Note which of the following four simple life cycle statements are correct and which are incorrect.
+A. A simple life cycle is characteristic of animals with a direct type of development.
–B. A simple life cycle is accompanied by a change of generations.
–V. This type of life cycle is typical for insects.
+G. With a simple life cycle, all subsequent generations are similar to each other.
Option 4
As you write down your responses to the test questions, circle the letters that correspond to the statements you think are correct and cross out the letters that correspond to the statements that you think are incorrect. For example, if you think statements A and C are correct and statements B and D are wrong, write down
+B. Gametes have a haploid set of chromosomes.
–V. The supply of nutrients for the embryo is placed in the sperm.
–G. All higher plants are monoecious.
3. Choose the correct statement regarding ontogeny.
–A. In most multicellular animals, only two germ layers are formed.
–B. Blastomeres are formed during immigration.
+V. Cleavage precedes gastrulation.
–G. In animals, the body is formed from educational tissue.
4. Which of the following organs and tissues are formed from the ectoderm? Mark which of the following four answers are correct and which are incorrect.
–A. The beginnings of the skeleton.
–B. Circulatory system.
+V. Epidermis.
–G. Liver.
5. Mark which of the following four statements regarding the postembryonic development of animals are correct and which are incorrect.
–A. Indirect development is characteristic of all animals.
+B. Embryonization is a phenomenon in which the embryonic period is lengthened due to the nutrition of the embryo with the resources of the mother's body.
+V. During reparative regeneration, structures lost due to accidental damage are restored.
–G. Some birds have a real live birth.
6. Note which of the following four statements about life cycles are correct and which are incorrect.
+A. Many plants have complex life cycles.
–V. Birds develop with metamorphosis.
–G. All invertebrates have alternating generations.
7. Mark which of the following four statements regarding the forms of reproduction of organisms are correct and which are incorrect.
–A. Individuals that have arisen as a result of sexual reproduction are genetically exact copies of the parental forms.
+B. The number of daughter organisms resulting from asexual reproduction is usually greater than due to sexual reproduction.
–V. Annelids can reproduce by multiple fragmentation.
–G. Fragmentation is the division of a cell into large and small daughter cells.
8. Mark which of the following four statements regarding the process of fertilization are correct and which are incorrect.
–A. During fertilization, a haploid zygote is formed.
–B. Insects are characterized by external fertilization.
–V. Amphibians are characterized by internal fertilization.
9. Mark which of the following four statements about embryogenesis are correct and which are incorrect.
+A. Embryogenesis in coelenterates ends at the stage of gastrulation.
–B. The method of gastrula formation, in which the blastula cavity arches, is called immigration.
–V. The method of gastrula formation, in which part of the blastomeres moves into the blastula cavity, is called invagination.
+G. In chordates, the mesoderm is formed by invagination of outgrowths of the wall of the primary intestine into the blastocoel.
10. Note which of the following four statements regarding organogenesis are correct and which are incorrect.
–A. At the neurula stage, the primary intestine is located between the notochord and the neural tube.
+B. The neural tube is formed from the ectoderm.
–V. The sex organs develop from the endoderm.
+G. The connective tissue layers of the skin develop from the mesoderm.
11. Note which of the following four statements about the growth of organisms are correct and which are incorrect.
+A. Growth is a progressive increase in the mass and size of an organism.
–B. In the process of animal growth, the processes of energy metabolism prevail over the processes of plastic metabolism.
+V. Most higher plants are characterized by unlimited growth .
–G. Continuous growth is characteristic of arthropods.
12. Note which of the following four statements about the complex life cycle are correct and which are incorrect.
+A. A complex life cycle is accompanied by alternation of generations.
+B. In coelenterates, there is an alternation of sexual and asexual generations.
–V. The asexual generation in angiosperms is significantly reduced.
–G. Dioecious and hermaphroditic generations alternate in the life cycle of Daphnia.
abstract
on the topic: "Reproduction"
Introduction 3
1. Types of reproduction 4
1.1 Asexual reproduction 4
1.2 Sexual reproduction 6
2. Individual development of organisms 10
2.1 Embryonic period of development 10
2.2 Postembryonic period of development 13
2.3 General patterns of development. Biogenetic Law 15
Conclusion 18
References 18
Introduction
The ability to reproduce, i.e. to produce a new generation of individuals of the same species is one of the main features of living organisms. In the process of reproduction, the genetic material is transferred from the parent generation to the next generation, which ensures the reproduction of traits not only of this species, but of specific parental individuals. For a species, the meaning of reproduction is to replace those of its representatives who die, which ensures the continuity of the existence of the species; in addition, under suitable conditions, reproduction allows you to increase the total number of the species.
Each new individual, before reaching the stage at which it will be able to reproduce, must go through a series of stages of growth and development. Some individuals die before reaching the reproductive stage (or sexual maturity) as a result of predation, disease, and various random events; therefore, the species can be preserved only on the condition that each generation produces more offspring than there were parental individuals that took part in reproduction. Populations fluctuate depending on the balance between reproduction and extinction of individuals. There are a number of different breeding strategies, each with distinct advantages and disadvantages; all of them will be described in this abstract.
1. Types of reproduction
Various forms of reproduction are known, but they can all be grouped into two types: sexual and asexual.
Sexual reproduction is called the change of generations and the development of organisms from specialized - sex - cells, which are formed in the sex glands. In this case, a new organism develops as a result of the fusion of two germ cells formed by different parents. However, in invertebrates, sperm and eggs are often formed in the body of one organism. This phenomenon - bisexuality - is called hermaphroditism. Flowering plants are also bisexual. In most species of angiosperms (flowering) plants, the bisexual flower includes both stamens, which form male germ cells - spermine, and pistils, which contain eggs. In about a quarter of the species, male (staminate) and female (pistillate) flowers develop independently, i.e. they have single flowers. An example is hemp. In some plants - corn, birch - both male and female flowers appear on the same individual.
Some animal and plant species develop
unfertilized egg. Such reproduction is called virginal or parthenogenetic.
Asexual reproduction is characterized by the fact that a new individual develops from non-sex, somatic (bodily) cells.
1.1 Asexual reproduction
With asexual reproduction, a new organism can arise from one cell or from several non-sex (somatic) cells of the mother. Only one parent is involved in asexual reproduction. Since the cells that give rise to daughter organisms arise as a result of mitosis, then all descendants will be similar in hereditary characteristics to the mother.
Rice. 1. Reproduction of euglena green
Many protozoa (ameba, green euglena, etc.), unicellular algae (chlamydomonas) reproduce by mitotic cell division (Fig. 1). Other unicellular - some lower fungi, algae (chlorella), animals, for example, the causative agent of malaria - malarial plasmodium, are characterized by sporulation. In this case, the cell breaks up into a large number of individuals, equal to the number of nuclei previously formed in the parent cell as a result of repeated division of its nucleus. Multicellular organisms are also capable of spore formation: these are mosses, higher fungi, multicellular algae, ferns and some others.
In both unicellular and multicellular organisms, budding also serves as a method of asexual reproduction. For example, in yeast fungi and some ciliates (sucking ciliates), when budding on the mother cell, a small tubercle containing a nucleus is initially formed - a kidney. It grows, reaches a size close to the size of the mother's organism, and then separates, moving on to an independent existence. In multicellular organisms (freshwater hydra), the kidney consists of a group of cells from both layers of the body wall. The kidney grows, lengthens, and a mouth opening appears at its front end, surrounded by tentacles. Budding ends with the formation of a small hydra, which then separates from the mother's organism.
In multicellular animals, asexual reproduction is carried out in the same way (jellyfish, annelids, flatworms, echinoderms). From each such part a full-fledged individual develops.
In plants, vegetative reproduction is widespread, i.e. parts of the body - cuttings, mustaches, tubers. So, potatoes reproduce by modified underground parts of the stem - tubers. In jasmine, willow shoots easily take root - cuttings. With the help of cuttings, grapes, currants, gooseberries are propagated.
Long creeping stems of strawberries - mustaches - form buds, which, taking root, give rise to a new plant. Few plants, such as begonias, can be propagated by leaf cuttings (leaf blade and petiole). On the underside of the leaf, in places where large veins branch, roots appear, on the upper side - buds, and then shoots.
The root is also used for vegetative propagation. In gardening, with the help of cuttings from lateral roots, raspberries, cherries, plums, and roses are propagated. Dahlias propagate with the help of root tubers. Modification of the underground part of the stem - the rhizome - also forms new plants. For example, thistle with the help of rhizomes can give more than a thousand new individuals per 1 m2 of soil.
1.2 Sexual reproduction
Sexual reproduction has very large evolutionary advantages over asexual reproduction. This is due to the fact that the genotype of the offspring arises by combining genes belonging to both parents. As a result, the ability of organisms to adapt to environmental conditions increases. Since new combinations are carried out in each generation, a much larger number of individuals can be adapted to the new conditions of existence than with asexual reproduction. The emergence of new combinations of genes provides a more successful and faster adaptation of the species to changing habitat conditions.
Thus, the essence of sexual reproduction lies in the combination in the hereditary material of the descendant of genetic information from two different sources - parents.
Sex cells develop in the gonads: male - spermatozoa, female - eggs (or eggs). In the first case, their development is called spermatogenesis, in the second - ovogenesis (from Latin ovo - egg).
In the process of formation of germ cells, a number of stages are distinguished. The first stage is the period of reproduction, in which the primary germ cells divide by mitosis, as a result of which their number increases.
The second stage is the period of growth. In immature male gametes, it is not pronounced. Their size increases slightly. On the contrary, future eggs - oocytes - sometimes increase in size by hundreds, and more often by thousands and even millions of times. The growth of oocytes is carried out at the expense of substances formed by other cells of the body. So, in fish, amphibians, and to a greater extent in reptiles and birds, the bulk of the egg is the yolk. It is synthesized in the liver, in a special soluble form it is transferred by the blood to the ovary, penetrates into the growing oocytes and is deposited there in the form of yolk plates. In addition, numerous proteins and a large number of various RNAs are synthesized in the future germ cell itself: transport, ribosomal and informational. The yolk is a set of nutrients (fats, proteins, carbohydrates, vitamins, etc.) necessary for the nutrition of the developing embryo, and RNA provides protein synthesis at an early stage of development, when its own disastrous information is not yet used.
The next stage, the maturation period, or meiosis, is shown in Figure 2. Cells entering the maturation period contain a diploid set of chromosomes and already double the amount of DNA.
Rice. 2. Maturation of germ cells (meiosis)
The essence of meiosis is that each sex cell receives a single, haploid, set of chromosomes. However, at the same time, meiosis is a stage during which new combinations of genes are created by combining different maternal and paternal chromosomes, recombination of hereditary inclinations also occurs as a result of crossing over - the exchange of sections between homologous chromosomes during meiosis.
Meiosis involves two consecutive divisions. As in mitosis, there are four stages in each meiotic division: prophase, metaphase, anaphase, and telophase.
First (I) meiotic division. Prophase I begins with the spiralization of chromosomes. As you remember, each chromosome consists of two chromatids connected at the centromere. Then the homologous chromosomes approach, each point of each chromatid of one chromosome is combined with the corresponding point of the chromatid of another, homologous chromosome. This process of exact and close approximation of homologous chromosomes in meiosis is called conjugation. In the future, crossing over can occur between such chromosomes - an exchange of identical, or homologous, i.e., containing the same genes, sections. By the end of prophase, repulsive forces arise between homologous chromosomes. Initially, they appear in the centromere region, and then in other areas.
In metaphase I, chromosome spiralization is maximal. Conjugated chromosomes are located along the equator, and the centromeres of homologous chromosomes are facing different poles of the cell. The spindle fibers are attached to them.
In anaphase I, the arms of the homologous chromosomes finally separate, and the chromosomes diverge to different poles. Consequently, only one of each pair of homologous chromosomes enters the daughter cell. The number of chromosomes is halved, the chromosome set becomes haploid. However, each chromosome consists of two chromatids, i.e., it still contains twice the amount of DNA.
In telophase I, the nuclear envelope forms for a short time. During interphase between the first and second divisions of meiosis, DNA replication does not occur. Cells formed as a result of the first division of maturation differ in the composition of paternal and maternal chromosomes and, consequently, in the set of genes.
For example, all human cells, including primary germ cells, contain 46 chromosomes. Of these, 23 are from the father and 23 from the mother. During the formation of germ cells after the first meiotic division, spermatocytes and oocytes also get 23 chromosomes. However, due to the random segregation of paternal and maternal chromosomes in anaphase I, the resulting cells receive a wide variety of combinations of parental chromosomes. For example, in one of them there may be 3 paternal and 20 maternal chromosomes, in another - 10 paternal and 13 maternal, in the third - 20 paternal and 3 maternal, etc. The number of possible combinations is very large. If we also take into account the exchange of homologous regions of chromosomes in the prophase of the first division of meiosis, then it is quite obvious that each resulting germ cell is genetically unique, since it carries its own unique set of genes.
