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Meiosis and its phases. Examples of solving a meiosis problem. §23. Meiosis and its biological significance In anaphase of meiosis II occurs

This article will help you learn about the type of cell division. We will talk briefly and clearly about meiosis, the phases that accompany this process, outline their main features, and find out what features characterize meiosis.

What is meiosis?

Reduction cell division, in other words, meiosis, is a type of nuclear division in which the number of chromosomes is halved.

Translated from ancient Greek, meiosis means reduction.

This process occurs in two stages:

  • Reducing ;

At this stage, during the process of meiosis, the number of chromosomes in the cell is halved.

  • Equational ;

During the second division, the cell haploidy is maintained.

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Feature this process is that it occurs only in diploid, as well as even polyploid cells. And all because as a result of the first division in prophase 1 in odd polyploids, it is not possible to ensure pairwise fusion of chromosomes.

Phases of meiosis

In biology, division occurs during four phases: prophase, metaphase, anaphase and telophase . Meiosis is no exception; the peculiarity of this process is that it occurs in two stages, between which there is a short interphase .

First division:

Prophase 1 is a rather complex stage of the entire process as a whole; it consists of five stages, which are listed in the following table:

Stage

Sign

Leptotene

Chromosomes shorten, DNA condenses and thin strands are formed.

Zygotene

Homologous chromosomes are connected in pairs.

Pachytena

The longest phase in duration, during which homologous chromosomes are tightly attached to each other. As a result, some areas are exchanged between them.

Diplotena

The chromosomes are partially decondensed, and part of the genome begins to perform its functions. RNA is formed, protein is synthesized, while the chromosomes are still connected to each other.

Diakinesis

DNA condensation occurs again, the formation processes stop, the nuclear envelope disappears, the centrioles are located at opposite poles, but the chromosomes are connected to each other.

Prophase ends with the formation of a fission spindle, the destruction of nuclear membranes and the nucleolus itself.

Metaphase The first division is significant in that the chromosomes line up along the equatorial part of the spindle.

During anaphase 1 Microtubules contract, bivalents separate, and chromosomes move to different poles.

Unlike mitosis, at the anaphase stage, entire chromosomes, which consist of two chromatids, move to the poles.

At the stage telophases chromosomes despiral and a new nuclear membrane is formed.

Rice. 1. Scheme of meiosis of the first stage of division

Second division has the following signs:

  • For prophase 2 characterized by condensation of chromosomes and division of the cell center, the division products of which diverge to opposite poles of the nucleus. The nuclear envelope is destroyed, and a new fission spindle is formed, which is located perpendicular to the first spindle.
  • During metaphases The chromosomes are again located at the equator of the spindle.
  • During anaphase chromosomes divide and chromatids are located at different poles.
  • Telophase indicated by despiralization of chromosomes and the appearance of a new nuclear membrane.

Rice. 2. Scheme of meiosis of the second stage of division

As a result, from one diploid cell through this division we obtain four haploid cells. Based on this, we conclude that meiosis is a form of mitosis, as a result of which gametes are formed from diploid cells of the gonads.

The meaning of meiosis

During meiosis, at prophase 1, the process occurs crossing over - recombination of genetic material. In addition, during anaphase, both the first and second division, chromosomes and chromatids move to different poles in a random order. This explains the combinative variability of the original cells.

In nature, meiosis is of great importance, namely:

  • This is one of the main stages of gametogenesis;

Rice. 3. Scheme of gametogenesis

  • Carries out the transfer of genetic code during reproduction;
  • The resulting daughter cells are not similar to the mother cell and also differ from each other.

Meiosis is very important for the formation of germ cells, since as a result of fertilization of gametes, the nuclei fuse. IN otherwise in a zygote the number of chromosomes would be twice as large. Thanks to this division, the sex cells are haploid, and during fertilization the diploidity of the chromosomes is restored.

What have we learned?

