Stages of gametogenesis. Spermatogenesis and ovogenesis are the processes of formation and formation of germ cells. How does the formation of germ cells occur in humans

In our article, we will get acquainted with such a concept as gametogenesis. This is a process that is characteristic only for organisms that reproduce sexually.

Concept definition

In the course of sexual reproduction of individuals, a new generation arises as a result of the fusion of specialized cells. They are called gametes.

In the animal body, they are formed in special organs - glands. Older organisms are hermaphroditic. In this case, two types of cells mature in one organism - female and male. These include coelenterates, all types of worms, and some molluscs. In the course of evolution, dioecious species appeared.

Gametogenesis is a process that also occurs in plants. For example, in angiosperms, the generative organ is a flower. Spores form gametophytes.

Stages of gametogenesis

Despite significant differences in the structure of female and male germ cells, they go through a number of identical stages of development. The stages of gametogenesis are as follows: reproduction, growth, maturation. In the first, the primary germ cells are repeatedly crushed. At the same time, the diploid set in the nuclei of their chromosomes is preserved, which ensures an increase in the number of gametes.

In the growth phase, an increase in cell size occurs. They also carry out the process of DNA replication. Its essence lies in the synthesis of a daughter nucleic acid molecule based on the parent. Strengthened growth is accompanied by the accumulation of nutrients necessary for further development.

As a result of the maturation stage, the emerging sex cells divide by meiosis. Since it is reduction, haploid gametes are formed from diploid gametes. They have a single set of chromosomes. This is very important because the sexual process involves cell fusion. Gametogenesis ends with the maturation stage.

Features of the formation of male germ cells

The process of gametogenesis has its own distinctive features. As a result of the maturation stage, four haploid male germ cells are formed. They divide by meiosis. As a result, haploid spermocytes are formed. At the stage of formation in their cells, the nucleus and cytoplasm become denser, and the sizes become much smaller - almost 30 times than in their predecessors.

The process of spermatogenesis lasts 80 days. The length of male reproductive cells of mammals reaches a maximum of 800 microns. They consist of a head, in which the nucleus is located, a neck and a tail. The last part ensures the movement of spermatozoa.

Formation of eggs

The process of formation of female gametes is called oogenesis. At the stage of egg growth, after the first meiotic division, two cells are formed. The cytoplasm is unevenly distributed between them, so they differ in structure. The first one is larger. It contains a large number of reserve substances and is called an oocyte. The second is a polar body, much smaller.

As a result of the second division, four cells are formed. Only one of them is viable. This is an ovum. The other three are called directional or polar bodies. They contain a minimal amount of nutrients, and are subsequently destroyed.

What is the meaning of such gametogenesis? This guarantees a reduction in the number of eggs that are fertile and provides the required amount of nutrients.

gametogenesis in animals

The process of formation of germ cells of animals occurs in specialized glands. In the male body, these are the testes, in the female body, the ovaries. After gametogenesis, fertilization follows - the fusion of germ cells. In animals, two types of this process are distinguished: external and internal.

In the first case, fertilization occurs outside the body of the female. It is characteristic of some worms, mollusks, as well as fish and amphibians. But the predominant number of terrestrial and some aquatic animals is characterized by internal fertilization. This is a more progressive type, since it provides protection and viability for the future organism.

Gametogenesis in plants

In different systematic groups of plants, gametogenesis and further fertilization have their own characteristics. This process is the most progressive in flowering, or angiosperms. From the mother cell in the lower expanded part of the pistil, which is called the ovary, four cells are formed during meiosis. They are called megaspores and have a haploid set of chromosomes. At the same time, only one of them continues to divide, and the remaining three die off. From a viable cell, an embryo sac develops, in which the central germ and ovum are located.

In the anthers of the stamen, as a result of reduction cleavage, microspores with a haploid set of chromosomes develop. They keep sharing. As a result, pollen grains are formed.

In flowering plants, fertilization is preceded by pollination. This is the transfer of sperm from the anther of the stamen to the stigma of the pistil, which is carried out with the help of wind, insects or humans. Two male gametes take part in the process of fertilization. One of them fuses with the female gamete to form an embryo. Another sperm connects with the central germ cell, forming a reserve nutrient - the endosperm. Therefore, this type of fertilization is called double.

So, gametogenesis is the formation of germ cells. During this process, which occurs in several stages, gametes with a single chromosome set are formed. This is because the gametes fuse later on. In this case, a diploid organism is formed.

The formation of germ cells. Fertilization.