Therefore, meiosis is the basis of combinative genotypic variability.
Second (II) meiotic division. The second division of meiosis generally proceeds in the same way as ordinary mitotic division, with the only difference being that the dividing cell is haploid. In anaphase II, the centromeres that connect the sister chromatids in each chromosome divide, and the chromatids, as in mitosis, from that moment on become independent chromosomes. With the completion of telophase II, the entire process of meiosis also ends: four haploid cells were formed from the original primary germ cell.
In males, all of them are converted into gametes - spermatozoa. In females, due to uneven meiosis, only one cell produces a viable egg. Three other daughter cells are much smaller, they turn into the so-called directional, or reduction, little bodies, which soon die. The formation of only one egg and the death of three genetically complete target bodies from a biological point of view is due to the need to preserve in one cell all the reserve nutrients that will be needed for the development of the future embryo.
The period of formation consists in the acquisition by cells of a certain shape and size, corresponding to their function.
In the process of maturation, female germ cells become covered with membranes and are ready for fertilization immediately after the completion of meiosis. In many cases, for example, in reptiles, birds and mammals, due to the activity of the cells surrounding the egg, a number of additional membranes arise around it. Their function is to protect the egg and the developing embryo from external adverse influences. Spermatozoa can vary in size and shape.
The function of spermatozoa is to deliver genetic information to the egg and stimulate its development. The formed sperm cell contains mitochondria, the Golgi apparatus, which secretes enzymes that dissolve the egg membrane during fertilization, i.e., during the fusion of the sperm and egg. The resulting diploid cell is called a zygote.
2. Individual development of organisms
Individual development, or ontogenesis, is called the entire period of an individual's life - from the moment the spermatozoon fuses with the egg and the formation of a zygote until the death of the organism. Ontogeny is divided into two periods: 1) embryonic - from the formation of a zygote to birth or exit from the egg membranes; 2) postembryonic - from the exit from the egg membranes or birth to the death of the organism.
The science that studies the patterns of individual development of organisms at the embryonic stage is called embryology (from the Greek embryo - embryo).
2.1 Embryonic period of development
In most multicellular animals, regardless of the complexity of their organization, the stages of embryonic development that the embryo goes through are the same. In the embryonic period, three main stages are distinguished: crushing, gastrulation and primary organogenesis.
Splitting up. The development of the organism begins with the stage of one cell. A fertilized egg is a cell and at the same time already an organism at the earliest stage of its development. As a result of repeated divisions, a unicellular organism turns into a multicellular one. The diploid nucleus, which arose during fertilization by the fusion of the spermatozoon and the egg, begins to divide after a few minutes, and the cytoplasm divides along with it. The resulting cells decrease in size with each division, so the division process is called crushing. During the period of crushing, cellular material accumulates for further development. Cleavage ends with the formation of a multicellular embryo - blastula. The blastula has a cavity filled with fluid, the so-called primary body cavity.
In cases where there is little yolk in the cytoplasm of the egg (like a lancelet) or relatively little (like a frog), crushing is complete, that is, the egg is divided entirely.
Otherwise, the period of crushing in birds proceeds. The yolk-free cytoplasm makes up only 1% of the total volume of the chicken egg; the rest of the cytoplasm of the egg, and hence the zygote, is filled with an array of yolk. If you look closely at a chicken egg, on one of its poles, directly on the yolk, you can see a small spot - a blastula, or an embryonic disc, formed as a result of crushing a section of the cytoplasm free from the yolk containing the nucleus. In such cases, crushing is called incomplete. Incomplete crushing is also characteristic of some fish and reptiles.
In all cases - in the lancelet, and in amphibians, and in birds, as well as in other animals - the total volume of cells at the blastula stage does not exceed the volume of the zygote. In other words, the mitotic division of the zygote is not accompanied by the growth of the resulting daughter cells to the volume of the mother, and their size progressively decreases as a result of a series of successive divisions. This feature of mitotic cell division during cleavage is observed during the development of fertilized eggs in all animals.
Some other features of crushing are also characteristic of various animal species. For example, all cells in the blastula have a diploid set of chromosomes, are identical in structure, and differ from each other mainly in the amount of yolk they contain. Such cells, devoid of signs of specialization to perform certain functions, are called non-specialized (or undifferentiated) cells. Another feature of cleavage is the extremely short mitotic cycle of blastomeres compared to the cells of an adult organism. During a very short interphase, only DNA duplication occurs.
Gastrulation. The blastula, as a rule, consisting of a large number of blastomeres (for example, in the lancelet of 3000 cells), in the process of development passes into a new stage, which is called the gastrula (from the Greek gaster - stomach). The embryo at this stage consists of clearly distinguishable layers of cells - the so-called germ layers: the outer, or ectoderm (from the Greek ectos - located outside), and the internal, or endoderm (from the Greek entos - located inside). The set of processes leading to the formation of a gastrula is called gastrulation.
In the lancelet, gastrulation is carried out by pushing one of the blastula poles inwards, towards the other; in other animals, either by stratification of the blastula wall or by fouling the massive vegetative pole with small cells of the animal pole.
In multicellular animals, except for intestinal cavities, in parallel with gastrulation or, like in the lancelet, after it, a third germinal layer appears - the mesoderm (from the Greek mesos - located in the middle), which is a collection of cellular elements located between the ecto- and endoderm in the primary body cavity is the blastocele. With the advent of the mesoderm, the embryo becomes three-layered.
Thus, the essence of the process of gastrulation is the movement of cell masses. The cells of the embryo practically divide and do not grow. However, at this stage, the use of the genetic information of the cells of the embryo begins, and the first signs of differentiation appear.
Differentiation, or differentiation, is the process of its occurrence and the growth of structural and functional differences between individual cells and parts of the embryo. From a morphological point of view, differentiation is expressed in the fact that several hundred types of cells of a specific structure are formed that differ from each other. From non-specialized cells of the blastula, epithelial cells of the skin, epithelium of the intestines, lungs gradually arise, nerve and muscle cells, etc. appear. From a biochemical point of view, cell specialization lies in the ability to synthesize certain proteins that are unique to this type of cell. Lymphocytes synthesize protective proteins - antibodies, muscle cells - the contractile protein myosin. Each cell type forms “its own” proteins peculiar only to it. Biochemical specialization of cells is ensured by selective - differential activity of genes, i.e., in the cells of different germ layers - the rudiments of certain organs and systems - begin to function different groups genes.
At different types animals, the same germ layers give rise to the same organs and tissues. This means that they are homologous. So, from the cells of the outer germ layer - the ectoderm - in arthropods, chordates, including fish, amphibians, reptiles, birds and mammals, skin integuments and their derivatives are formed, as well as the nervous system and sensory organs. The homology of the germ layers of the vast majority of animals is one of the proofs of the unity of the animal world.
Organogenesis. After completion of gastrulation, a complex of axial organs is formed in the embryo: neural tube, notochord, intestinal tube. In the lancelet, the axial organs are formed as follows: the ectoderm on the dorsal side of the embryo bends along the midline, turning into a groove, and the ectoderm located to the right and left of it begins to grow on its edges. The groove - the rudiment of the nervous system - plunges under the ectoderm, and its edges close. The neural tube is formed. The rest of the ectoderm is the rudiment of the skin epithelium.
The dorsal part of the endoderm, located directly under the nerve bud, separates from the rest of the endoderm and folds into a dense cord - a chord. From the rest of the endoderm, the mesoderm and intestinal epithelium develop. Further differentiation of the cells of the embryo leads to the emergence of numerous derivative germ layers - organs and tissues. In the process of specialization of the cells that make up the germ layers, the nervous system, sensory organs, skin epithelium, and tooth enamel are formed from the ectoderm; from the endoderm - the epithelium of the intestine, the digestive glands - the liver and pancreas, the epithelium of the gills and lungs; from the mesoderm - muscle tissue, connective tissue, including loose connective tissue, cartilage and bone tissue, blood and lymph, as well as the circulatory system, kidneys, sex glands.
2.2 Postembryonic period of development
At the moment of birth or release of the organism from the egg membranes, the embryonic period ends and the postembryonic period of development begins. Postembryonic development can be direct or accompanied by transformation (metamorphosis).
With direct development (in reptiles, birds, mammals), an organism of small size comes out of the egg shells or from the mother's body, but with all the main organs already inherent in an adult animal. Postembryonic development in this case is reduced mainly to growth and puberty.
During development with metamorphosis, a larva emerges from the egg, usually arranged more simply than an adult animal, with special larval organs that are absent in the adult state. The larva feeds, grows, and over time, the larval organs are replaced by organs characteristic of adults. Consequently, during metamorphosis, the larval organs are destroyed and organs appear that are inherent in adult animals.
Let us examine several examples of indirect postembryonic development. The larva of ascidians (type Chordates, subtype Larval-chordates) has all the main features of chordates: notochord, neural tube, gill slits in the pharynx. She swims freely, then attaches to some solid surface on the bottom of the sea, where metamorphosis takes place: her tail, chord, muscles disappear, and the neural tube breaks up into separate cells, most of which are phagocytosed. Only a group of cells remains from the nervous system of the larva, giving rise to the nerve ganglion. The structure of the body of an adult ascidian, leading an attached lifestyle, does not at all resemble the usual features of the organization of chordates. Only knowledge of the features of ontogenesis makes it possible to determine the systematic position of ascidians: the structure of the larva indicates their origin from chordates that led a free lifestyle. In the process of metamorphosis, ascidians switch to a sedentary lifestyle, and therefore their organization is simplified.
The larval form of amphibians is a tadpole, which is characterized by gill slits, a lateral line, a two-chambered heart, and one circle of blood circulation. In the process of metamorphosis, which occurs under the influence of thyroid hormone, the tail dissolves, limbs appear, the lateral line disappears, the lungs and the second circle of blood circulation develop. Attention is drawn to the similarity of a number of structural features of tadpoles and fish (lateral line, structure of the heart and circulatory system, gill slits).
An example of metamorphosis is also the development of insects. Butterfly caterpillars or dragonfly larvae differ sharply in structure, lifestyle and habitat from adult animals and resemble their ancestors - annelids.
The postembryonic period of development has a different duration. For example, mayflies in the larval state live for 2-3 years, and in the mature state - from 2-3 hours to 2-3 days, depending on the species. In most cases, the postembryonic period is longer. In humans, it includes the stage of puberty, the stage of maturity and the stage of old age.
In mammals and humans, there is a well-known dependence of life expectancy on the duration of puberty and pregnancy. Life expectancy usually exceeds
pre-reproductive period of ontogenesis by 5-8 times.
Postembryonic development is accompanied by growth. Distinguish growth indefinite, continuing throughout life, and certain, limited by some period. Indefinite growth is observed in woody forms of plants, some mollusks, and in vertebrates - in fish and rats.
In many animals, growth stops shortly after reaching puberty. In humans, growth ends by 20-25 years.
2.3 General patterns of development. biogenetic law
All multicellular organisms develop from a fertilized egg. The development of embryos in animals belonging to the same type is largely similar. In all chordates, in the embryonic period, an axial skeleton, the notochord, is laid, a neural tube appears, and gill slits form in the anterior part of the pharynx. The plan of the structure of chordates is also the same. In the early stages of development, vertebrate embryos are very similar (Fig. 3). These facts confirm the validity of the law of germinal similarity formulated by K. Baer: "Embryos reveal, already from the earliest stages, a certain general similarity within the limits of the type." The similarity of the embryos of different systematic groups indicates the commonality of their origin. Later, in the structure of the embryos, signs of a class, genus, species, and, finally, signs characteristic of a given individual appear. The divergence of signs of embryos in the process of development is called embryonic divergence and reflects the evolution of a particular systematic group of animals, the history of the development of a given species.
Rice. 3. Embryonic similarity in vertebrates: 1 - monotremes (echidna), 2 - marsupials (kangaroo), 3 - artiodactyls (deer), 4 - predatory (cat), 5 - primates (monkey), 6 - man
Great similarity of embryos in the early stages of development and
the phenomenon of differences at later stages have their own explanation.
The organism is subject to variability throughout development.