Meiosis is a type of division of a eukaryotic cell in which four haploid cells are formed from one diploid cell by reducing the number of chromosomes. The whole process takes place in two stages - reduction and equation, each of which consists of four phases - prophase, metaphase, anaphase and telophase. Meiosis is very important for the formation of gametes, for the transmission of genetic information to future generations, and also carries out the recombination of genetic material.

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Gametogenesis - process of egg formation ovogenesis) and spermatozoa ( spermatogenesis) - the order of stages is subdivided (Fig. 5.4).

In the breeding stage diploid cells from which gametes are formed are called spermatogonia And oogonia. These cells undergo a series of successive mitotic divisions, as a result of which their number increases significantly. Spermatogonia reproduce throughout the entire period of male puberty. The reproduction of oogonia is confined mainly to the period of embryogenesis. In humans, in the female body, this process occurs most intensively in the ovaries between the 2nd and 5th months. intrauterine development. By the 7th month, most of the oocytes enter prophase I of meiosis.

Since the method of reproduction of female progenitor cells and male gametes is mitosis, then oogonia and spermatogonia, like all somatic cells, are characterized by diploidity. During the mitotic cycle, their chromosomes have either a single-stranded structure (after mitosis and before the completion of the synthetic period of interphase) or a double-stranded structure (postsynthetic period, prophase and metaphase of mitosis), depending on the number of DNA double-strands. If in a single, haploid set the number of chromosomes is denoted as P, and the amount of DNA is as With, then the genetic formula of cells at the reproduction stage corresponds to 2 P 2With to S-period and 2 n 4c after him.

Rice. 5.4. Gametogenesis scheme:

1 - spermatogenesis, 2 - ovogenesis, n- number of chromosome sets,

With- amount of DNA, RT - reduction bodies

On growth stages there is an increase in cell size and the transformation of male and female germ cells into spermatocytes And oocytes of the first order, and the latter reach larger sizes than the former. One part of the accumulated substances represents nutritional material (yolk in oocytes), the other is associated with subsequent divisions. An important event of this period is DNA replication while maintaining the same number of chromosomes. The latter acquire a double-stranded structure, and the genetic formula of spermatocytes and oocytes of the first order takes on the form 2 n 4With.

Main events maturation stages are two successive divisions: reduction and equational, which together make up meiosis(see section 5.3.2). After the first division, they are formed spermatocytes And second order oocytes(formula n 2With), and after the second - spermatids and mature egg(ps).

As a result of divisions at the maturation stage, each first-order spermatocyte produces four spermatids, whereas each oocyte of the first order - one a full-fledged egg And reduction bodies, which do not participate in reproduction. Thanks to this in female gamete the maximum amount of nutritional material - the yolk - is concentrated.



The process of spermatogenesis is completed stage of formation, or spermiogenesis. The nuclei of spermatids become denser due to the supercoiling of chromosomes, which become functionally inert. The lamellar complex moves to one of the poles of the nucleus, forming the acrosomal apparatus, which plays a major role in fertilization. Centrioles occupy a place at the opposite pole of the nucleus, and from one of them a flagellum grows, at the base of which mitochondria are concentrated in the form of a spiral sheath. At this stage, almost all of the cytoplasm of the spermatid is rejected, so that the head of the mature sperm is practically devoid of it.

The central event of gametogenesis is a special form of cell division - meiosis. Unlike the widespread mitosis, which maintains a constant diploid number of chromosomes in cells, meiosis leads to the formation of haploid gametes from diploid cells. During subsequent fertilization, the gametes form a new generation organism with a diploid karyotype ( ps + ps == 2n 2c). This is the most important biological significance meiosis, which arose and became established in the process of evolution in all species that reproduce sexually (see section 3.6.2.2).

Meiosis consists of two divisions that quickly follow one another, occurring during the period of maturation. DNA doubling for these divisions occurs once during the growth period. The second meiotic division follows the first almost immediately so that the hereditary material is not synthesized in the interval between them (Fig. 5.5).