Completed by a group of biologists: Martemyanova Anastasia

Sokolova Ekaterina Tereshchenko Maria 10 "A" class students

Checked by: Evgenia Semyonovna Matrosova biology teacher

Gametes(from the Greek γᾰμετή - wife, γᾰμέτης - husband) - reproductive cells that have a haploid (single) set of chromosomes and participate in gamete, in particular, sexual reproduction. When two gametes merge in the sexual process, a zygote is formed, which develops into an individual (or group of individuals) with the hereditary characteristics of both parental organisms that produced gametes.

Sperm.

Spermatozoon - male sex cell, the male gamete, which serves to fertilize the female gamete, the egg.

Egg.

Egg(scientific oocyte, less often oocyte) - female gamete of animals, higher plants, as well as many algae and other protists that are oogamous. As a rule, eggs are haploid cells, but may have a different ploidy in polyploid organisms.

external fertilization

With external fertilization sperm live from 0.5 to 5 minutes. Productivity aquarium fish varies from tens to several thousand pieces. The number of developing eggs is influenced by the age, condition and size of females, hereditary factors, habitat and good nutrition. Less fecundity is characteristic of species that take care of their offspring.

Internal fertilization.

With internal fertilization, the fusion of germ cells takes place inside the body. In this case, the probability of fertilization increases, and the possibility of death of gametes from the influence of adverse environmental conditions decreases.

double fertilization

Two sperm penetrate the ovary of angiosperms, one of them merges with the egg, giving rise to a diploid embryo. The other connects to the central diploid cell. A triploid cell is formed, from which the endosperm will arise - the nutrient material for the developing embryo.

double fertilization

This process, characteristic of all angiosperms, was discovered at the end of the last century by S.G. Navashin and was called double fertilization. The significance of double fertilization lies in the fact that the active development of nutrient tissue is ensured after fertilization. Therefore, the ovule in angiosperms does not store nutrients for the future and, consequently, develops much faster than in many other plants, such as gymnosperms.

Gametogenesis is the process of formation of germ cells. Multicellular animals have a diploid set of chromosomes (2n). In the process of gametogenesis, which is based on meiosis, the resulting gametes have a haploid set of chromosomes (n).

Sex cells develop in the sex glands or specialized cells - in the testes in males and in the ovaries in females. These cells are laid down at an early stage embryonic development.

Gametogenesis proceeds sequentially, in three stages and ends with the maturation of gametes.

breeding stage. The initial primary germ cells with a diploid set of chromosomes are formed in the genital organs. During this period, cells divide - mitosis occurs, which leads to an increase in their number. Cells have a diploid set of chromosomes.

growth stage. The resulting cells grow, actively synthesize and store nutrients. This period corresponds to the interphase before meiotic division.

ripening stage. At this stage, meiosis occurs, as a result of which gametes with a haploid set of chromosomes are finally formed and mature.

Formation of male sex cells

Spermatogenesis is the process of formation of male germ cells - spermatozoa.

During the reproduction period, from the cells of the spermatogenic tissue, as a result of mitosis, numerous cells are formed - spermatogonia with a diploid set of chromosomes. The laying of the primary cells of spermatogonia occurs even in embryonic development, that is, before the birth of the organism, and intensive division occurs only after reaching puberty.

During the period of growth, spermatogonia slightly increase in size, and a spermatocyte of the 1st order develops from each cell, ready for division.

At the stage of maturation, as a result of the first division of meiosis, two cells are formed - spermatocytes of the 2nd order, and after the second division, four cells of the same size develop - spermatids with a haploid set of chromosomes. All four cells undergo complex cellular differentiation and develop into four spermatozoa.

Thus, four gametes are formed from each primary male germ cell. The hormone responsible for spermatogenesis in mammals is called testosterone.

The formation of female germ cells

Ovogenesis is the process of formation of female reproductive cells - eggs.

In the ovogenic tissue of the ovaries at the stage of reproduction, the primary germ cells - ovogonia with a diploid set of chromosomes divide several times by mitosis. Due to this, the growth of ovogenic tissue occurs. Further, each oogonia turns into an oocyte of the 1st order, which at the next stage begins to grow intensively, accumulating nutrients in the form of yolk grains.

The growth process of an oocyte takes much longer than that of a spermatocyte.

After growth, maturation of the 1st order oocyte occurs. The cell starts meiosis, but the process of division is delayed for a long time. For example, in mammals, division begins in the embryonic state, but stops at prophase I until the female's puberty, i.e. for several weeks, months or years, depending on the type of organism. Later, under the influence of sex hormones, meiosis continues further.