The mutation process affects the genes that determine the structure and metabolism of the youngest embryos. But the structures that arise in them (ancient features characteristic of distant ancestors) play a very important role in the processes of further development. As indicated, the notochord rudiment induces the formation of the neural tube, and its loss leads to the cessation of development. Therefore, changes in the early stages usually lead to underdevelopment and death of the individual. On the contrary, changes in the later stages, affecting less significant characters, may be favorable for the organism and in such cases are picked up by natural selection.
The appearance in the embryonic period of development of modern animal features characteristic of their distant ancestors reflects evolutionary transformations in the structure of organs.
In its development, the organism passes through a unicellular stage (the zygote stage), which can be considered as a repetition of the phylogenetic stage of the primitive amoeba. In all vertebrates, including their higher representatives, a chord is laid, which is then replaced by a spine, and in their ancestors, judging by the lancelet, the chord remained for life. During the embryonic development of birds and mammals, including humans, gill slits appear in the pharynx and their corresponding septa. The fact that parts of the gill apparatus are formed in the embryos of terrestrial vertebrates is explained by their origin from fish-like ancestors that breathe with gills. The structure of the heart of the human embryo in the early period of formation resembles the structure of this organ in fish: it has one atrium and one ventricle. Toothless whales develop teeth during the embryonic period. These teeth do not erupt, they are destroyed and resorbed.
The examples given here and many others point to a deep connection between the individual development of organisms and their historical development. This connection found its expression in the biogenetic law formulated by F. Müller and E. Haeckel in the 19th century: the ontogenesis (individual development) of each individual is a brief and quick repetition of the phylogenesis (historical development) of the species to which this individual belongs.
Conclusion
Completing the work on the abstract, we can conclude that the ability to reproduce, or self-reproduction, is one of the most important characteristics of organic nature. Reproduction is a property inherent in all living organisms without exception, from bacteria to mammals.
The existence of any kind of animals and plants, bacteria and fungi, the continuity between parent individuals and their offspring is maintained only through reproduction. Closely related to self-reproduction is another property of livingorganisms - development. ItIt is also inherent in all life on Earth: the smallest unicellular organisms, and multicellular plants and animals.
Bibliography
Bogen G. Modern biology. - M.: Mir, 1970.
Green N., Stout W., Taylor D. Biology: in 3 vols. T. 3: per. from English / ed. R. Sopera. - M.: Mir, 1990.
Mamontov S.G. Biology. General patterns. – M.: Bustard, 2002.
From animals to man. – M.: Nauka, 1971.
Slyusarev A.A. Biology with general genetics. - M.: Medicine, 1978.
PAGE_BREAK--At the diplonema stage, sister chromatids shorten, thicken, and mutually repel each other, resulting in the chromatids in the bivalent being nearly separated. The separation is incomplete for the reason that the centromere has not yet been split in each pair of chromosomes. As for the bivalents, they are held in various places along their length by chiasmata, which are structures formed between homologous chromatids as a result of previous crossing over between synaptically related homologues. In good preparations, one to several chiasmata may be observed, depending on the length of the bivalent. Each chiasma observed at this stage is the result of an exchange that occurred between non-sister chromatids during the pachynema stage. As the contraction and repulsion of the bivalents increases, the chiasmata move towards the ends of the chromosomes, i.e., chromosome terminalization occurs. At the end of the diplonema, chromosome despiralization occurs; homologues continue to repel each other.
At the stage of diakinesis, which is similar to diplotene, the shortening of the bivalents continues and the weakening (reduction) of the chiasma occurs, as a result of which discrete units are formed in the form of chromatids (four). Immediately after completion of this stage, the dissolution of the nuclear membrane occurs.
In metaphase I, the bivalents reach their maximum condensation. Becoming oval, they are located in the equatorial part of the nucleus, where they form the equatorial plates of meiotic metaphase I. The shape of each bivalent is determined by the number and localization of chiasmata. In males, the number of chiasmata per bivalent in metaphase I is usually 1-5. Bivalent XY becomes rod-shaped as a result of a single terminally located chiasm.
In anaphase I, the movement of opposite centromeres to opposite poles of the cell begins. As a result, homologous chromosomes separate. Each chromosome now consists of two chromatids held together by a centromere that does not divide and remains intact. In this, anaphase I of meiosis differs from anaphase mitosis, in which the centromere undergoes division. It is important to note that due to crossing over, each chromatid is genetically different.
In the telophase I stage, the chromosomes reach the poles, which ends the first meiotic division. After telophase I, a short interphase (interkinesis) occurs, in which the chromosomes despiralize and become diffuse, or telophase I passes directly into prophase II of the second meiotic division. In neither case, DNA replication was observed. After the first meiotic division, the cells are called spermatocytes of the second order. The number of chromosomes in each such cell decreases from 2n to n, but the DNA content does not change yet.
The second meiotic division occurs over several phases (prophase II, metaphase II, anaphase II, telophase II) and is similar to mitotic division. In prophase II, the chromosomes of secondary spermatocytes remain at the poles. In metaphase P, the centromere of each of the double chromosomes divides, providing each new chromosome with its own centromere. In anaphase II, the formation of the spindle begins, to the pole of which new chromosomes move. In telophase II, the second meiotic division ends, as a result of which each second-order spermatocyte produces two spermatids, from which spermatozoa then differentiate. As in the secondary spermatocyte, the number of chromosomes in the spermatid is haploid (n). However, spermatid chromosomes are single, while those of secondary spermatocytes II are double, being built from two chromatids. Therefore, the nucleus of each spermatid has a single set of non-homologous chromosomes. Secondary meiotic division is a division of the mitotic type (equatorial division). It separates double sister chromatids and differs from reduction division, in which homologous chromosomes are separated. The only significant difference from classical mitosis is that there is a haploid set of chromosomes.
So, the first meiotic division of spermatocytes of the first order leads to the formation of two secondary spermatocytes (second order). Both chromatids of structures formed as a result of reduction division are sister chromatids. The latter arise as a result of replication preceding the first meiotic division. The second meiotic division of each secondary spermatocyte results in the formation of four spermatids. Thus, in typical meiosis, cells divide twice, while chromosomes only divide once.
The final stage in spermatogenesis is associated with differentiation, which ends with each of the relatively large, spherical immobile spermatids turning into a small elongated motile spermatozoon.
In most adult (sexually mature) male animals, spermatogenesis occurs in the testes constantly or periodically (seasonally). For example, in insects, it takes only a few days to complete the cycle of spermatogenesis, while in mammals this cycle drags on for weeks and even months. In an adult, spermatogenesis takes place throughout the year. The development time of primitive spermatogonia in mature spermatozoa is about 74 days.
Male sex cells, produced by organisms of different species, are characterized by mobility and extreme diversity in size and structure. For example, the length of D. melanogaster sperm is 1.76 mm, which is 300 times the length of human sperm. Moreover, the length of D. bifurca spermatozoa is more than 28 mm, which exceeds the length of the insects of this species by twenty times.
Each human spermatozoon consists of three sections - the head, middle part and tail. The nucleus is located in the head of the spermatozoon. It contains a haploid set of chromosomes. The head is equipped with an acrosome, which contains lytic enzymes necessary for the sperm to enter the egg. Two centrioles are also localized in the head - the proximal one, which induces the division of the egg fertilized by the spermatozoon, and the distal one, which gives rise to the axial tail shaft. The basal body of the tail and mitochondria are located in the middle part of the spermatozoon. The tail (process) of the spermatozoon is formed by an internal axial rod and an external sheath, which is of cytoplasmic origin. Human spermatozoa are characterized by considerable mobility.
Ovogenesis is the process of egg formation. Its functions are to provide a haploid set of chromosomes in the nucleus of the egg and to provide the nutritional needs of the zygote. Ovogenesis in its manifestation is basically comparable to spermatogenesis.
In mammals and humans, oogenesis begins in the prenatal period (before birth). Owogonia, which are small cells with a fairly large nucleus and localized in the ovarian follicles, begin to differentiate into primary oocytes in the follicles. The latter are formed already in the third month of intrauterine development, after which they enter the prophase of the first meiotic division. By the time the girl is born, all primary oocytes are already in the prophase of the first meiotic division. Primary oocytes remain in prophase until the female individual reaches puberty. When ovarian follicles mature at the onset of puberty, meiotic prophase resumes in primary oocytes. The first meiotic division for each developing egg is completed shortly before the time of that egg's ovulation. As a result of the first meiotic division and the uneven distribution of the cytoplasm, one formed cell becomes a secondary oocyte, the other becomes a polar (reduction) body.
Secondary meiotic division in humans occurs when a secondary oocyte (developing egg) passes from the ovary into the fallopian tube. However, this division is not completed until the nuclear contents of the sperm enter the secondary oocyte, which usually occurs in the fallopian tube. When the sperm nucleus enters the secondary oocyte, the latter divides, resulting in the formation of an ovotid (mature egg) with a pronucleus containing a single set of 23 maternal chromosomes. In some other species, eggs are formed that determine both the male and female sex. It is important to emphasize that splitting and recombination of genes also occurs here, the basis of which is the divergence of chromosomes. Another cell, which is formed as a result of the second meiotic division in humans, is the second polar body, incapable of further development. At this time, the polar (reduction) body undergoes division in two. Thus, the development of one first-order oocyte is accompanied by the formation of one ovotid and three reduction bodies. In the ovaries, 300-400 oocytes usually mature in this way during a lifetime, but only one oocyte matures per month. During the differentiation of eggs, membranes are formed, their nucleus decreases in size.
In some species of animals, oogenesis proceeds rapidly and continuously and leads to the production of a large number of eggs.
Despite the similarities with spermatogenesis, oogenesis is characterized by some specific features. The nutrient material (yolk) of the primary oocyte is not distributed equally among the four cells that are formed as a result of meiotic divisions. The main amount of yolk is stored in one large cell, while the polar bodies contain very little of this substance. The first and second polar bodies receive, as a result of divisions, the same chromosome sets as the secondary oocytes, but they do not become germ cells. Therefore, eggs are much richer in nutritious material compared to sperm. This difference is especially pronounced in the case of oviparous animals.
Mammalian eggs are oval or somewhat elongated and are characterized by typical features. cellular structure. They contain all the structures characteristic of somatic cells, however, the intracellular organization of the egg is very specific and is determined by the fact that the egg is also the environment that ensures the development of the zygote. One of the characteristic features of the eggs is the complexity of the structure of their membranes. In many animals, primary, secondary and tertiary egg membranes are distinguished. The primary shell (inner) is formed at the stage of the oocyte. Representing the surface layer of the oocyte, it has a complex structure, since it is permeated with outgrowths of the follicular cells adjacent to it. The secondary (middle) shell is completely formed by follicular cells, and the tertiary (outer) is formed by substances that are secretion products of the oviduct glands through which the eggs pass. In birds, for example, the protein, subshell and shell membranes serve as the tertiary membranes of the eggs. Mammalian eggs are characterized by the presence of two membranes. The structure of the intracellular components of the eggs is species-specific, and sometimes even has individual characteristics.
Fertilization
Fertilization is the process of combining male and female gametes, which leads to the formation of a zygote and the subsequent development of a new organism. In the process of fertilization, the establishment of a diploid set of chromosomes in the zygote occurs, which determines the outstanding biological significance of this process.
Depending on the species of organisms in animals that reproduce sexually, there are external and internal fertilization.
External fertilization occurs in environment, which receives male and female germ cells. For example, fertilization in fish is external. The male (milk) and female (caviar) sex cells secreted by them enter the water, where they “meet” and unite. Data on fertilization in sea urchins indicate that as early as 2 seconds after the contact of the spermatozoa and the egg, changes occur in the electrical properties of the plasma membrane of the egg. The fusion of the content of the gametes occurs after 7 seconds.
internal fertilization is ensured by the transfer of sperm from the male to the female body as a result of sexual intercourse. Such fertilization occurs in mammals, and the outcome of the meeting between the germ cells is the central point here. It is believed that the nuclear contents of only one sperm enter the egg of these animals. As for the cytoplasm of the sperm, in some animals it enters the egg in a small amount, in others it does not enter the egg at all.
In humans, fertilization occurs in the upper part of the fallopian tube, and in fertilization, as in other mammals, only one spermatozoon is involved, the nuclear contents of which enter the egg. Sometimes in the fallopian tube there may be not one, but two or more eggs, as a result of which the birth of twins, triplets, etc. is possible. For example, in the 18th century. a case of birth in Russia by one mother (the wife of a peasant Fyodor Vasiliev) of 16 twins, 7 triplets and 4 quadruples (69 children in total) was registered.
As a result of fertilization, the diploid set of chromosomes is restored in the fertilized egg. Eggs are capable of fertilization for about 24 hours after ovulation, while sperm cells remain fertile for up to 48 hours.