The first meiotic division is called reduction since it leads to the formation of diploid cells (2 P 2With) haploid cells P 2With. This result is ensured due to the peculiarities of the prophase of the first division of meiosis. In prophase I of meiosis, as well as in ordinary mitosis, compact packaging of genetic material (chromosome spiralization) is observed. At the same time, an event occurs that is absent in mitosis: homologous chromosomes conjugate with each other, i.e. are closely approximated by the corresponding areas.

As a result of conjugation, chromosome pairs are formed, or bivalents, number P. Since each chromosome entering meiosis consists of two chromatids, the bivalent contains four chromatids. The formula of the genetic material in prophase I remains 2 n 4c. Towards the end of prophase, the chromosomes in bivalents, strongly spiraling, shorten. As in mitosis, in prophase I of meiosis, the formation of the spindle begins, with the help of which chromosomal material will be distributed between daughter cells (Fig. 5.5).

Rice. 5.5. Stages of meiosis

Paternal chromosomes are indicated in black, maternal chromosomes are uncolored. The figure does not show metaphase I, in which the bivalents are located in the equatorial plane of the spindle, and telophase I, which quickly turns into prophase II

The processes occurring in prophase I of meiosis and determining its results determine the longer duration of this phase of division compared to mitosis and make it possible to distinguish several stages within it (Fig. 5.5).

Leptotene - the earliest stage of prophase I of meiosis, in which the spiralization of chromosomes begins, and they become visible under the microscope as long and thin threads. Zygotene characterized by the beginning of conjugation of homologous chromosomes, which are united by the synaptonemal complex into a bivalent (Fig. 5.6). Pachytena - the stage in which, against the background of ongoing spiralization of chromosomes and their shortening, between homologous chromosomes occurs crossing over - intersection with the exchange of corresponding sections. Diplotena characterized by the emergence of repulsive forces between homologous chromosomes, which begin to move away from each other primarily in the centromere region, but remain connected in the areas of past crossing over - chiasmach(Fig. 5.7).

Diakinesis - the final stage of prophase I of meiosis, in which homologous chromosomes are held together only at individual points of the chiasmata. Bivalents take on the bizarre shape of rings, crosses, eights, etc. (Fig. 5.8).

Thus, despite the repulsive forces that arise between homologous chromosomes, the final destruction of bivalents does not occur in prophase I. A feature of meiosis in oogenesis is the presence of a special stage - dictyotens, absent in spermatogenesis. At this stage, reached in humans during embryogenesis, the chromosomes, having taken on a special morphological form"lamp brushes" stop any further structural changes for many years. Upon reaching female body During reproductive age, under the influence of the luteinizing hormone of the pituitary gland, as a rule, one oocyte monthly resumes meiosis.

IN metaphase I Meiosis completes the formation of the spindle. Its threads are attached to the centromeres of chromosomes, united in bivalents, in such a way that from each centromere only one thread goes to one of the spindle poles. As a result, the threads associated with the centromeres of homologous chromosomes, heading to different poles, establish bivalent positions in the equatorial plane of the spindle.

IN anaphase I During meiosis, the bonds between homologous chromosomes in bivalents are weakened and they move away from each other, heading to different poles of the spindle. In this case, a haploid set of chromosomes, consisting of two chromatids, goes to each pole (see Fig. 5.5).

IN telophase In meiosis I, a single, haploid set of chromosomes is assembled at the spindle poles, each of them containing double the amount of DNA.

The formula of the genetic material of the resulting daughter cells corresponds to P 2With.

Second meiotic(equational)division leads to the formation of cells in which the content of genetic material in the chromosomes will correspond to their single-stranded structure ps(see Fig. 5.5). This division proceeds like mitosis, only the cells entering it carry a haploid set of chromosomes. In the process of such division, the maternal double-stranded chromosomes, splitting, form single-stranded daughter chromosomes.

One of the main tasks of meiosis is creation of cells with a haploid set of single-stranded chromosomes - is achieved due to a single DNA reduplication for two successive meiotic divisions, as well as due to the formation of pairs of homologous chromosomes at the beginning of the first meiotic division and their further divergence into daughter cells.