The first division of meiosis occurs asymmetrically: one large cell is formed - an oocyte of the 2nd order, where all the nutrients and organelles pass, and one small cell - the primary polar, or directional, body - in which there is only a nucleus.

The second division of meiosis is also asymmetric. From the oocyte of the 2nd order, one large cell is formed - the egg, in which all the nutrients are located, and one secondary polar (directive) body. Two small secondary polar bodies are formed from the primary polar body. In most vertebrates, the second division of meiosis stops at the stage of metaphase II of meiosis, and the formation of the egg is completed only after fertilization.

Thus, during oogenesis, from each primary female germ cell - ovogonia, one large egg with a haploid set of chromosomes and three polar bodies are formed, which are reduced. They serve only for the uniform division of the nucleus and the distribution of chromosomes in meiosis. Ovogenesis in mammals is controlled by the hormone progesterone.

The process of formation of male and female cells has a number of differences.

• The number of oogonia that have entered maturation is laid at the stage of embryonic development, and spermatogonia begin to actively divide at the onset of puberty, and this process goes on continuously.
• In the process of spermatogenesis, 4 gametes are formed, and in the process of oogenesis, only one.
• Oogenesis is finally completed after fertilization.

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1. Anatomy of the genital organs of males

Anatomy of the genital organs of male ruminants and ungulates

1.1 The general structure of the genital organs of males

The genital organs of males (Fig. 1) include testes, epididymis, sperm ducts, accessory sex glands, penis (penis) and prepuce. The testicles are a paired gonad...

Anatomy of the genital organs of male ruminants and ungulates

2. Physiology of the male genital organs

Anatomy of the genital organs of male ruminants and ungulates

3. Features of the structure of the genital organs of male ruminants and one-hoofed animals

Sexual organs of males different types are characterized by some features. In a bull, the testicles weighing about 300 g are located vertically in the scrotum. The appendage lies behind, the tail is lowered downwards The ampullae of the spermatic ducts are well developed (length 12 cm ...

Anatomy of farm animals

5. Features of the anatomical structure and topography of the genital organs of pigs and mares

Genital organs of pigs Ovaries (ovaria) The ovaries are larger than those of cows with a tuberous surface, surrounded by a highly developed ovarian bursa. Their length is inconsistent, the shape of a mulberry or blackberry ...

Aspects of the biotechnological process

9. The use of plant cells for the transformation of medicinal substances. Advantages of obtaining target products using plant cell cultures in comparison with traditional ones. Economic aspects of the plant tissue culture bioindustry

Plants are producers of many biologically active substances - compounds capable of influencing biological processes in the body. These compounds include cardiac glycosides, saponins, sterols, carotenoids, polyphenols, alkaloids, vitamins…

Biological substantiation of artificial reproduction of pink salmon in Kamchatka

3.2 Obtaining reproductive products and insemination

Selection of sex products of pink salmon should be carried out with the following requirements. The air temperature must match the water temperature. The selection of sexual products should be carried out in the early morning hours ...

The concept of modern natural science

1.

Gametogenesis is the process of formation of germ cells

Formation of the chromosome theory

By the end of the nineteenth century. as a result of improving the optical qualities of microscopes and improving cytological methods, it became possible to observe the behavior of chromosomes in gametes and zygotes. Back in 1875, Hertwig drew attention to the fact ...

2. Characteristics of the gonads

Sex glands (synonymous with gonads), organs that form sex cells and sex hormones. They are part of the genital organs. They perform mixed functions, as they produce products not only external (potential offspring) ...

Morphology and hormonal activity of the gonads

2.1 Morphology of the female gonads

The ovary (ovarium; Greek oophoron) is a paired organ, the female sex gland, located in the pelvic cavity behind the broad ligament of the uterus. The ovaries develop and mature female sex cells (ova) ...

Morphology and hormonal activity of the gonads

2.2 Morphology of the male gonads

Testicle (testis; Greek orchis, s.didymis) is a paired male gonad. The function of the testicles is the formation of male sex cells and hormones, so the testicles are also the glands of external and internal secretion. Testicles or testicles...

Morphology and hormonal activity of the gonads

3.

Hormonal activity of the gonads

Sex hormones are hormones of a steroid nature that determine the development of the genital organs and the nature of secondary sexual characteristics in humans and higher animals ...

Properties of ethylene in plants

2.7 Formation and ripening of fruits

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Endocrine system

8 Endocrine part of the gonads

The testis in men and the ovary in women, in addition to germ cells, produce and secrete into the blood sex hormones, under the influence of which secondary sexual characteristics are formed. The endocrine function in the testicle is possessed by the interstitium ...