Much remains unclear about the mechanisms of fertilization. It is assumed that the penetration of the nuclear material into the ovum of only one of the many spermatozoa is associated with changes in the electrical properties of the ovum plasma membrane. There are two hypotheses regarding the reasons for the activation of egg metabolism by spermatozoa. Some researchers believe that the binding of the sperm to external receptors on the cell surface is a signal that enters the egg through the membrane and activates inositol triphosphate and calcium ions there. Others believe that spermatozoa contain a special initiating factor.
A fertilized egg gives rise to a zygote, the development of organisms through the formation of zygotes is called zygogenesis. Experimental developments carried out in recent years have shown that fertilization of eggs of mammals, including humans, is possible in a test tube, after which the embryos that have developed in a test tube can be implanted in the uterus of a woman, where they can develop further. To date, numerous cases of the birth of “test-tube” children are known. It has also been established that not only spermatozoa, but also spermatids are capable of fertilizing a human egg. Finally, it is possible to fertilize the eggs (artificially devoid of nuclei) of mammals with the nuclei of their somatic cells.
Unlike zygogenesis, many animal organisms are capable of reproduction in natural conditions by parthenogenesis (from the Greek parthenos - virgin and genesis - birth). There are obligate and facultative parthenogenesis. Obligate parthenogenesis is the reproduction of organisms from an unfertilized egg. Such parthenogenesis serves as a way of reproduction of animals of more than 90 species, including some vertebrates. An example of obligate parthenogenesis is the reproduction of the Caucasian rock lizard, represented only by females. On the contrary, facultative parthenogenesis consists in the fact that eggs are able to develop both without fertilization and after fertilization. Facultative parthenogenesis, in turn, is female and male. Female parthenogenesis is frequent in bees, ants, rotifers, in which males develop from unfertilized eggs. Male parthenogenesis occurs in some isogamous algae.
In plants, cases are also known when the embryo develops from an unfertilized egg. As noted above, this phenomenon is called apomix. It is very widely found in many angiosperms, including cultivated ones, such as beets, cotton, flax, tobacco, and others.
Along with natural parthenogenesis, artificial (induced) parthenogenesis is distinguished, which can be caused by irritation of the oocytes with the help of physical or chemical factors, which leads to the activation of the eggs and, as a result, to the development of unfertilized eggs. Artificial parthenogenesis has been observed in the case of animals belonging to many systematic groups - echinoderms, worms, mollusks, and even some mammals.
continuation
--PAGE_BREAK--There is a known form of parthenogenesis called androgenesis (from Greek andros - male, genesis - birth). If the nucleus is inactivated in the egg and if after that several spermatozoa penetrate into it, then a male organism develops from such an egg as a result of the fusion of male (sperm) nuclei. The experiments of V. L. Astaurov (1904-1974), who showed androgenesis on the silkworm, are widely known. These experiences were as follows. In eggs of a silkworm of one species (Bombyx mandarina), the nuclei were inactivated using high temperature, and then such eggs were fertilized with spermatozoa of a silkworm of another species (B. mori). Penetrating into the eggs, the latter merged with each other, which gave rise to new organisms, which in their properties turned out to be paternal organisms (B. mori). Crosses of these organisms with B. mori females produced offspring belonging to B. mori.
The role of parthenogenesis and its forms in nature is small, since it does not provide wide adaptive capabilities of organisms. However, its use is of practical importance. In particular, B. L. Astaurov developed a method for obtaining parthenogenetic offspring from the silkworm, which is widely used in the industrial production of silk.
Unlike zygogenesis and parthenogenesis, there is gynogenesis (from the Greek gyne - woman), which is pseudogamy, which consists in the fact that the sperm meets the egg and activates it, but the sperm nucleus does not merge with the nucleus of the egg. In this case, the permissive offspring consists of only females. In certain species of roundworms, fish and amphibians, gynogenesis serves as a normal form of reproduction, producing offspring consisting only of females. Gynogenesis can also be induced artificially with the help of factors capable of destroying cell nuclei (radiation, temperature, etc.). In particular, cases of artificial gynogenesis in the silkworm, in some species of fish and amphibians are described. Obtaining such forms may be of some practical importance in the case of economically useful species.
As noted above, fertilization in flowering (angiosperms) has a significant distinguishing feature in the form of double fertilization (S. G. Navashin, 1896), which boils down to the fact that in the embryo sac a haploid egg and a diploid central cell are fertilized by sperm, resulting in the formation of a diploid embryo and a triploid cell that develops into endosperm cells
Parthenogenesis, androgenesis and gynogenesis are forms of violation of sexual reproduction. It is assumed that these forms arose in the course of evolution as a result of particular evolutionary adaptations.
Generation alternation
Organisms that reproduce only sexually are characterized by the alternation of haploid and diploid phases in their development. In many organisms, including mammals, this alternation has a regular character, and the preservation of species characteristics of organisms is based on it. Diploidy promotes the accumulation of different alleles. On the contrary, for organisms that can reproduce both sexually and asexually, alternation (change) of generations is characteristic, when one or more asexual generations of organisms is replaced by a generation of organisms that reproduce sexually.
Distinguish between primary and secondary alternation of generations. The primary alternation of generations is observed in organisms that have developed sexual progress in the course of evolution, but retained the ability to asexual reproduction, and consists in the regular alternation of sexual and asexual generations. It is found in animals (protozoa), in algae and in all higher plants. In the simplest, a classic example of the primary alternation of generations is the asexual reproduction of the malarial plasmodium in the human body (schizogony) and the sexual reproduction in the body of the malarial mosquito. In plants, the sexual generation is represented by the gametophyte, the asexual generation by the sporophyte. The mechanism of primary alternation is that spores develop on plants of the sporophyte generation, which, on the basis of meiosis, give rise to haploid male and female gametophytes. On the latter, sperm and eggs develop. Fertilization of the egg gives rise to the diploid sporophyte. Thus, gametophyte cells contain a haploid set of chromosomes, and sporophyte cells contain a diploid set, i.e., in plants, the alternation of generations is associated with a change in haploid and diploid states.
If we follow the ratio between the sporophyte and the gametophyte in plants of different levels of organization, we can see that in the course of evolution the sporophyte underwent development, while the gametophyte was characterized by reduction. For example, in mosses, the gametophyte (haploid generation) is predominant, on which the sporophyte lives. But already in ferns, the sporophyte (diploid generation) is predominant in the form of a well-developed plant with stems and roots, and the gametophyte is represented by a layer of cells that form a plate attached to the soil with the help of rhizoids. Further, in gymnosperms, the gametophyte is reduced to small numbers of cells, and in angiosperms, the male hematophyte is represented by only two cells, the female - by seven, while the sporophyte in gymnosperms are trees (pine, spruce, and others), and angiosperms - trees, shrubs, herbs.
Between the gametophyte and the sporophyte, there can be both similarities in morphology and lifespan, and differences in these characters. In the first case, this is called isomorphic alternation of generations, in the second - heteromorphic.
Secondary alternation of generations is widely found in animals. It is noted in the forms of heterogony and metagenesis. Heterogony consists in the primary alternation of the sexual process and parthenogenesis. For example, trematodes sexual reproduction regularly replaced by parthenogenesis. In many other organisms, heterogony depends on the season. So, rotifers, daphnia and aphids reproduce in autumn by zygogenesis (by fertilization of eggs and the formation of zygotes), and in summer by parthenogenesis. Metagenesis consists in the alternation of sexual reproduction and vegetative (asexual) reproduction. For example, hydras usually reproduce by budding, but when the temperature drops, they form germ cells. In coelenterates, at some stages of development, there is a transition from sexual reproduction to vegetative. In some marine coelenterates, the polypoid generation regularly alternates with the medusa. The polypoid generation is characterized by reproduction by the so-called strobilation (transverse constrictions), for the medusoid generation - sexually (fertilization of eggs, the formation of larvae and the development of polyps).
Sexual dimorphism. Hermaphroditism
Male and female animals are characterized by differences in specific phenotypic traits (size, body structure, color and other properties), as well as in behavior. Differences between females and males in their properties are called sexual dimorphism. In animals, it is already found at the lower stages of evolutionary development, for example, in round helminths, arthropods, and reaches its greatest expression in vertebrate animals, in which external differences between males and females are very expressive. In plants of those species that are characterized by the presence of male and female individuals, sexual dimorphism also takes place, but it is very slightly expressed.
If in animals male and female sex cells are produced by the same individual, which has both male and female sex glands, then this phenomenon is called hermaphroditism. The term "hermaphroditism" is a combination of the Greek names Hermes (god of male beauty) and Aphrodite (goddess of female beauty). There are true and false hermaphroditism. True hermaphroditism is most often found in organisms at low levels of evolution, such as flatworms and annelids, as well as molluscs. In flatworms, the male and female gonads function throughout the life of the individual. In contrast, in mollusks, the gonads produce eggs and sperm alternately. However, the phenomenon of true hermaphroditism is also found in more organized creatures. In particular, it is found in mammals. For example, in pigs, the development of ovaries on one side of the body is sometimes noted, and the development of testis on the other, or the development of combined structures (ovotestis), and in both cases there is a synthesis of functionally active eggs and spermatozoa. Such animals are classified as "intermediate" sex, and most individuals of the intermediate sexual type are female individuals with two XX chromosomes. Some of them are characterized by aggressive behavior, which indicates that although their tecticular tissue does not contain germ cells, the secretion of testosterone that affects behavior does take place. A similar phenomenon was observed in goats.
True hermaphroditism also occurs in humans, arising as a result of developmental disorders. Hermaphrodite genotypes are 46XX or 46XY, with most cases being XX (about 60%). XX genotypes are most common in hermaphrodites of Negroid African populations, while XY genotypes are more common among Japanese. In hermaphrodites of both types, a tendency towards bilateral asymmetry of the gonads was noted. Among true hermaphrodites, there are also chromosome mosaics, in which somatic cells contain a pair of XX chromosomes, others - a pair of XY chromosomes.
False hermaphroditism is also known, when individuals have external genital organs and secondary sexual characteristics characteristic of both sexes, but produce sex cells of only one type - male or female.
Most flowering plants are characterized by hermaphroditic flowers, which are usually called bisexual, since each flower has a pistil and stamens. For this reason, fruits develop from all flowers. Bisexual are wheat, cherry, apple tree and many other plant species. In addition to the bisexuals, in the course of evolution, plants developed with a division of the sexes within the same species, i.e., monoecious and dioecious plants arose. Plants containing both pistillate (female) and staminate (male) flowers are called monoecious. In monoecious plants, fruits develop only from pistillate flowers. Monoecious are corn, cucumber, pumpkin and others. In contrast, plants containing either pistillate or staminate flowers (within the same species) are dioecious. In dioecious plants, only those that have pistillate flowers (females) are fruitful. Poplar, strawberry and other types of woody and herbaceous plants are dioecious.
Hermaphroditism in humans is one of the pathological conditions. As for plants, knowledge of their hermaphroditism is extremely important for the practice of agriculture.
Ontogeny, its types and periodization
Ontogeny (from the Greek ontos - a being, genesis - development) is a complete history (cycle) of the development of an individual organism (animal or plant), starting with the formation of the germ cells that gave rise to it and ending with its death. Ideas about ontogeny (an individual history of the development of an organism) are based on data on the growth of an organism, differentiation of its cells, and morphogenesis. Consequently, ontogeny is an individual category.
In contrast to ontogenesis, the species category is phylogeny (from the Greek phyle - tribe, genesis - development) under which, since the time of E. Haeckel, who first substantiated this term, they understand the history of the emergence and development of a species (animals or plants). There is a close relationship between ontogeny and phylogeny, which is reflected in the so-called biogenetic law (E. Haeckel, F. Müller), which, as studies have shown, is in principle fair. Since the ontogenesis of an individual is determined by certain features of the phylogenetic development of the species to which the given individual belongs, it can be said that ontogenesis is the basis of phylogenesis, on the one hand, and the result of phylogenesis, on the other.
The study of the fundamental foundations of ontogeny is essential for understanding the biology and evolution of organisms. However, in order to better understand the current state of the doctrine of ontogeny, let us first consider how the growth and development of the organism was understood in past times using the example of the human body.
The first ideas about growth and development date back to the times of the ancient world. Even Hippocrates (460-377 BC) assumed that the eggs already contain a fully formed organism, but in a very reduced form. This idea was then continued in the doctrine of preformism (from Latin divformatio - preformation), which turned out to be especially popular in the 17th-18th centuries. Supporters of preformism were Harvey, Malpighi and many other prominent biologists and physicians of that time. For the preformists, the controversial issue was only in which germ cells the organism was preformed - female or male. Those who gave preference to eggs were called ovists, and those who attached great importance to male germ cells were called animalculists. Preformism is a metaphysical doctrine from beginning to end, for it denied development. The decisive blow to preformism was dealt by C. Bonnet (1720-1793), who discovered parthenogenesis in 1745 using the development of aphids from unfertilized eggs as an example. After that, preformism could no longer recover and began to lose its significance.