The processes occurring in reduction division also provide an equally important consequence - genetic diversity of gametes, formed by the body. Such processes include crossing over, divergence of homologous chromosomes into different gametes And independent behavior of bivalents in the first meiotic division(see section 3.6.2.3).

Crossing over ensures recombination of paternal and maternal alleles in linkage groups (see Fig. 3.72). Due to the fact that the crossing of chromosomes can occur in different areas, crossing over in each individual case leads to the exchange of different amounts of genetic material. It is also necessary to note the possibility of several crossovers occurring between two chromatids (Fig. 5.9) and participation in the exchange of more than two bivalent chromatids (Fig. 5.10). The noted features of crossing over make this process an effective mechanism for the recombination of alleles.

Divergence of homologous chromosomes into different gametes in the case of heterozygosity, it leads to the formation of gametes that differ in the alleles of individual genes (see Fig. 3.74).

Random arrangement of bivalents in the equatorial plane of the spindle and their subsequent divergence in anaphase I meiosis ensure the recombination of parental linkage groups in the haploid set of gametes (see Fig. 3.75).

All four chromatids of a bivalent can enter into crossing over; mutant alleles are indicated by Latin letters; "+" sign - normal alleles

The last stages of oogenesis are also reproduced outside the woman’s body, in an artificial nutrient medium. This made it possible to conceive a person “in vitro”. Before ovulation, the egg is surgically removed from the ovary and transferred to a medium containing sperm. The zygote resulting from fertilization, when placed in a suitable environment, undergoes fragmentation. At the stage of 8-16 blastomeres, the embryo is transferred to the uterus of the recipient woman, who carries out pregnancy and childbirth. The number of successful results of such a transfer in Lately increases.

Gametogenesis is different high performance. During sexual life, a man produces at least 500 billion sperm. In the fifth month of embryogenesis, in the rudiment of the female reproductive gland there are 6-7 million egg precursor cells. By the beginning of the reproductive period, approximately 100,000 oocytes are found in the ovaries. From the moment of puberty until the cessation of gametogenesis, 400-500 oocytes mature in the ovaries.

Spermatogenesis. Morphologically, the testis consists of many seminiferous tubules. Lobed structure. Between the seminiferous tubules, Leiding cells (begin to work at 12-14 years of age) synthesize testosterone - the development of secondary sexual characteristics. The testis very early becomes an endocrine organ; under the influence of androgens, the formation of male genital organs occurs. The seminiferous tubule has zones:

Reproduction,

Maturation and formation.

There are periods of growth of the same name. The reproduction zone is in the outer part of the testis. The cells are round, there is a lot of cytoplasm, the nucleus is large - spermatogonia. They multiply by mitosis, and the testis increases in size until puberty, after which only stem cells divide. The supply of cells does not decrease and the testis does not decrease either. In the breeding zone 2n2c. the next phase is growth. The size of the nucleus and cytoplasm increases, DNA replication occurs (interphase 1), the cells are first-order spermatocytes 2n4c. These cells enter the zone of formation and maturation at the seminiferous tubules. Meiosis consists of 2 mitotic divisions, after the first division n2c, after the second - nc.

Oogenesis (ovaries). The gonads are formed in the 2nd month of embryonic development. In humans, the yolk sac is formed very early (the function of forming primary germ cells, providing nutrients). Germ cells (primary) migrate into the developing gonad, and the yolk sac degenerates. During embryogenesis, the ovaries are not active. The formation of female germ cells is passive. Primary germ cells are oogonia, they divide. First order oocytes are formed. The division period ends by the 7th month of embryogenesis - 7,000,000 primary cells. 400-500 mature during life, the rest are unclaimed. The development of eggs in humans is blocked in the prophase of the first meiotic division (at the diplotene stage). With the onset of puberty, the oocyte increases in size, and the size of the yolk also increases. Pigments accumulate, biochemical and morphological changes occur. Each oocyte is surrounded by small follicular cells that mature in the follicle. The egg, maturing, approaches the periphery. Follicular fluid surrounds it at all stages. The follicle ruptures. The egg enters abdominal cavity. Then into the oviduct funnel. Continuation of meiosis in 2/3 of the oviduct as a result of contact of the egg with the sperm.