Stages of gametogenesis and types of crossing

2. Sexual reproduction. Development of sex cells (gametogenesis)

In more complexly organized animals, either exclusively, or at least predominantly, the sexual method of reproduction is practiced, at least the rudiments of this form of reproduction of offspring, so to speak, a hint of it ...

50. Gametogenesis in animals

process of formation of male sex cells

Answer options for your crossword puzzle

SPERMATOGENESIS

• The process of formation of male germ cells
• Formation of differentiated male germ cells

GAMETOGENESIS

• The process of formation of germ cells
• The process of development and formation of germ cells
• The process of development and formation of germ cells (gametes) - female (oogenesis) and male (spermatogenesis)

AMPHIMIXIS

• The usual type of sexual process in animals and plants, in which the nuclei of male and female germ cells merge

karyogamy

• Fusion of the nuclei of male and female germ cells into the nucleus of the zygote (syncaryon) during fertilization
• Fusion of the nuclei of male and female gametes during fertilization

HEMOPOIESIS

HERMAPHRODITISM

• The presence of female and male traits in an individual
• The presence of male and female reproductive organs in the same organism
• The presence in the body of male and female genital organs fertilization fusion of male and female germ cells

hematopoiesis

• The process of formation, development and maturation of blood cells - leukocytes, erythrocytes, platelets
• (hematopoiesis) formation, development and maturation of blood cells

OVOGENESIS

FERTILIZATION

INSEMINATION

• The process by which sex cells meet in animals

These words were also found by queries:

Sexual reproduction in multicellular organisms. Features of germ cells. Fertilization process, biological significance

Sexual reproduction is called reproduction, in which an increase in the number of individuals is carried out with the help of specialized sex cells - gametes: female - eggs and male - spermatozoa.

Male sex cells (gametes) - spermatozoa - are formed as a result of spermatogenesis.

This process takes place in three stages:

1. reproduction in the testes of diploid cells of spermatogenic tissue, as a result of which spermatocytes (2n) are formed;

2. growth of spermatocytes, accompanied by DNA synthesis and completion of the second chromatid;

3. maturation of spermatocytes, which divide by meiosis to form haploid (n) spermatozoa.

Chromosomal sets of spermatozoa (human and other mammals) differ in sex chromosomes: some carry the X-chromosome, while others carry the Y-chromosome.

female sex cells(gametes) - eggs - are formed as a result of oogenesis.

This process also takes place in the ovaries in three stages.:

1. reproduction in the ovaries of diploid cells of ooogenic tissue, as a result of which oocytes are formed (2n);

2. growth of oocytes, accompanied by DNA synthesis and the construction of the second chromatid of chromosomes;

3. maturation of oocytes and their division by meiosis.

As a result, one haploid egg with single chromatid chromosomes (1n1c) and three reduction bodies are formed from the oocyte. In the future, the egg participates in the sexual process, and the reduction bodies die off.

The process of formation of male and female gametes is called gametogenesis Differences in the structure of sperm and eggs are associated with their functions. In the process of maturation, the eggs become covered with shells (in some cases, for example, in reptiles, birds and mammals, a number of additional shells appear).

Sperm function It consists in delivering genetic information to the egg and stimulating its development. In this regard, a significant restructuring occurs in the spermatozoa: the Golgi apparatus is located at the anterior end of the head, transforming into a ring body (acrosome), which secretes enzymes that act on the egg shell.

Gametogenesis is the process of formation of germ cells

Mitochondria are compactly packed around the emerging flagellum, forming a neck. The formed spermatozoon also contains centrioles.

Fertilization - This is the process of fusion of a spermatozoon with an egg, followed by the fusion of their nuclei and the formation of a diploid zygote. biological significance

20. Spermatogenesis and ovogenesis. Cytological and cytogenetic characteristics. The biological significance of sexual reproduction. Spermatogenesis - the formation of differentiated male germ cells - spermatozoa; in humans and animals - in the testes, in lower plants - in the antheridia. In most higher plants, spermatozoa are formed in the pollen tube, more often they are called spermatozoa. Spermatogenesis begins simultaneously with the activity of the testis under the influence of sex hormones during the puberty of a teenager and then proceeds continuously (in most men almost until the end of life), has a clear rhythm and uniform intensity.