In the ancient world, another doctrine arose that was opposite to preformism and later received the name of epigenesis (from the Greek epi - after, genesis - development). Like preformism, epigenesis also became widespread in the 17th-18th centuries. In the spread of epigenesis, the views of K. F. Wolf (1733-1794), summarized in his book The Theory of Development (1759), were of great importance. K.F. Wolf believed that the egg does not contain either a re-formed organism or its parts, and that the egg consists of an initially homogeneous mass. In contrast to the preformists, the views of KF Wolf and other supporters of epigenesis were progressive for their time, since they contained the idea of development. However, in the future, new moments appeared. In particular, in 1828, K. Baer published his work "History of the Development of Animals", in which he showed that the contents of the egg are not homogeneous, that is, structured, and the degree of structure increases with the development of the embryo. Thus, K. Baer showed the failure of both preformism and epigenesis.
In our time, the growth of an organism is understood as a gradual increase in its mass as a result of an increase in the number of cells. Growth can be measured by plotting body size, weight, dry weight, cell count, nitrogen content, and other parameters from measurements. As for cell differentiation, this is the process by which some cells become morphologically, biochemically and functionally different from other cells. The reproduction and differentiation of some cells is always coordinated with the growth and differentiation of others. Both of these processes occur throughout the life cycle of an organism. Since differentiating cells change their shape, and groups of cells are involved in shape changes, this is accompanied by morphogenesis, which is a set of processes that determine the structural organization of cells and tissues, as well as the general morphology of organisms. Thus, growth is the result of quantitative changes in the form of an increase in the number of cells (body weight) and qualitative changes in the form of cell differentiation and morphogenesis.
The concepts of the growth of organisms (cell reproduction), cell differentiation, and morphogenesis make it possible to formulate a conclusion about development as a fundamental feature of ontogeny.
Development is a qualitative change in organisms, which is determined by cell differentiation and morphogenesis, as well as biochemical changes in cells and tissues that provide progressive changes in individuals during ontogenesis. Within the framework of modern ideas, the development of an organism is understood as a process in which the structures formed earlier stimulate the development of subsequent structures. The process of development is determined genetically and is closely connected with the environment. Consequently, development is determined by the unity of internal and external factors. Ontogeny, depending on the nature of the development of organisms, is typed into direct and indirect, in connection with which a distinction is made between direct and indirect development.
Direct development of organisms in nature occurs in the form of non-larval and intrauterine development, while indirect development occurs in the form of larval development.
Larval development is understood as indirect development, since organisms in their development have one or more larval stages. Larval development is widespread in nature and is typical for insects, echinoderms, and amphibians. The larvae of these animals lead an independent lifestyle, then undergoing transformations. Therefore, this development is also called development with metamorphoses (see below).
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--PAGE_BREAK--Non-larval development is characteristic of organisms that develop directly, such as fish, reptiles and birds, whose eggs are rich in yolk (nutrient material). Due to this, a significant part of ontogenesis takes place in eggs laid in the external environment, the metabolism of embryos is provided by developing provisional organs, which are embryonic membranes (yolk sac, amnion, allantois).
Intrauterine development is also characteristic of organisms that develop in a direct way, for example, for mammals, including humans. Since the eggs of these organisms are very poor in nutrients, all the vital functions of the embryos are provided by the maternal organism through provisional organs formed from the tissues of the mother and the embryo, among which the placenta is the main one. Evolutionarily, intrauterine development is the latest form, but it is most beneficial for the embryos, since it most effectively ensures their survival.
Ontogeny is divided into proembryonic, embryonic and postembryonic periods. In the case of humans, and sometimes even higher animals, the period of development before birth is often called prenatal or antenatal, and after birth, postnatal. Within the prenatal period, the initial (first week of development), embryonic and fetal periods are distinguished. The developing embryo before the formation of the rudiments of organs is called the embryo, after the formation of the rudiments of organs - the fetus.
Prombryonic and embryonic periods
The proembryonic (from Greek pro - to, embryon - embryo) period in the individual development of organisms is associated with the formation of germ cells in the process of gametogenesis. As noted above, the male germ cells of animals do not differ significantly from other (somatic) cells in their structure, while the ovules are characterized by an important distinguishing feature, which consists in the fact that they contain a lot of yolk. Given the amount of yolk and its topography in the eggs, the latter are classified into three types, namely:
1. Isolecithal cells. These eggs contain some yolk, which is localized evenly throughout the cell. Isolecithal eggs are produced by echinoderms (sea urchins), lower chordates (lancelets), and mammals.
2. Telolecithal eggs. These eggs contain a large amount of yolk, which is concentrated at one of the poles - the vegetative one. Such eggs are produced by molluscs, amphibians, reptiles, birds. For example, frog eggs consist of 50% yolk, chicken eggs (in everyday life chicken eggs) - by 95%. At the other pole (animal) of telolecithal eggs, the cytoplasm and nucleus are concentrated.
3. Centrolecithal oocytes. There is little yolk in these eggs, but it occupies a central position. The periphery of such eggs is occupied by the cytoplasm. An example of centrolecithal eggs are eggs produced by arthropods.
The proembryonic period is also characterized by the fact that during this period metabolic processes occur in gametes associated with the accumulation of intensively synthesized RNA molecules.
The embryonic period or embryogenesis (from the Greek embryon - embryo, genesis - development), begins with the fusion of the nuclei of the male and female germ cells, which is the process of fertilization of eggs. In organisms that are characterized by intrauterine development, the embryonic period ends with the birth of offspring, and in organisms that are characterized by larval and non-larval types of development, the embryonic period ends with the release of offspring from egg or embryonic membranes, respectively.
Within the embryonic period, the stages of zygote, crushing, blastula, gastrula, formation of germ layers, histogenesis and organogenesis are distinguished. As noted above, taking into account the time factor in mammals and humans, the embryo until the formation of the rudiments of organs is called an embryo, and after that, until birth, it is called a fetus. In humans, the development of the embryo (embryo) ends by the end of the second month. Starting from the 9th week, the fetal period follows, characterized by further growth and development of the organism (fetus) in the prenatal state until birth.
Zygote. In mammals, the zygote is formed as a result of fertilization, which begins with the fact that one of the male germ cells reaches the egg and initiates its development. In an egg activated by a male germ cell, a number of physical and chemical processes occur, including the movement of protoplasm, which leads to the establishment of bilateral symmetry of the egg, as well as the rearrangement of the plasma membrane, which excludes the fusion of other (additional) male germ cells with the egg. This is followed by the fusion of the plasma membranes of the egg and sperm, followed by the destruction of the nuclear membranes, which ensures the fusion of the nuclei of the two cells. Cell nuclei merge, and the diploid set of chromosomes is restored. Fertilization of the egg is accompanied by the activation of protein synthesis in it. Thus, an essentially single-celled organism is formed.
Splitting up. Morula formation. Cleavage is the initial period of development of the zygote (fertilized egg). Since the eggs have centrioles, it consists in the division of the zygote by mitosis, which begins, for example, in humans, 30 hours after insemination. In humans, division begins with the movement of a fertilized egg through the fallopian tube and consists in the appearance of a furrow on the surface of the egg. The first furrow leads to the formation of two cells - two blastomeres, the second - four blastomeres, the third - eight blastomeres, etc. The group of cells formed as a result of successive divisions of the zygote is called morula (from Greek morum - mulberry).
All multicellular animals that reproduce sexually go through the morula stage. Depending on the species, the division proceeds in different ways. Distinguish between radial (vertebrates, echinoderms), bilateral (fungiformes, some chordates) and spiral crushing (nemerteans, annelids, many mollusks), and these forms of crushing depend on the planes of crushing. Therefore, their morulae consist of a different number of cells. In addition, a structure called a trophoblast is formed from a part of the cells, the cells of which nourish the embryo, and thanks to enzymes, they also ensure the introduction of the latter into the uterine wall. In humans, attachment of the morula to the wall of the uterus occurs on the 7th day after fertilization. Later, the trophoblast cells exfoliate from the embryo and form a vesicle, which is filled with fluid from the tissues of the uterus.
A characteristic feature of cleavage is that significant cell growth does not occur during it. Therefore, the biological significance of this stage lies in the fact that from a large cell, which is an egg, smaller cells are formed in which the ratio of the cytoplasm to the nucleus is reduced. As a result, the topology of cytoplasmic complexes in blastomeres changes, which creates a new cytoplasmic environment for the nuclei.
Cleavage of the zygote ends with the formation of a multicellular structure called blastula (from the Greek blastos - sprout). This structure is shaped like a vesicle, consisting of a single layer of cells called the blastoderm. Now these cells are called embryonic. The size of the blastula is similar to that of an egg. During the period of crushing, the number of nuclei increases, the total amount of DNA increases. At the end of the blastula stage, a small amount of mRNA and tRNA is also synthesized, but new ribosomes and ribosomal RNA are not yet detected before gastrulation begins, or if they are detected, then in negligible amounts.
Gastrulation. Gastrulation (from the Greek gastre - vessel cavity) is the process of movement of embryonic cells following the formation of the blastula, which is accompanied by the formation of two or three (depending on the type of animal) layers of the embryo or the so-called germ layers
Gastrulation is characterized by an increase in the intensity of metabolism compared to crushing by 2-3 times. The synthesis of mRNA, rRNA, ribosomes and proteins increases sharply.
The development (gastrulation) of isolecithal eggs occurs by invagination (invagination) of the vegetative pole into the blastula, as a result of which the opposite poles almost merge, and the blastocoel (blastula cavity) almost disappears or completely disappears. The outer layer of embryonic cells is called the ectoderm (from Greek ectos - outside, derma - skin) or outer germ layer, while the inner layer is called endoderm (from Greek entos - inside) or inner germ layer. The resulting cavity is called the gastrocoel, or primary intestine, the entrance to which is called the blastopore (primary mouth).
The development of two germ layers is typical for sponges and coelenterates. However, chordates during gastrulation are characterized by the development of the third germ layer - the mesoderm (from the Greek mesos - middle), which is formed between the ectoderm and endoderm
Gastrulation is a necessary prerequisite for the subsequent stages of development, as it brings cells into a position that opens up the possibility of forming organs. The embryonic material differentiated into three embryonic anlages gives rise to all tissues and organs of the developing embryo.
Histogenesis and organogenesis
The development (differentiation) of germ layers during embryogenesis is accompanied by the fact that various tissues and organs are formed from them. In particular, the epidermis of the skin, nails and hair, sebaceous and sweat glands, the nervous system (brain, spinal cord, ganglia, nerves), receptor cells of the sense organs, the lens of the eye, the epithelium of the mouth, nasal cavity and anus, dental enamel. From the endoderm, the epithelium of the esophagus, stomach, intestines, gall bladder, trachea, bronchi, lungs, urethra, as well as the liver, pancreas, thyroid, parathyroid and goiter glands develop. From the mesoderm, smooth muscles, skeletal and cardiac muscles, dermis, connective tissue, bones and cartilage, dental dentin, blood and blood vessels, mesentery, kidneys, testes and ovaries develop. In humans, the brain and spinal cord are the first to separate. 26 days after ovulation, the length of the human fetus is about 3.5 mm. At the same time, the rudiments of the arms are already visible, but the rudiments of the legs are just beginning to develop. 30 days after ovulation, the length of the embryo is already 7.5 mm. At this time, segmentation of limb buds, eye cups, cerebral hemispheres, liver, gallbladder, and even the division of the heart into chambers can already be distinguished.
In an eight-week-old human embryo, with a length of about 40 mm and a weight of about 5 g, almost all body structures appear. Organogenesis ends by the end of the embryonic period. At this time, the embryo in appearance acquires features of resemblance to a person.
The length of a 12-week-old human fetus is already about 87 mm, and the weight is about 45 g. Further growth and development of the fetus continues. For example, in the 4th month of development, hair appears, and in the 20th week, blood cells begin to form.
If the definitive oral opening is formed at the site of the primary mouth (blastopore), then these animals are called protostomes (worms, mollusks, arthropods). If the definitive mouth is formed in the opposite place, then these animals are called deuterostomes (echinoderms, chordates).