During meiosis, chromosomes are distributed. The result is 4 cores. Chromosome conjugation occurs (due to highly repetitive DNA sequences in 1 gene). During gametogenesis, each of the 4 nuclei receives only 1 chromatid from a pair. As a result of meiosis during spermatogenesis, from each first-order spermocyte, 4 chromatids are obtained and 4 sperm are formed. From one first-order oocyte, 2 nuclei with a haploid set of chromosomes are formed. One of them is with a large amount of cytoplasm (since during cytokinesis the division is uneven) and the other is a reduction (directing) body. Subsequent division produces an egg and a guide body. During oogenesis, 1 egg and 3 guiding bodies are formed from each oocyte, which degenerate and disappear. The egg contains all the necessary reserves of nutrients.

Meiosis– a method of distribution of chromosomes and genes, ensuring their independent and random recombination. During oogenesis, it serves to redistribute cytoplasm between cells. Crossing over is a method that brings together and redistributes the genes of individual homologous chromosomes.

1. How many daughter cells and with what set of chromosomes are formed from one diploid cell as a result of: a) mitosis; b) meiosis?

Two haploid, two diploid, four haploid, four diploid.

a) As a result of mitosis - two diploid cells.

b) As a result of meiosis, there are four haploid cells.

2. What is chromosome conjugation? In what phase of meiosis does crossing over occur? What is the significance of this process?

Chromosome conjugation is observed in prophase of meiosis I. This is the process of bringing together homologous chromosomes. During conjugation, the chromatids of homologous chromosomes intersect in some places. Crossing over also occurs in prophase of meiosis I and is an exchange of regions between homologous chromosomes. Crossing over leads to recombination of hereditary material and is one of the sources of combinative variability, due to which descendants are not exact copies of their parents and differ from each other.

3. What events occurring in meiosis ensure that the number of chromosomes in daughter cells is halved?

A decrease in the chromosome set occurs in anaphase I of meiosis due to the fact that not sister chromatids (as in anaphase of mitosis and anaphase II of meiosis), but bichromatid homologous chromosomes diverge to different poles of the dividing cell. Consequently, from each pair of homologous chromosomes only one will end up in the daughter cell. At the end of anaphase I, the set of chromosomes at each pole of the cell is already haploid (1n2c).

4. What is the biological significance of meiosis?

In animals and humans, meiosis leads to the formation of haploid germ cells - gametes. During the subsequent process of fertilization (fusion of gametes), the organism of the new generation receives a diploid set of chromosomes, which means it retains its inherent this species organisms karyotype. Therefore, meiosis prevents the number of chromosomes from increasing during sexual reproduction. Without such a division mechanism, chromosome sets would double with each subsequent generation.

In plants, fungi and some protists, spores are formed through meiosis.

The processes occurring in meiosis (crossing over, independent divergence of chromosomes and chromatids) serve as the basis for the combinative variability of organisms.

5. Compare mitosis and meiosis, identify similarities and differences. What is the main difference between meiosis and mitosis?

The main difference is that as a result of meiosis, the set of chromosomes in daughter cells decreases by 2 times compared to the mother cell.

Similarities:

● They are methods of dividing eukaryotic cells and require energy.

● Accompanied by an accurate and uniform distribution of hereditary material between daughter cells.

● Similar processes of cell preparation for division (replication, doubling of centrioles, etc.).

● Similar processes occurring in the corresponding phases of division (spiralization of chromosomes, disintegration of the nuclear membrane, formation of the division spindle, etc.) and, as a consequence, the same names of the phases (prophase, metaphase, anaphase, telophase). The second division of meiosis proceeds by the same mechanism as mitosis of a haploid cell.

Differences:

● As a result of mitosis, daughter cells retain the set of chromosomes inherent in the mother cell. As a result of meiosis, the set of chromosomes in daughter cells decreases by 2 times.