spermatogonia, containing a double set of chromosomes, divide by mitosis, leading to the emergence of subsequent cells - spermatocytes of the 1st order. Further, as a result of two successive divisions (meiotic divisions), spermatocytes of the 2nd order are formed, and then spermatids (cells of spermatogenesis immediately preceding the spermatozoon). With these divisions, a decrease (reduction) in the number of chromosomes by half occurs. Spermatids do not divide, enter the final period of spermatogenesis (the period of sperm formation) and, after a long phase of differentiation, turn into spermatozoa. This happens by gradual elongation of the cell, changes, elongation of its shape, as a result of which the cell nucleus of the spermatid forms the head of the spermatozoon, and the membrane and cytoplasm form the neck and tail. In the last phase of development, the spermatozoa heads closely adjoin the Sertoli cells, receiving nutrition from them until full maturation. After that, the spermatozoa, already mature, enter the lumen of the testicular tubule and further into the epididymis, where they are accumulated and excreted from the body during ejaculation.

Ovogenesis- the process of development of female germ cells of gametes, ending with the formation of eggs. A woman only has one egg during her menstrual cycle. The process of oogenesis has a fundamental similarity with spermatogenesis and also goes through a series of stages: reproduction, growth and maturation. Oocytes are formed in the ovary, developing from immature germ cells - ovogonia containing a diploid number of chromosomes. Owogonia, like spermatogonia, undergo successive mitotic divisions, which are completed by the time the fetus is born. Then comes the period of growth of oogonia, when they are called oocytes of the first order. They are surrounded by a single layer of cells - the granulosa membrane - and form the so-called primordial follicles. The female fetus on the eve of birth contains about 2 million of these follicles, but only about 450 of them reach the stage of ovocytes of the second order and exit the ovary during ovulation. The maturation of the oocyte is accompanied by two successive divisions, leading to a halving of the number of chromosomes in the cell. As a result of the first division of meiosis, a large oocyte of the second order and the first polar body are formed, and after the second division, a mature egg cell capable of fertilization and further development with a haploid set of chromosomes and the second polar body is formed. Polar bodies are small cells that do not play a role in oogenesis and are eventually destroyed.

biological significance This process consists in the fact that when male and female gametes merge, a new organism is formed that carries the characteristics of both parental organisms. During the formation of gametes in meiosis, cells with different combinations of chromosomes arise, therefore, after fertilization, new organisms combine the characteristics of the father and mother in various combinations. As a result, the hereditary diversity of organisms increases significantly.

21. Fertilization. Parthenogenesis. Forms and prevalence in nature. sexual dimorphism.

Fertilization- this is the process of fusion of a sperm with an egg, followed by the fusion of their nuclei and the formation of a diploid zygote. Parthenogenesis- a form of sexual reproduction, the development of an egg without fertilization. It is characteristic of many invertebrate animals (daphnia, rotifers, aphids, bees, many seed and spore plants. Examples of natural parthenogenesis in mammals are unknown; they are occasionally found in lower vertebrates and are very common in invertebrates, especially insects .

There are two types of parthenogenesis: mandatory and optional. The first is characteristic of species in which males either do not exist at all, or they are rare and unable to function. These species include some aphids, stick insects, crickets, butterflies; populations without males are occasionally found in fish, such as silver carp.

With facultative parthenogenesis, eggs can develop both parthenogenetically and as a result of fertilization, and parthenogenetic reproduction may prevail in conditions where contacts of individuals of different sexes are too rare, for example, at the border of the distribution range of the species.

Special forms of parthenogenesis- gynogenesis and androgenesis, as well as pedogenesis. sexual dimorphism. Both sexes have fundamental differences in gametogenesis, in the final stages - in adults. The differences in the processes of formation of eggs and spermatozoa lie in the fact that in men an exceptionally dynamic and constantly renewing process of the formation of new cells occurs, while in women there is a periodic, monthly induction of the development of only one cell from a once formed pool.

Thus, the male body is able to perceive the influences of the environment, and the female body is able to transmit these influences, perceived by the male sex, to the next generation. Sexual dimorphism of gametogenesis allows realizing the so-called population functions of the sexes

20. Formation of sex cells. Meiosis

Remember!

Where does sex cells form in the human body?

What set of chromosomes do gametes contain? Why?

Specialized cells are required for sexual reproduction. gametes containing a single (haploid) set of chromosomes. When they merge (fertilize), a diploid set is formed, in which each chromosome has a pair - a homologous chromosome. In each pair of homologous chromosomes, one chromosome comes from the father and the other from the mother.

In animals, the process of formation of germ cells - gametogenesis- proceeds in special organs - the sex glands (gonads). In most animals, male sex cells (spermatozoa) are formed in the testes, female gametes (eggs) in the ovaries. The development of the oocyte is called ovogenesis or oogenesis, and spermatozoa spermatogenesis.