To ensure the connection of the embryo with the environment, it develops the so-called provisional organs, which exist temporarily. Depending on the type of oocytes, provisional organs are different structures. In fish, reptiles and birds, the yolk sac is the provisional organ. In mammals, the yolk sac is formed at the beginning of embryogenesis, but does not develop. Later it is reduced. In the course of evolution, reptiles, birds, and mammals have developed embryonic membranes that provide protection and nutrition for embryos (Fig. 91). In mammals, including humans, these germinal membranes are sheets of tissue that develop from the body of the embryo. There are three such membranes - amnion, chorion and allantois. The outer shell of the embryo is called the chorion. She grows into the uterus. The place of greatest growth in the uterus is called the placenta. The fetus is connected to the placenta through the umbilical cord or umbilical cord, in which there are blood vessels that provide placental blood circulation. The amnion develops from the inner leaf, and the allantois develops between the amnion and the chorion. The space between the embryo and the amnion, called the amniotic cavity, contains fluid (amniotic). This fluid contains the embryo, and then the fetus until birth. The metabolism of the fetus is provided through the placenta.
At the heart of the formative interaction of the parts of the embryo are coordinated metabolic processes in a certain way. The pattern of development is heterochrony, which is understood as the formation of organ anlages different in time and the different intensity of their development. Those organs and systems that should start functioning earlier develop faster. For example, in humans, the rudiments of the upper limbs develop faster than the rudiments of the lower ones.
The development of the embryo and fetus is greatly influenced by the living conditions of the mother. The embryo is extremely sensitive to various influences. Therefore, the so-called critical periods are distinguished, i.e., the periods in which the embryos, and then the fruits, are most sensitive to damaging factors. In the case of humans, the critical periods of embryonic ontogenesis are the first days after fertilization, the time of placenta formation and childbirth, and the damaging factors are alcohol, toxic substances, lack of oxygen, viruses, bacteria, pathogenic protozoa, helminths and other factors. These factors have a teratogenic effect and lead to deformities and disruption of normal development.
Ever since the time of Hippocrates (5th century AD), the question of the causes that initiate the birth of the fetus has been discussed. In particular, Hippocrates himself assumed that the development of the fetus initiates its own birth. The latest experimental work of British researchers, carried out on sheep, showed that in sheep the initiation of lambing is controlled by the hypothalamus + pituitary gland + adrenal glands of the fetus. Damage to the nuclei of the hypothalamus, removal of the anterior pituitary gland or adrenal glands prolongs the pregnancy of sheep. On the contrary, administration of adenocorticotropic hormone (anterior pituitary secretion) or cortisol (adrenal gland secretion) to sheep shortens their pregnancies.
A fairly common developmental disorder is the separation of the embryo at a very early stage of development, which is accompanied by the development of identical (monozygous) twins. The so-called Siamese twins, which are unseparated organisms, are also known. Non-separation occurs in different ways - from a slight connection to an almost complete merger of two organisms with separated heads or legs. Sometimes, of two Siamese twins, one is normal, but the other is extremely altered, being attached to the normal, being, in essence, a parasite.
So, in the process of development of higher eukaryotes, a single fertilized zygote cell in the course of further development as a result of mitosis gives rise to cells of various types - epithelial, nervous, bone, blood cells and others, which are characterized by a variety of morphology and macromolecular composition. However, cells of different types are also characterized by the fact that they contain the same sets of genes, but are highly specialized, performing only one or several specific functions, i.e. some genes "work" in cells, others are inactive. For example, only erythrocytes are specific in the synthesis and storage of hemoglobin.
Similarly, only epidermal cells synthesize keratin. Therefore, questions have long arisen about the genetic identity of somatic cell nuclei and about the control mechanisms of the development of fertilized oocytes as a prerequisite for understanding the mechanisms underlying cell differentiation.
Since the 1950s, experiments have been carried out in many laboratories on the successful transplantation of somatic cell nuclei into eggs artificially devoid of their own nuclei. The study of DNA from the nuclei of different differentiated cells showed that in almost all cases the genomes contain the same sets of nucleotide pair sequences. Only exceptions are known when mammalian erythrocytes lose their nuclei during the last stage of differentiation. But by this time, pools of persistent hemoglobin mRNAs have already been synthesized, so the nuclei are no longer needed by red blood cells. Other examples are immunoglobulin and T cell genes that are modified during development.
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--PAGE_BREAK--One of the major milestones in the direction of knowledge of the control mechanisms of embryonic ontogenesis were the results of experiments performed in 1960-70. English researcher D. Gurdon in order to find out whether the nuclei of somatic cells have the ability to ensure the further development of eggs, if these nuclei are introduced into eggs from which their own nuclei have been previously removed. a scheme of one of these experiments is given, in which the nuclei of tadpole somatic cells were transplanted into frog eggs with previously removed nuclei. These experiments showed that the nuclei of somatic cells are indeed capable of ensuring the further development of eggs, since they were able to fertilize eggs and “forced” them to develop further. This showed the possibility of animal cloning.
Later, other researchers performed experiments in which it was shown that the transfer of individual blastomeres from 8- and 16-day-old sheep embryos of one breed into the nuclear-free half of the egg (after dissection of the latter in half) of another breed was accompanied by the formation of viable embryos, followed by the birth of lambs.
In early 1997, British authors showed that the introduction of somatic cell nuclei (cells of embryos, fetuses and udders of adult sheep) into artificially deprived nuclei of sheep eggs, and then the implantation of eggs fertilized in this way into the uterus of sheep, is accompanied by the onset of pregnancy followed by the birth of lambs.
The evaluation of these results shows that mammals can be propagated asexually, producing offspring of animals whose cells contain nuclear material of paternal or maternal origin, depending on the sex of the donor sheep, in such cells only the cytoplasm and mitochondria are of maternal origin. This conclusion is of extremely important general biological significance, expanding our views on the potential for animal reproduction. But it is also important to add that we are talking about genetic manipulations that are absent in nature. On the other hand, in practical terms, these genetic manipulations represent a direct way of cloning highly organized animals with desired properties, which is of great economic importance. In medical terms, this path may be used in the future to overcome male infertility.
So, the genetic information necessary for the normal development of the embryo is not lost during cell differentiation. In other words, somatic cells have a property called totipotency, i.e., their genome contains all the information they received from the fertilized egg that gave rise to them as a result of differentiation. The presence of these data certainly means that cell differentiation is subject to genetic control.
It has been established that intensive protein synthesis following fertilization in most eukaryotes is not accompanied by mRNA synthesis. The study of oogenesis in vertebrates, in particular. In amphibians, it has been shown that intense transcription occurs even during prophase I (especially diplotene) of meiosis. Therefore, gene transcripts in the form of mRNA or pro-mRNA molecules are stored in the egg in an inactive state. It has been established that the so-called asymmetric division takes place in embryonic cells, which consists in the fact that the division of the embryonic cell gives rise to two cells, of which only one inherits the proteins involved in transcription. Thus, the unequal distribution of transcription factors between daughter cells leads to the expression of different sets of genes in them after division, i.e., to cell differentiation.
In amphibians, and perhaps most vertebrates, the genetic programs that control early development (up to the blastula stage) are established during oogenesis. Later stages of development, when cellular differentiation begins (approximately from the gastrula stage), require new programs for gene expression. Thus, cell differentiation is associated with the reprogramming of genetic information in one direction or another.
A characteristic feature of cell differentiation is that it irreversibly leads to one or another cell type. This process is called determination and is also under genetic control, and as it is now assumed, cell differentiation and determination is regulated by cell interaction based on signals carried out by peptide growth factors through tyrosine kinase receptors. There are probably many such systems. One of them is that the differentiation of muscle and nerve cells is regulated by neuroregulins, which are membrane proteins that act through one or more tyrosine kinase receptors.
genetic control determinations also demonstrated by the existence of so-called homeiotropic or homeotic mutations, which have been shown in insects to cause changes in determination in specific imaginal discs. As a result, some parts of the body develop out of place. For example, in Drosophila, mutations transform the determination of the antennal disc into a disc that develops into an appendix of a limb extended from the head. In insects of the genus Ophthalmoptera, wing structures may develop from the disc for the eyes. In mice, the existence of the Hox gene cluster (complex) has been shown, which consists of 38 genes and controls the development of limbs.
Of independent importance is the question of the regulation of gene activity during embryonic development. It is believed that during differentiation, genes act at different times, which is expressed in transcription in different differentiated cells of different mRNAs, i.e., repression and derepression of genes take place. For example, the number of genes transcribed into RNA in sea urchin blastocytes is 10%, in rat liver cells it is also 10%, and in cattle thymus cells it is 15%. It is assumed that nonhistone proteins are involved in the control of the transcriptional status of genes. The following data support this assumption. When cell chromatin in phase is transcribed in the in vitro system, only histone mRNA is synthesized, followed by histones. In contrast, when G1-phase cell chromatin is used, no histone mRNA is synthesized. When non-histone proteins are removed from the G1-phase chromatin and replaced by non-histone chromosomal proteins synthesized in the S phase, histone mRNA is synthesized after transcription of such chromatin in vitro. Moreover, when non-histone proteins originate from the G1 phase of cells, and DNA and histones from the S phase of cells, no histone mRNA is synthesized. These results indicate that non-histone proteins contained in chromatin determine the possibility of transcription of genes encoding histones. Therefore, it is believed that non-histone chromosomal proteins can play an important role in the regulation and expression of genes in eukaryotes.
The available data suggest that protein and steroid hormones are involved in the regulation of transcription in animals. Protein (insulin) and steroid (estrone and testosterone) hormones are two signaling systems used in cell-to-cell communication. In higher animals, hormones are synthesized in specialized secretory cells. Being released into the bloodstream, they enter the tissues, since the molecules of protein hormones are relatively large, they do not penetrate into the cells. Therefore, their effects are mediated by receptor proteins localized in target cell membranes and by intracellular levels of cyclic AMP (cAMP). On the contrary, steroid hormones are small molecules, as a result of which they easily penetrate into cells through membranes. Once inside the cells, they bind to specific receptor proteins that are present in the cytoplasm of only target cells. It is believed that hormone + protein receptor complexes, concentrating in the nuclei of target cells, activates the transcription of specific genes through interaction with certain non-histone proteins that bind to the promoter regions of specific genes. Therefore, the binding of the hormone + protein complex (protein receptor) to non-histone proteins releases the promoter regions for the movement of RNA polymerase. Summarizing the data on the genetic control of the embryonic period in the ontogeny of organisms, it can be concluded that its course is controlled by the differential switching on and off of the action of genes in different cells (tissues) through their derepression and repression.
Postembryonic period
After the birth of an organism, its postembryonic development (postnatal for humans) begins, which in different organisms proceeds from several days to hundreds of years, depending on their species. Therefore, life expectancy is a species trait of organisms, independent of their level of organization.
In postembryonic ontogenesis, juvenile and pubertal periods are distinguished, as well as the period of old age ending in death.
Juvenile period. This period (from lat. juvenilis - young) is determined by the time from the birth of the organism to puberty. In different organisms, it proceeds differently and depends on the type of ontogenesis of organisms. This period is characterized by either direct or indirect development.
In the case of organisms that are characterized by direct development (many invertebrates, fish, reptiles, birds, mammals, humans), hatched from egg shells or newborns are similar to adult forms, differing from the latter only in smaller sizes, as well as underdevelopment of individual organs and imperfect proportions body
A characteristic feature of the growth in the juvenile period of organisms subject to direct development is that there is an increase in the number and size of cells, and body proportions change. The growth of different human organs is uneven. For example, the growth of the head ends in childhood, the legs reach a proportional size by about 10 years. The external genitalia grow very quickly at the age of 12-14 years. Distinguish between definite and indefinite growth. A certain growth is characteristic of organisms that stop growing by a certain age, for example, insects, mammals, humans. Indefinite growth is characteristic of organisms that grow throughout their lives, such as molluscs, fish, amphibians, reptiles, and many plant species.
In the case of indirect development, organisms undergo transformations called metamorphoses (from Latin metamorphosis - transformation). They represent modifications of organisms in the process of development. Metamorphoses are widely found in coelenterates (hydras, jellyfish, coral polyps), flatworms (fasciolae), roundworms (roundworms), molluscs (oysters, mussels, octopuses), arthropods (crayfish, river crabs, lobsters, shrimps, scorpions, spiders, ticks, insects) and even in some chordates (tunicates and amphibians). At the same time, complete and incomplete metamorphoses are distinguished. The most expressive forms of metamorphoses are observed in insects that undergo both incomplete and complete metamorphoses.