● Mitosis is one cell division, and meiosis is two successive cell divisions (meiosis I and meiosis II). Therefore, as a result of mitosis, two daughter cells are formed from one mother cell, and as a result of meiosis, four are formed.

● Unlike mitosis, meiosis involves conjugation of homologous chromosomes and crossing over. Note: in fact, mitotic crossing over also exists (discovered by K. Stern in 1936), but its study is not included in the school curriculum.

● In anaphase of mitosis, sister chromatids diverge to different poles of the cell, and in anaphase I of meiosis, homologous chromosomes diverge.

And (or) other significant features.

6. A birch root cell contains 18 chromosomes.

1) The diploid cell of the birch anther has undergone meiosis. The resulting microspores divided by mitosis. How many cells were formed? How many chromosomes does each of them contain?

2) Determine the number of chromosomes and the total number of chromatids in birch cells during meiotic division:

a) in the equatorial plane of the cell in metaphase I;

b) in metaphase II;

c) at each cell pole at the end of anaphase I;

d) at each cell pole at the end of anaphase II.

1) The birch root cell is somatic, which means that birch has 2n = 18. As a result of meiosis, 4 cells are formed from one mother cell with a halved set of chromosomes. Consequently, 4 haploid microspores (n = 9) were formed from the diploid anther cell.

Each microspore then divided by mitosis. As a result of mitosis, two daughter cells with the same set of chromosomes were formed from each microspore. Thus, a total of 8 haploid cells were formed.

Answer: 8 cells were formed, each containing 9 chromosomes.

2) The formula of the hereditary material located in the equatorial plane of the cell in metaphase I is 2n4c, which for birch is 18 chromosomes, 36 chromatids. A cell in metaphase II has a set of 1n2c - 9 chromosomes, 18 chromatids. At the end of anaphase I, at each pole of the cell there is a set of 1n2c - 9 chromosomes, 18 chromatids, and at the end of anaphase II - 1n1c - 9 chromosomes, 9 chromatids.

Answer: a) 18 chromosomes, 36 chromatids; b) 9 chromosomes, 18 chromatids; c) 9 chromosomes, 18 chromatids; d) 9 chromosomes, 9 chromatids.

7. Why is meiosis not observed in organisms that are not characterized by sexual reproduction?

In the development cycle of all organisms that are characterized by sexual reproduction, the process of fertilization takes place - the fusion of two cells (gametes) into one (zygote). In fact, fertilization doubles the chromosome number. Therefore, there must also be a mechanism that reduces the number of chromosomes by 2 times, and this mechanism is meiosis. Without meiosis, chromosome sets would double with each successive generation.

In organisms that do not reproduce sexually, there is no process of fertilization. Therefore, they do not have meiosis, there is no need for it.

8. Why is the second division of meiosis necessary, since a decrease in the number of chromosomes by 2 times has already occurred as a result of the first division?

Daughter cells formed as a result of the first meiotic division have a set of 1n2c, i.e. are already haploid. However, each chromosome of such a cell consists not of one chromatid, as it should be in a young cell entering a new cell cycle, but of two, as in a mature cell ready to divide. Consequently, cells with the 1n2c set will not be able to normally go through the cell cycle (and, above all, replication in the S period). Therefore, almost immediately after the first meiotic division, the second begins, during which sister chromatids diverge with the formation of “normal” single-chromatid chromosomes, characteristic of young daughter cells.

In addition, as a result of meiosis, gametes are formed in animals and humans, and spores are formed in plants. Due to the fact that meiosis is not one, but two successive divisions, the number of gametes (or spores) formed increases by 2 times.

Meiosis (Greek meiosis - decrease, decrease) or reduction division. As a result of meiosis, the number of chromosomes decreases, i.e. from a diploid set of chromosomes (2p) a haploid set (p) is formed.
Meiosis consists of 2 consecutive divisions:
The first division is called reduction or diminutive.
II division is called equational or equalizing, i.e. proceeds according to the type of mitosis (which means the number of chromosomes in the mother and daughter cells remains the same).