The structure of germ cells.

Oocytes- These are relatively large fixed cells of a rounded shape. In some fish, reptiles and birds, they contain a large supply of nutrients in the form of yolk and range in size from 10 mm to 15 cm. The eggs of mammals, including humans, are much smaller (0.1–0.3 mm) and the yolk is practically do not contain.

Spermatozoa - small mobile cells, in humans their length is only about 60 microns. In different organisms, they differ in shape and size, but, as a rule, all spermatozoa have a head, neck and tail, which ensures their mobility. In the head of the sperm cell there is a nucleus containing chromosomes, and an acrosome - a special vial with enzymes necessary to dissolve the egg cell membrane. Mitochondria are concentrated in the neck, which provide the moving sperm with energy (Fig. 63).

Rice. 63. Mammalian sperm: A - electronic photography; B - structure diagram

Spermatozoa were first described by the Dutch naturalist A. Leeuwenhoek in 1677. He also introduced this term - sperm (from the Greek. sperma- seed and zoon- a living being), i.e. a living seed. The mammalian egg was discovered in 1827 by the Russian scientist K. M. Baer.

The formation of germ cells. The development of germ cells is divided into several stages: reproduction, growth, maturation, and in the process of spermatogenesis, the formation stage is also distinguished (Fig. 64).

Rice. 64. Human gametogenesis

Rice. 65. Phases of meiosis

breeding stage. At this stage, the cells that form the walls of the gonads actively divide by mitosis, forming immature germ cells. This stage in men begins with the onset of puberty and lasts almost a lifetime. In women, the formation of primary germ cells is completed even in the embryonic period, that is, the total number of eggs that a woman will mature during her reproductive period is determined already at an early stage in the development of the female body. At the stage of reproduction, the primary germ cells, like all other cells of the body, are diploid.

growth stage. At the stage of growth, which is much better expressed in oogenesis, there is an increase in the cytoplasm of cells, the accumulation of necessary substances and DNA reduplication (doubling of chromosomes).

maturation stage. The third stage is meiosis. Meiosis- This is a special way of cell division, leading to a halving of the number of chromosomes and to the transition of the cell from a diploid state to a haploid one.

Future gametes at the stage of maturation divide twice. Cells that begin meiosis contain a diploid set of already doubled chromosomes. In the process of two meiotic divisions, four haploid cells are formed from one diploid cell.

Meiosis consists of two successive divisions preceded by a single duplication of DNA carried out during the growth stage. In each division of meiosis, four phases are distinguished, which are also characteristic of mitosis (prophase, metaphase, anaphase, telophase), but they differ in some features (Fig. 65).

Prophase of the first meiotic division ( prophase I) is much longer than the prophase of mitosis. At this time, the doubled chromosomes, each of which already consists of two sister chromatids, spiralize and acquire a compact size. Then the homologous chromosomes are arranged parallel to each other, forming the so-called bivalents or tetrads, consisting of two chromosomes (four chromatids). An exchange of the corresponding homologous regions (crossing over) can occur between homologous chromosomes, which will lead to the recombination of hereditary information and the formation of new combinations of paternal and maternal genes in the chromosomes of future gametes (Fig. 66).

By the end of prophase I, the nuclear envelope is destroyed.

IN metaphase I homologous chromosomes in pairs in the form of bivalents, or tetrads, are located in the equatorial plane of the cell, and spindle threads are attached to their centromeres.

IN anaphase I homologous chromosomes from a bivalent (tetrad) diverge towards the poles. Consequently, only one of each pair of homologous chromosomes gets into each of the two resulting cells - the number of chromosomes is halved, the chromosome set becomes haploid. However, each chromosome still consists of two sister chromatids.

Rice. 66. Crossing of chromosomes and exchange of homologous regions

IN telophase I cells are formed that have a haploid set of chromosomes and a double amount of DNA.

After a short period of time, the cells begin the second meiotic division, which proceeds like a typical mitosis, but differs in that the cells participating in it are haploid.

IN prophase II the nuclear envelope breaks down. IN metaphase II Chromosomes line up in the equatorial plane of the cell, spindle fibers connect to the centromeres of chromosomes. IN anaphase II centromeres connecting sister chromatids divide, chromatids become independent daughter chromosomes and diverge to different poles of the cell. Telophase II completes the second division of meiosis.

As a result of meiosis, from one initial diploid cell containing doubled DNA molecules, four haploid cells are formed, each chromosome of which consists of a single DNA molecule.