Incomplete transformation is such a development in which an organism comes out of the egg membranes, the structure of which is similar to the structure of an adult organism, but the dimensions are much smaller. Such an organism is called a larva. In the process of growth and development, the size of the larvae increases, but the existing chitinized cover prevents a further increase in body size, which leads to molting, i.e., shedding of the chitinized cover, under which there is a soft cuticle. The latter straightens out, and this is accompanied by an increase in the size of the animal. After several molts, the animal reaches maturity. Incomplete transformation is typical, for example, in the case of the development of bedbugs
Complete transformation is such a development in which a larva is released from the egg membranes, which differs significantly in structure from adults. For example, in butterflies and many insects, caterpillars are larvae. Caterpillars are subject to molting, and they can molt several times, then turning into pupae. From the latter, adult forms (imago) develop, which do not differ from the original ones.
In vertebrates, metamorphoses are found among amphibians and bony fishes. The larval stage is characterized by the presence of provisional organs, which either repeat the characteristics of their ancestors, or have a clearly adaptive value. For example, a tadpole, which is the larval form of a frog and repeats the features of the original form, is characterized by a fish-like shape, the presence of gill breathing, and one circle of blood circulation. Adaptive features of tadpoles are their suckers, long intestines. It is also characteristic of larval forms that, in comparison with adult forms, they are adapted to life in completely different conditions, occupying a different ecological niche and a different place in the food chain. For example, frog larvae have gill breathing, while adult forms are pulmonary. Unlike adult forms, which are carnivorous creatures, frog larvae feed on plant foods.
The sequence of events in the development of organisms is often referred to as life cycles, which can be simple or complex. The simplest cycles of development are typical, for example, for mammals, when an organism develops from a fertilized egg, which again produces eggs, etc. Complex biological cycles are cycles in animals, which are characterized by development with metamorphoses. Knowledge of biological cycles is of practical importance, especially in cases where organisms are causative agents or vectors of pathogens in animals and plants.
Development and differentiation associated with metamorphosis are the result of natural selection, due to which many larval forms, for example, insect caterpillars and frog tadpoles, are better adapted to the environment than adult sexually mature forms.
puberty. This period is also called mature, and it is associated with the sexual maturity of organisms. The development of organisms during this period reaches a maximum.
Environmental factors have a great influence on growth and development in the postembryonic period. For plants, the decisive factors are light, humidity, temperature, and the quantity and quality of nutrients in the soil. For animals, complete feeding is of paramount importance (the presence of proteins, carbohydrates, lipids, mineral salts, vitamins, microelements in the feed). Oxygen, temperature, light (vitamin D synthesis) are also important.
The growth and individual development of animal organisms are subject to neurohumoral regulation by humoral and nervous mechanisms of regulation. Hormone-like active substances, called phytohormones, have been found in plants. The latter affect the vital functions of plant organisms.
In the cells of animals in the process of vital activity, chemically active substances are synthesized that affect the processes of vital activity. The nerve cells of invertebrates and vertebrates produce substances called neurosecrets. The glands of endocrine, or internal, secretion also secrete substances that are called hormones. The endocrine glands, in particular those related to growth and development, are regulated by neurosecretions. In arthropods, the regulation of growth and development is very well illustrated by the effect of hormones on molting. The synthesis of larval secretion by cells is regulated by hormones that accumulate in the brain. In a special gland in crustaceans, a hormone is produced that inhibits molting. The levels of these hormones determine the frequency of shedding. In insects, hormonal regulation of egg maturation and diapause have been established.
In vertebrates, the endocrine glands are the pituitary, pineal, thyroid, parathyroid, pancreas, adrenals and sex glands, which are closely related to one another. The pituitary gland in vertebrates produces a gonadotropic hormone that stimulates the activity of the sex glands. In humans, the pituitary hormone affects growth. With a deficiency, dwarfism develops, with an excess - gigantism. The pineal gland produces a hormone that affects the seasonal fluctuations in the sexual activity of animals. Thyroid hormone affects the metamorphosis of insects and amphibians. In mammals, underdevelopment of the thyroid gland leads to growth retardation, underdevelopment of the genital organs. In humans, due to a defect in the thyroid gland, ossification, growth (dwarfism) is delayed, puberty does not occur, mental development stops (cretinism). The adrenal glands produce hormones that affect cell metabolism, growth, and differentiation. The gonads produce sex hormones that determine secondary sexual characteristics. Removal of the gonads leads to irreversible changes in a number of signs. For example, in castrated roosters, the growth of the comb stops, the sexual instinct is lost. A castrated man acquires an outward resemblance to a woman (a beard and hair do not grow on the skin, fat is deposited on the chest and in the pelvic region, the timbre of the voice is preserved, etc.).
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Reproduction and individual development of organisms
Introduction
Reproduction, or the ability to reproduce itself, is one of the basic properties of all living organisms, from bacteria to mammals and flowering plants. Thanks to it, the existence of each species is ensured, continuity is maintained between parent individuals and their offspring. The forms of reproduction of organisms are diverse and will be discussed below.
All forms of reproduction are based on cell division, which proceeds quite similarly in plants and animals. Since the complex processes associated with sexual reproduction arose on the basis of cell division, we will first of all consider the process leading to the formation of two cells from one cell.
- mitotic cell division
Interphase and various methods of cell division. There are two methods of division: I) the most common, complete division - mitosis (indirect division) and 2) amitosis (direct division). During mitotic division, the cytoplasm is restructured, the nuclear envelope is destroyed, and chromosomes are identified. In the life of a cell, there is a period of mitosis itself and an interval between divisions, which is called interphase. However, the period of interphase (non-dividing cells) in its essence can be different. In some cases, during interphase, the cell functions and simultaneously prepares for the next division. In other cases, cells enter interphase, function, but no longer prepare for division. As part of a complex multicellular organism, there are numerous groups of cells that have lost the ability to divide. These include, for example, nerve cells. Cell preparation for mitosis occurs in interphase. In order to imagine the main features of this process, remember the structure of the cell nucleus.
The basic structural unit of the nucleus is the chromosomes, which are made up of DNA and protein. In the nuclei of living nondividing cells, as a rule, individual chromosomes are indistinguishable, but most of the chromatin, which is found on stained preparations in the form of thin filaments or grains of various sizes, corresponds to the chromosomes. In some cells, individual chromosomes are also clearly visible in the interphase nucleus, for example, in rapidly dividing cells of a developing fertilized egg and in the nuclei of some protozoa. At different periods of life, chromosome cells undergo cyclical changes that can be traced from one division to another.
Chromosomes during mitosis are elongated dense bodies, along the length of which two strands can be distinguished - chromatids containing DNA, which are the result of chromosome doubling. Each chromosome has a primary constriction, or centromere. This narrowed part of the chromosome can be located either in the middle or closer to one of the ends, but for each particular chromosome its place is strictly constant. During mitosis, the chromosomes and chromatids are tightly coiled helical filaments (a spiralized or condensed state). In the interphase nucleus, the chromosomes are strongly elongated, i.e., despiralized, due to which they become difficult to distinguish. Consequently, the cycle of chromosome changes consists in spiralization, when they shorten, thicken and become clearly distinguishable, and despiralization, when they are strongly elongated, intertwined, and then it becomes impossible to distinguish each separately. Spiralization and despiralization are associated with the activity of DNA, since it functions only in a despiralized state. The release of information, the formation of RNA on DNA in a spiralized state, that is, during mitosis, stops.
The fact that chromosomes are present in the nucleus of a non-dividing cell is also proved by the constancy of the amount of DNA, the number of chromosomes, and the preservation of their individuality from division to division.
Preparation of the cell for mitosis. During interphase, a number of processes occur that enable mitosis. Let's name the most important of them: 1) centrioles are doubled, 2) chromosomes are doubled, i.e. the amount of DNA and chromosomal proteins, 3) proteins are synthesized from which the achromatin spindle is built, 4) energy is accumulated in the form of ATP, which is consumed during division, 5) cell growth ends.
Of paramount importance in preparing a cell for mitosis is the synthesis of DNA and duplication of chromosomes.
The doubling of chromosomes is associated primarily with the synthesis of DNA and the simultaneous synthesis of chromosome proteins. The doubling process lasts 6-10 hours and occupies the middle part of the interphase. Chromosome duplication proceeds in such a way that each old single strand of DNA builds a second one for itself. This process is strictly ordered and, starting at several points, spreads along the entire chromosome.
Mitosis. Phases of mitosis
Mitosis is a universal method of cell division in plants and animals, the main essence of which is the exact distribution of duplicated chromosomes between both formed daughter cells. The preparation of a cell for division, as we can see, occupies a significant part of the interphase, and mitosis begins only when the preparation in the nucleus and cytoplasm is completely completed. The whole process is divided into four phases. During the first of them - prophase - centrioles divide and begin to diverge in opposite directions. Around them, achromatin filaments are formed from the cytoplasm, which, together with centrioles, form an achromatin spindle. When the divergence of the centrioles ends, the whole cell is polar, both centrioles are located at opposite poles, and the middle plane can be called the equator. The filaments of the achromatin spindle converge at the centrioles and are widely distributed at the equator, resembling a spindle in shape. Simultaneously with the formation of a spindle in the cytoplasm, the nucleus begins to swell, and a ball of thickened threads - chromosomes - is clearly distinguished in it. During prophase, chromosomes spiralize, shortening and thickening. Prophase ends with the dissolution of the nuclear envelope, and the chromosomes are found to be lying in the cytoplasm. At this time, it can be seen that all chromosomes are already double.
Then comes the second phase - metaphase. Chromosomes, randomly arranged at first, begin to move towards the equator. All of them are usually located in the same plane at an equal distance from the centrioles. At this time, part of the spindle threads is attached to the chromosomes, while the other part of them still stretches continuously from one centriole to another - these are the supporting threads. Pulling, or chromosomal, threads are attached to centromeres (primary constrictions of chromosomes), but it must be remembered that both chromosomes and centromeres are already double. Pulling threads from the poles are attached to those chromosomes that are closer to them. There is a short pause. This is the central part of mitosis, after which the third phase begins - anaphase.
During anaphase, the pulling filaments of the spindle begin to contract, stretching the chromosomes to different poles. In this case, the chromosomes behave passively, they, bending like a hairpin, move forward by centromeres, for which they are pulled by a spindle thread. At the beginning of anaphase, the viscosity of the cytoplasm decreases, which contributes to the rapid movement of chromosomes.
Consequently, the threads of the spindle ensure the exact divergence of chromosomes (doubling even in interphase) to different poles of the cell.
Mitosis ends with the last stage, telophase. Chromosomes, approaching the poles, are closely intertwined with each other. At the same time, their stretching (despiralization) begins, and it becomes impossible to distinguish between individual chromosomes. Gradually, the nuclear envelope is formed from the cytoplasm, the nucleus swells, the nucleolus appears, and the previous structure of the interphase nucleus is restored.
At the end of anaphase or at the beginning of telophase, the division of the cytoplasm begins. In animal cells, a constriction appears on the outside in the form of a ring, which, deepening, divides the cell into two smaller ones. In plants, the cytoplasmic membrane arises in the middle of the cell and spreads to the periphery, dividing the cell in half. After the formation of the plasma membrane, a cellulose membrane appears in plant cells. Consequently, both the nucleus and the cytoplasm take an active part in cell division. The nucleus contains unique cell structures - chromosomes, and the achromatin spindle, formed from the cytoplasm, distributes them correctly and equally between both daughter cells.
Length of mitosis and interphase
Mitosis is a relatively short period in the life of a cell; interphase lasts much longer, as can be seen from the table.
In rapidly reproducing cells, mitosis can last only a few minutes. Consequently, the duration of mitosis varies from several minutes to 2-3 hours. Interphase lasts from 8-10 hours to several days.
The speed at which the individual phases of mitosis proceed is also different:
- The constancy of the number and individuality of chromosomes
Chromosomes consist of DNA and protein, i.e., in their own way chemical composition they are all similar, but differ in shape and size, the location of the primary constriction, and the presence of secondary constrictions. The study of the chromosomes of many plants and animals has shown that they have a certain individuality. In addition, it was found that all chromosomes (with the exception of the so-called sex chromosomes) form homologous pairs. The paired set is characteristic of somatic cells (non-sex) and is called diploid. Six chromosomes of a skerda plant belonging to the Compositae family. These six chromosomes form three distinct pairs. However, chromosomes are not always well distinguishable; 3 pairs of mosquito chromosomes are difficult to distinguish by external signs. The number of chromosomes and their individuality are preserved in all cells and are characteristic features for each species. The table shows data on the number of chromosomes in some plant and animal species:
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Diploid number of chromosomes |
Diploid number of chromosomes |
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house fly |
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fruit drosifila |
Chimpanzee |
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Amitosis. Amitosis is a division of the nucleus in the interphase state without previous spiralization of chromosomes and rearrangement of the nucleus. For example, in some connective tissue cells, the nucleus is elongated, a constriction appears in the middle, which deepens, and there are two nuclei in the cell. Then the same constriction begins to divide the cytoplasm, and two cells are obtained. In many cases, only the nucleus divides, and as a result, the cell becomes two- or multi-nuclear (if there were several such divisions). Sometimes the nucleus during amitosis is divided into two unequal parts: one is large, and the other is smaller. Apparently, during amitosis, DNA is unevenly distributed between the daughter nuclei.