The biological meaning of meiosis is that from one mother cell with a diploid set of chromosomes, four haploid cells are formed, thus the number of chromosomes is reduced by half, and the amount of DNA is reduced by four times. As a result of this division, sex cells (gametes) are formed in animals and spores in plants.
The phases are called the same as in mitosis, and before the start of meiosis, the cell also goes through interphase.

Prophase I– the longest phase and it is conventionally divided into 5 stages:
1) Leptonema (leptotene) – or the stage of thin filaments. Chromosomes are spiraling, a chromosome consists of 2 chromatids, and on the still thin strands of chromatids thickenings or clumps of chromatin are visible, which are called chromomeres.
2) Zygonema (zygotene, Greek merging threads) - the stage of paired threads. At this stage, homologous chromosomes (identical in shape and size) come together in pairs, they attract and adhere to each other along the entire length, i.e. conjugate in the chromomere region. It's similar to a zipper lock. A pair of homologous chromosomes is called bivalents. The number of bivalents is equal to the haploid set of chromosomes.
3) Pachynema (pachytene, Greek thick) – the stage of thick threads. Further spiralization of chromosomes occurs. Then each homologous chromosome is split in the longitudinal direction and it becomes clearly visible that each chromosome consists of two chromatids; such structures are called tetrads, i.e. 4 chromatids. At this time, crossing over occurs, i.e. exchange of homologous regions of chromatids.
4) Diplonema (diplotene) – stage of double filaments. Homologous chromosomes begin to repel each other, move away from each other, but maintain their relationship with the help of bridges - chiasmata, these are the places where crossing over will occur. At each chromatid junction (i.e., chiasma), sections of chromatids are exchanged. Chromosomes spiral and shorten.
5) Diakinesis – the stage of isolated double threads. At this stage, the chromosomes are completely condensed and intensely stained. The nuclear membrane and nucleoli are destroyed. Centrioles move to the cell poles and form spindle filaments.
The chromosome set of prophase I is 2n4c.
Thus, in prophase I:
1. conjugation of homologous chromosomes;
2. formation of bivalents or tetrads;
3. crossing over.
Depending on the conjugation of chromatids, there may be different kinds crossing over: 1 – correct or incorrect; 2 – equal or unequal; 3 – cytological or effective; 4 – single or multiple.

Metaphase I– chromosome spiralization reaches its maximum. The bivalents line up along the equator of the cell, forming a metaphase plate. The spindle strands are attached to the centromeres of homologous chromosomes. The bivalents find themselves connected to different poles of the cell.
The chromosome set of metaphase I is 2n4c.

Anaphase I– the centromeres of the chromosomes do not divide, the phase begins with the division of the chiasmata. Entire chromosomes, not chromatids, disperse to the poles of the cell. Only one of a pair of homologous chromosomes enters the daughter cells, i.e. they are randomly redistributed. At each pole, it turns out, there is a set of chromosomes - 1n2c, and in general the chromosome set of anaphase I is - 2n4c.

Telophase I– at the poles of the cell there are whole chromosomes, consisting of 2 chromatids, but their number has become 2 times less.
In animals and some plants, chromatids despiral. A nuclear membrane forms around them at each pole.
Next comes cytokinesis.
The chromosome set of cells formed after the first division is n2c.

There is no S-period between divisions I and II and DNA replication does not occur, because the chromosomes are already duplicated and consist of sister chromatids, therefore interphase II is called interkinesis - i.e. there is a movement between two divisions.

Prophase II– very short and proceeds without any special changes; if the nuclear envelope is not formed in telophase I, then spindle filaments are immediately formed.

Metaphase II– chromosomes line up along the equator. The spindle filaments are attached to the centromeres of the chromosomes.
The chromosome set of metaphase II is - n2c.

Anaphase II– the centromeres divide and the spindle filaments move the chromatids to different poles. Sister chromatids are called daughter chromosomes (or mother chromatids will be daughter chromosomes).
The chromosome set of anaphase II is - 2n2c.