During spermatogenesis at the maturation stage, as a result of meiosis, four identical cells are formed - the precursors of spermatozoa, which at the formation stage acquire the characteristic appearance of a mature spermatozoon and become mobile.

Meiotic divisions in oogenesis are characterized by a number of features. Prophase I ends in the embryonic period, that is, by the time the girl is born, her body already has a complete set of future eggs. The remaining events of meiosis continue only after puberty in the female. Every month, in one of the woman's ovaries, one of the cells that have stopped dividing continues to develop. As a result of the first division of meiosis, a large cell is formed - the precursor of the egg and a small, so-called polar, body, which enter the second division of meiosis. At the stage of metaphase II, the precursor of the egg ovulates, that is, it leaves the ovary into the abdominal cavity, from where it enters the oviduct. If fertilization occurs, the second meiotic division is completed - a mature egg and a second polar body are formed. If the fusion with the sperm does not occur, the cell that has not finished dividing dies and is excreted from the body.

Polar bodies serve to remove excess genetic material and redistribute nutrients in favor of the egg. Some time after division, they die.

Significance of gametogenesis. As a result of gametogenesis, germ cells are formed containing a haploid set of chromosomes, which makes it possible to restore the number of chromosomes characteristic of the species during fertilization. In the absence of meiosis, the fusion of gametes would double the number of chromosomes in each successive generation resulting from sexual reproduction. This does not happen due to the existence of a special process - meiosis, during which the diploid number of chromosomes ( 2n) is reduced to haploid (1 n). Thus, the biological role of meiosis is to maintain the constancy of the number of chromosomes in a number of generations of the species.

Review questions and assignments

1. Compare the structure of male and female germ cells. What are their similarities and differences?

2. What determines the size of the eggs? Explain why mammalian eggs are among the smallest.

3. What periods are distinguished in the development of germ cells?

4. Describe how the maturation period (meiosis) proceeds in the process of spermatogenesis; ovogenesis.

5. List the differences between meiosis and mitosis.

6. What is the biological meaning and significance of meiosis?

Think! Execute!

1. The organism developed from an unfertilized egg. Are his hereditary characteristics an exact copy of the characteristics of the mother's organism?

2. Explain why there are two terms for male germ cells: sperm (for example, in angiosperms) and spermatozoa.

Work with computer

Refer to the electronic application. Study the material and complete the assignments.

Repeat and remember!

Human

sex cells. Sperm formation in men begins at puberty. The duration of all four phases of spermatogenesis is about 80 days. For a lifetime, a huge amount of spermatozoa is formed in a man's body - up to 10 10.

Despite the fact that a lot of eggs are laid in the female embryo, only a few of them mature. During the reproductive period, that is, when a woman is capable of childbearing, about 400 eggs are finally formed.

The development of germ cells (ovogenesis and spermatogenesis) determines the health of the future generation. smoking, drinking alcoholic beverages, narcotic drugs can have an irreversible effect on the developing germ cells, which will later lead to infertility or the birth of a child with hereditary or congenital disorders.

Consider figures 84, 85, 86. How do male germ cells differ from female ones? Think about how cells divide. What is mitosis? What processes occur in each stage of mitosis?

The basis of sexual reproduction is the process of fusion of germ cells - gametes. Unlike non-sex cells, sex cells always have a single set of chromosomes, which prevents an increase in the number of chromosomes in a new organism. The formation of cells with a single set of chromosomes occurs in the process of a special type of division - meiosis.

Meiosis. Meiosis (from the Greek meiosis - decrease, decrease) is a cell division in which the chromosome set in the newly formed daughter cells is halved.

Both mitosis and meiosis are preceded by an interphase in which DNA replication occurs. Before division begins, each chromosome consists of two DNA molecules, which form two sister chromatids linked by centromeres. Thus, before the start of division, the chromosome set of the cell is 2 liters, and the amount of DNA is doubled.

The process of meiosis consists of two successive divisions - meiosis I and meiosis II, which are divided into the same stages as mitosis. As a result, not two, but four cells are formed (Fig. 82).

Rice. 82. Stages of meiosis: 1 - prophase I; 2 - metaphase I; 3 - anaphase I; 4 - telophase I; 5 - metaphase II; 6 - apaphase II; 7 - telophase II

Prophase I. This stage is much longer than in mitosis. Chromosomes coil and thicken. Homologous chromosomes are connected in pairs with each other, that is, they are conjugated (from Latin conjugation - connection). As a result, a complex of double chromosomes is formed in the cell (Fig. 83). Then, between the sections of homologous chromosomes, genes are exchanged - crossing over (from the English. Crossing over - crossing, crossing). This leads to new combinations of genes in the chromosomes (Fig. 83). After that, the nuclear membrane in the cell disappears, the centrioles diverge towards the poles, and a fission spindle is formed.