Amitosis is often observed in pathological conditions or under the action of unfavorable factors on the cell, for example, after the action of low temperature or X-rays, i.e., such influences that disrupt mitosis. After ligation of nuclei during amitosis, in most cases the cytoplasm does not divide, and the very presence of ligation of the nucleus, as a rule, indicates irreversible changes in the cell, which will sooner or later lead to its death.
Mitosis is the primary method of cell division, the most common and physiologically complete. Amitosis should be considered as its modification, i.e., a secondary phenomenon. Amitosis is relatively rare and is an inferior way of nuclear and cell division.
- Lifespan, aging and cell death
The growth and development of multicellular organisms are associated with an increase in mass, which is carried out by cell division. For example, the development of a rat, which began with a single cell. On the 12-13th day of development, the embryo contains 50 million cells. By the time of birth, a baby rat already consists of 6 billion, and a three-month-old rat - of about 67 billion cells.
In mammals and many other animals, in addition to growth associated with an increase in the number of cells, there is a constant death and replacement of some cells by others through their division. For example, keratinized cells of the skin epithelium are constantly exfoliated and replaced by new ones. The same thing happens with blood cells. So, it is estimated that in an adult person of average weight, about 2 billion red blood cells - erythrocytes - die off in one second and are replaced by new ones coming from the bone marrow, where their loss is constantly replenished by division. Therefore, the lifespan of reproducing cells is determined by the duration of interphase, that is, the time that lasts from one division to another. But there is also another segment of the life of the cell - from the last division to its death, that is, the period when the cell lives and functions, but no longer divides. Thus, nerve cells in mammals cease to multiply by the time of birth or shortly after birth, their life expectancy is on average equal to the life expectancy of the organism. In other tissues, the function is associated with the constant death and renewal of cells; for example, erythrocytes, getting into the bloodstream, live and function there for about 120 days, and then die. The same happens with leukocytes, which live and function for only a few days. The tissues whose function is associated with cell renewal include various epithelia. The examples given show that mitotic cell division in an adult organism is associated with normal cell renewal, i.e., physiological regeneration. Cell division also provides tissue repair during regeneration after cuts, burns or any other damage. Naturally, during the growth of the organism, the number of reproducing cells is greater than the number of dying ones, which ensures a general increase in the mass of cells.
Aging and cell death
Aging and death of cells can be directly, and not associated with aging and death of the organism. In erythrocytes, the loss of the nucleus, which makes protein synthesis impossible, predetermines the inevitable death of the cell, which depends on the aging of its own proteins. When the cells of the skin epithelium become keratinized, a special protein accumulates in the cytoplasm, which leads the cells to death. In all cases, the onset of aging is associated with the cessation of division and the accumulation of specific proteins in the cytoplasm, which leads to cell death. The situation is different with long-living cells, for example, nerve cells. With aging, metabolism is disturbed, pigment grains, sometimes drops of fat, accumulate in the cytoplasm. In these cases, the death of a mass of cells is associated with aging and death of the organism. From the above examples, it can be seen that the signs of aging are detected, as a rule, in the cytoplasm. When cells are placed in an artificial nutrient medium (tissue culture), they can multiply indefinitely. To do this, it is necessary to constantly change the nutrient medium and remove excess cells. For example, a chicken tissue culture existed for about 50 years. A number of other tissue cultures have been maintained for decades.
One might think that the nucleus has nothing to do with cell aging. However, it is not. Cells arising after abnormal mitoses may contain an incomplete set of chromosomes, which will inevitably lead to cell death both in the body and in tissue culture. Consequently, signs of aging can be carried by: 1) the nucleus and its genetic apparatus, 2) the entire cell as a whole, or 3) only the cytoplasm.
- Forms of reproduction of organisms
As mentioned above, there are several forms of reproduction of organisms, of which we consider the main ones: 1) sexual reproduction, 2) asexual and 3) vegetative reproduction.
Asexual and vegetative reproduction. Asexual reproduction is widespread in nature in animals and plants. For example, the division of ciliates is the same as the division of other unicellular organisms. Among plants, asexual reproduction is characteristic of spores: algae, fungi, mosses and ferns. In all cases of asexual reproduction of a plant, it is carried out at the expense of spores. Therefore, asexual reproduction is called reproduction with the help of one cell, which does not carry the characteristics characteristic of germ cells. During vegetative reproduction, a group of somatic cells is separated from the mother organism, from which the daughter organism develops. A typical example is the reproduction of freshwater hydra. A slight thickening appears on the side of her body, which then turns into an outgrowth (kidney). This outgrowth consists of endoderm and ectoderm cells. Gradually, the outgrowth lengthens, at the front end a mouth is formed, around which tentacles appear. The whole process ends with the formation of a small daughter hydra.
Vegetative propagation is especially widespread in plants. So, individual willow branches, taking root, develop into a new plant. Propagation by cuttings is widespread and is used in the propagation of a number of plants. Another example is the vegetative propagation of strawberries. The aerial parts of the stem, growing and strongly stretching, form the so-called mustache. Once in the soil, the ends of the mustache take root, and a new plant is formed from them.
Sexual reproduction. Unlike vegetative reproduction, both in plants and animals, sexual reproduction always occurs due to specialized germ cells - eggs and spermatozoa, which are formed in the sex glands. Sex cells contain a haploid (half) number of chromosomes, and hence half the amount of DNA. In such a haploid set of each pair of chromosomes present in somatic cells, only one chromosome is present. The eggs of various animals are usually large and immobile. Their sizes vary greatly. For example, among mammals in a rabbit, the egg cell diameter is 0.2 mm. The size of the egg is determined by the content of the reserve nutrient in the cytoplasm - the yolk. Large eggs contain a large amount of yolk, which is a striking example of a huge egg of a bird. The egg of a bird is that part of the egg that is usually called the yolk in the hostel (its diameter is about 3 cm). On one side of the yolk is a white speck representing the active cytoplasm with a nucleus. It is from this small area that the embryo develops, and the rest of the mass contains reserve nutrients that ensure the development of the chicken in the egg. Such an egg is surrounded by shells - protein and shell, which are additional formations. These shells ensure the development of the embryo in the air. Smaller eggs in fish and amphibians. These are eggs with a diameter of several millimeters. They contain quite a lot of yolk in the cytoplasm, but much less than in birds. Small eggs contain very little yolk, and it is evenly distributed throughout the egg. The egg cell's own membrane, formed by the surface of the cytoplasm, is called the yolk membrane. In addition to it, a more or less developed protein coat arises, which is secreted by the cells of the oviducts. Either in the center of the egg, or at the edge, there is one relatively large nucleus.
The sperm is always many times smaller than the egg. Mammalian spermatozoa have a typical shape for many animals, which consist of three sections: head, neck and tail. The nucleus is located in the head, in addition to it, at the anterior end there is a small area of compacted cytoplasm, with the help of which the sperm enters the egg. The neck - the narrowed part behind the head - contains the centriole and passes into a thin elongated cytoplasmic thread - the tail. The tail is similar to the flagellum of the flagellate or ciliate cilium. Thanks to its movement, spermatozoa actively move.
Development of germ cells
Both the testis, in which spermatozoa are formed, and the ovary, in which eggs are formed, can be represented as a tube, inside which the entire process of formation of germ cells takes place. At the very beginning of the tube there are primary germ cells, which divide by ordinary mitosis, due to which their number increases all the time. This area of the gonad is called the reproductive zone. Moving to the next zone, the cells begin to grow, forming a growth zone. The growth process is more pronounced during the formation of female germ cells - ovogenesis ("ovum" - egg, "genesis" - development, lat.). Less pronounced period of growth during the formation of male germ cells - spermatogenesis.
During growth, in addition to an increase in the mass of the cytoplasm, there is also an increase in the size of the nucleus. The grown cells (during spermatogenesis) are called spermatocytes of the 1st order, they enter the period of maturation and pass into the zone of maturation.
During this process, spermatocytes divide twice, i.e. four cells are formed from one spermatocyte. Each of them further turns into a spermatozoon.
During oogenesis, the growth period usually lasts longer than during spermatogenesis, the cell that has passed into the growth zone is called the 1st order oocyte. During growth, it increases hundreds, and sometimes thousands of times due to the accumulation of reserve nutrients. For example, from an oocyte with a diameter of 20-30 microns, as a result of growth, a frog egg cell with a diameter of 3-4 mm is formed.
The grown oocytes begin maturation, which consists of two divisions (as in spermatogenesis), but outwardly these divisions proceed differently. When dividing, a 1st-order oocyte separates a small cell (director body), and a large cell remains. Then the second division takes place, in which the next directional body is released and a large, already mature egg is formed. While the second division is taking place, the first directional body has time to divide, and in total four cells are formed from the oocyte: three small and one large - the egg, which retains all the yolk accumulated during growth, which is necessary for the development of the embryo.
maturation of germ cells (meiosis). The number of chromosomes for the cells of each plant or animal species is constant. This constancy in all cells is maintained by mitosis, which is preceded by chromosome doubling. How is the constancy of the number of chromosomes maintained during sexual reproduction, when a new organism arises from the fusion of two germ cells? Mature germ cells contain only half the (haploid) number of chromosomes, and, accordingly, half the amount of DNA. The table shows two examples illustrating the ratio of the number of chromosomes and the amount of DNA in the somatic and germ cells of a cat and a rabbit.
The decrease in the number of chromosomes by half occurs in the process of maturation of germ cells. Externally, the maturation process consists of two subsequent divisions: the first and second. In this case, four cells are formed from one spermatocyte, and each of them turns further into a spermatozoon. In oogenesis, only one egg and three directional bodies are formed from an oocyte, i.e., also four cells. A decrease in the number of chromosomes occurs during meiosis and is determined by the fact that of each pair of homologous chromosomes, only one remains in a mature germ cell. Preparation for meiosis, especially during the formation of eggs, begins long before the first division of maturation occurs. Meiosis begins with DNA synthesis and the corresponding doubling of the number of chromosomes, which proceeds in the same way as during mitosis. Further, the chromosomes in the prophase of meiosis shorten, become clearly distinguishable, each of them is doubled, but they do not diverge, remaining connected, and behave as a single whole (2).
Following the duplication of chromosomes, their conjugation occurs, which consists in the fact that paired homologous and already duplicated chromosomes closely approach and temporarily unite. Conjugation occurs along the entire length of the chromosomes - from one end to the other. At the same time, they twist, and it seems that the number of chromosomes has halved (3). It is important to emphasize that temporary pairing (conjugation) of chromosomes always occurs only between homologous (paired) chromosomes. After conjugation, the chromosomes diverge, but in some places they are connected so tightly that when they diverge, breaks occur in the transverse direction and mutual exchange of sites. This process is of great importance for understanding some of the laws of inheritance of traits, which will be discussed in detail in chapter IX.
After the end of conjugation, the chromosomes diverge and the metaphase of the first division of maturation occurs, outwardly similar to the metaphase of mitosis, but the divergence of chromosomes occurs differently than during mitosis (4). During the anaphase of meiosis, homologous, already doubled chromosomes diverge to opposite poles. Thus, only one (5) of each pair of homologous chromosomes enters the daughter cell. If we take into account that each pair of homologous chromosomes (in the diagram of the same size) consists of one paternal and the other maternal, which are indicated in the diagram in different colors, it becomes clear that after division, either the paternal or maternal chromosome enters the spermatocyte.
Following the first comes the second division of maturation. Now division is no longer preceded by DNA synthesis (6). All chromosomes are double, they are located in metaphase, as in mitosis, and in anaphase they diverge to opposite poles, and both daughter cells (spermatids) have the same set of chromosomes. Therefore, before meiosis begins, there is only one duplication of chromosomes, followed by two maturation divisions, resulting in a halving of the number of chromosomes. However, the main difference between meiosis and mitosis is not only this. The duplicated chromosomes conjugate and exchange separate sections. During mitosis, chromosomes double and are evenly distributed between daughter cells. During reduction division, the chromosomes from each homologous pair fall into different daughter cells.
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