Telophase II– chromosomes despiral, stretch and are then poorly distinguishable. Nuclear membranes and nucleoli are formed. Telophase II ends with cytokinesis.
The chromosome set after telophase II is – nc.

The meaning of meiosis

1. A constant number of chromosomes is maintained in species that reproduce sexually, because When haploid cells fuse, the diploid set of chromosomes is restored.
2. A large number of different combinations of paternal and maternal chromosomes are formed due to the independent divergence of homologous chromosomes in anaphase I. The number of combinations of chromosome pairs is defined as 2n, where n is the haploid set of chromosomes. For a person, the number of combinations is 223 = 8388608.
3. Recombination of genetic material occurs due to crossing over, which occurs in prophase I, at the pachynema stage.

Let's look at examples of solving problems on meiosis

Problem 1
The somatic cell of Drosophila has 2n=8 chromosomes. How many chromosomes, chromatids and DNA will the cells formed as a result of spermatogenesis have? Name the periods of spermatogenesis and the resulting cells. Draw schematically.
Solution:

Problem 2
Under the influence of radiation, a woman did not go through the anaphase II stage during puberty. How many eggs were formed, and with what set of chromosomes? What consequences can you expect? Draw diagrammatically.
Solution:

In anaphase II, the centromeres divide and the spindle filaments move the chromatids towards the poles. If this anaphase has not passed, then the chromosomes cannot disperse to the poles, so one nucleus is formed with a double set of chromosomes, i.e., as a result of the second meiotic division, an oocyte with a set of 46 was formed from an oocyte of the second order chromosomes (46 chr-m, 92 chr-dy, 4c) and one reduction body with the same set of chromosomes. When an egg (n = 46 chromosomes, 2c) is fertilized with a normal sperm (n = 23 chromosomes, 1c), a triploid is formed; such an organism is not viable for early stages embryonic development.

Meiosis is a type of cell division in which the number of chromosomes is halved and cells transition from a diploid to a haploid state.

Meiosis is a sequence of two divisions.

Stages of meiosis

The first division of meiosis (reduction) leads to the formation of haploid cells from diploid cells. In prophase I, as in mitosis, chromosome spiralization occurs. At the same time, homologous chromosomes come together with their identical sections (conjugate), forming bivalents. Before entering meiosis, each chromosome has doubled genetic material and consists of two chromatids, so the bivalent contains 4 strands of DNA. In the process of further spiralization, crossing over can occur - the crossing of homologous chromosomes, accompanied by the exchange of corresponding sections between their chromatids. In metaphase I, the formation of the division spindle is completed, the threads of which are attached to the centromeres of chromosomes, united into bivalents in such a way that only one thread goes from each centromere to one of the poles of the cell. In anaphase I, the chromosomes diverge to the poles of the cell, with each pole having a haploid set of chromosomes, consisting of two chromatids. In telophase I, the nuclear envelope is restored, after which the mother cell divides into two daughter cells.

The second division of meiosis begins immediately after the first and is similar to mitosis, but the cells entering it carry a haploid set of chromosomes. Prophase II is very short in time. This is followed by metaphase II, during which the chromosomes are located in the equatorial plane, and a spindle is formed. In anaphase II, centromeres separate and each chromatid becomes an independent chromosome. The daughter chromosomes separated from each other are directed to the division poles. In telo-phase II, cell division occurs, in which 4 daughter haploid cells are formed from two haploid cells.

Thus, as a result of meiosis, one diploid cell produces four cells with a haploid set of chromosomes.

During meiosis, two mechanisms of recombination of genetic material are carried out.

1. Non-permanent (crossing over) is an exchange of homologous regions between chromosomes. Occurs in prophase I at the pachytene stage. The result is recombination of allelic genes.

2. Constant - random and independent divergence of homologous chromosomes in anaphase I of meiosis. As a result, gametes receive a different number of chromosomes of paternal and maternal origin.

Biological significance of meiosis

1) is the main stage of gametogenesis;

2) ensures the transfer of genetic information from organism to organism during sexual reproduction;

3) daughter cells are not genetically identical to the mother and to each other.