Rice. 83. Conjugation and crossing over between homologous chromosomes (letters indicate genes located on chromosomes)

Metaphase I Homologous chromosomes are located in pairs in the equatorial zone of the cell above and below the equatorial plane. The centromeres of chromosomes are connected to the spindle fibers.

Anaphase I Homologous chromosomes diverge to the poles of the cell. This is the main difference between meiosis and mitosis, where sister chromatids separate. Thus, each of the poles has only one chromosome from a homologous pair. The number of chromosomes at the poles is halved - it is reduced.

Telophase I. All the rest of the contents of the cell divides, a constriction is formed and two cells with a single set of chromosomes (l) appear. Each chromosome consists of two sister chromatids - two DNA molecules. The formation of two cells does not always occur. Sometimes telophase is accompanied only by the formation of two nuclei.

There is no interphase before the second division of meiosis. Both formed cells after a period of rest or immediately begin the second division of meiosis. Meiosis II is completely identical to mitosis and occurs in two cells (nuclei) synchronously.

Prophase II is much shorter than prophase I. The nuclear envelope disappears again, and a fission spindle is formed.

In metaphase II, the chromosomes line up in the plane of the equator. The spindle fibers connect to the centromeres of chromosomes. In anaphase II, as in mitosis, sister chromatids - chromosomes - diverge to the poles of the cell. At each pole, a single set of chromosomes (p.) is formed, with each chromosome consisting of one DNA molecule. Telophase II ends with the formation of four cells (nuclei) with a single set of chromosomes and one DNA molecule each.

The biological significance of meiosis is the formation of cells with a single set of chromosomes. The gametes that then develop from them during sexual reproduction merge and the double set of chromosomes is restored as a result. In addition, crossing over leads to new combinations of genes in the chromosomes of cells, which serves as the basis for the combinative variability of organisms.

The formation of germ cells in animals. The process of formation of germ cells is called gametogenesis (from gamete and Greek genesis - birth). In animals, gametes are formed in the genital organs: in the testes in males and in the ovaries in females.

Gametogenesis proceeds sequentially, in three stages in the respective zones and ends with the formation of spermatozoa and eggs. At the stage of reproduction, primary germ cells intensively divide by mitosis, which significantly increases their number. In the next stage of growth, cells grow and store nutrients. This period corresponds to the interphase before meiosis. Next, the cell enters the stage of maturation, where meiosis occurs, cells with a single set of chromosomes are formed, gametes are finally formed and mature.

Rice. 85. Mammalian spermatozoa: A - structure diagram: 1 - head; 2 - vial with enzymes: 3 - core: 4 - neck; 5 - mitochondria; 6 - centrioles; 7 - ponytail. B - photo in a light microscope

Spermatogenesis is characterized by the formation of male germ cells - spermatozoa. From one primary germ cell, four gametes of the same size are formed - spermatozoa.

Oogenesis (from the Greek un - egg and genesis) is characterized by the formation of female germ cells - eggs. The process of formation of an egg is much longer than that of spermatozoa. Mitochondria, concentrated in the neck, provide the moving sperm with energy.

The egg is a rounded, large, immobile cell containing a nucleus, all organelles and a lot of nutrients in the form of a yolk (Fig. 86). The egg in any animal species is always much larger than its sperm. Thanks to its nutrients, the development of the embryo is ensured at the initial stage (in fish, amphibians and mammals) or throughout the embryonic development (in reptiles and birds).

Rice. 86. The structure of the egg of a mammal: 1 - nucleus; 2 - yolk grains

The size of the eggs in different animal species varies significantly. In mammals, they average 0.2 mm. Amphibians and fish have 2-10 mm, while reptiles and birds reach several centimeters.

Lesson learned exercises

1. What type of cell division underlies the sexual reproduction of animals? What cells are formed as a result of such division?
2. What is the main difference between meiosis and mitosis? 3. Explain why the division of meiosis always precedes the sexual reproduction of animals. 4. What is the biological significance of meiosis? 5. What are the differences in the processes of spermatogenesis and oogenesis?
3. Use a microscope to examine ready-made micropreparations of spermatozoa and eggs of mammals. Compare the structure of a sperm and an egg. What is the reason for the difference?