Essence, mechanism and biological significance of meiosis. Reproductive function and biological significance of meiosis

During sexual reproduction, a daughter organism arises as a result of the fusion of two sex cells ( gametes) and subsequent development from a fertilized egg - zygotes.

The reproductive cells of the parents have a haploid set ( n) chromosomes, and in a zygote, when two such sets are combined, the number of chromosomes becomes diploid (2 n): each pair of homologous chromosomes contains one paternal and one maternal chromosome.

Haploid cells are formed from diploid ones as a result of a special cell division - meiosis.

Meiosis - a type of mitosis, as a result of which from diploid (2p) somatic cells the samehaploid gametes are formed (1n). During fertilization, the gamete nuclei fuse and the diploid set of chromosomes is restored. Thus, meiosis ensures that the set of chromosomes and the amount of DNA remain constant for each species.

Meiosis is a continuous process consisting of two successive divisions called meiosis I and meiosis II. In each division, prophase, metaphase, anaphase and telophase are distinguished. As a result of meiosis I, the number of chromosomes is halved ( reduction division): During meiosis II, cell haploidy is preserved (equational division). Cells entering meiosis contain 2n2xp genetic information (Fig. 1).

In prophase of meiosis I, gradual spiralization of chromatin occurs to form chromosomes. Homologous chromosomes come together to form a common structure consisting of two chromosomes (bivalent) and four chromatids (tetrad). The contact of two homologous chromosomes along the entire length is called conjugation. Then repulsive forces appear between homologous chromosomes, and the chromosomes first separate at the centromeres, remaining connected at the arms, and form decussations (chiasmata). The divergence of chromatids gradually increases, and the crosshairs move towards their ends. During the process of conjugation, an exchange of sections can occur between some chromatids of homologous chromosomes - crossing over, leading to recombination of genetic material. By the end of prophase, the nuclear envelope and nucleoli dissolve, and an achromatic spindle is formed. The content of genetic material remains the same (2n2хр).

In metaphase In meiosis I, chromosome bivalents are located in the equatorial plane of the cell. At this moment, their spiralization reaches its maximum. The content of genetic material does not change (2n2xr).

In anaphase Meiosis I homologous chromosomes, consisting of two chromatids, finally move away from each other and diverge to the poles of the cell. Consequently, from each pair of homologous chromosomes, only one gets into the daughter cell - the number of chromosomes is halved (reduction occurs). The content of genetic material becomes 1n2xp at each pole.

In telophase Nuclei are formed and the cytoplasm is divided - two daughter cells are formed. Daughter cells contain a haploid set of chromosomes, each chromosome contains two chromatids (1n2хр).

Interkinesis- a short interval between the first and second meiotic divisions. At this time, DNA replication does not occur, and the two daughter cells quickly enter meiosis II, which proceeds as mitosis.

Rice. 1. Diagram of meiosis (one pair of homologous chromosomes is shown). Meiosis I: 1, 2, 3. 4. 5 - prophase; 6 - metaphase; 7 - anaphase; 8 - telophase; 9 - interkinesis. Meiosis II; 10 - metaphase; II - anaphase; 12 - daughter cells.

In prophase In meiosis II, the same processes occur as in prophase of mitosis. In metaphase, chromosomes are located in the equatorial plane. There are no changes in the content of genetic material (1n2хр). In anaphase of meiosis II, the chromatids of each chromosome move to opposite poles of the cell, and the content of genetic material at each pole becomes lnlxp. In telophase, 4 haploid cells (lnlxp) are formed.

Thus, as a result of meiosis, 4 cells with a haploid set of chromosomes are formed from one diploid mother cell. In addition, in prophase of meiosis I, recombination of genetic material (crossing over) occurs, and in anaphase I and II, chromosomes and chromatids randomly move to one or the other pole. These processes are the cause of combinational variability.

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.

Also, the biological significance of meiosis lies in the fact that a decrease in the number of chromosomes is necessary during the formation of germ cells, since during fertilization the nuclei of the gametes fuse. If this reduction did not occur, then in the zygote (and therefore in all cells of the daughter organism) there would be twice as many chromosomes. However, this contradicts the rule of a constant number of chromosomes. Thanks to meiosis, the sex cells are haploid, and upon fertilization, the diploid set of chromosomes is restored in the zygote (Fig. 2 and 3).

Rice. 2. Gametogenesis scheme: ? - spermatogenesis; ? - ovogenesis

Rice. 3.Diagram illustrating the mechanism of maintaining the diploid set of chromosomes during sexual reproduction

Meiosis- This is a special method of cell division, as a result of which the number of chromosomes is reduced by half. It was first described by W. Flemming in 1882 in animals and by E. Sgrasburger in 1888 in plants. With the help of meiosis, spores and germ cells - gametes are formed. As a result of the reduction of the chromosome set, each haploid spore and gamete receives one chromosome from each pair of chromosomes present in a given diploid cell. During the further process of fertilization (fusion of gametes), the organism of the new generation will again receive a diploid set of chromosomes, i.e. The karyotype of organisms of a given species remains constant over generations. Thus, the most important significance of meiosis is to ensure the constancy of the karyotype in a number of generations of organisms of a given species during sexual reproduction.

Meiosis involves two divisions that quickly follow one another. Before the onset of meiosis, each chromosome is replicated (doubled in the S period of interphase). For some time, its two resulting copies remain connected to each other by the centromere. Therefore, each nucleus in which meiosis begins contains the equivalent of four sets of homologous chromosomes (4c).

The second division of meiosis follows almost immediately after the first, and DNA synthesis does not occur in the interval between them (i.e., in fact, there is no interphase between the first and second division).

The first meiotic (reduction) division leads to the formation of haploid cells (n) from diploid cells (2n). It starts with prophaseI, in which, as in mitosis, the packaging of hereditary material (spiralization of chromosomes) is carried out. At the same time, homologous (paired) chromosomes come together with their identical sections - conjugation(an event that is not observed in mitosis). As a result of conjugation, chromosome pairs are formed - bivalents. Each chromosome, entering meiosis, as noted above, has a double content of hereditary material and consists of two chromatids, so the bivalent consists of 4 strands. When the chromosomes are in a conjugated state, their further spiralization continues. In this case, individual chromatids of homologous chromosomes intertwine and cross each other. Subsequently, homologous chromosomes are somewhat repelled from one another. As a result, in places where chromatids are intertwined, chromatid breaks can occur, and as a result, in the process of reuniting chromatid breaks, homologous chromosomes exchange corresponding sections. As a result, the chromosome that came to a given organism from the father includes a section of the maternal chromosome, and vice versa. The crossing of homologous chromosomes, accompanied by the exchange of corresponding sections between their chromatids, is called crossing over. After crossing over, the already changed chromosomes subsequently diverge, that is, with a different combination of genes. Being a natural process, crossing over each time leads to the exchange of sections of different sizes and thus ensures the effective recombination of chromosome material in gametes.

Biological significance of crossing over extremely high, since genetic recombination allows the creation of new, previously non-existent combinations of genes and increases the survival of organisms in the process of evolution.

IN metaphaseI The formation of the fission spindle is completed. Its threads are attached to the kinetochores of chromosomes, united in bivalents. As a result, the threads associated with the kinetochores of homologous chromosomes establish bivalents in the equatorial plane of the spindle.

IN anaphase I homologous chromosomes separate from each other and move to the poles of the cell. In this case, a haploid set of chromosomes goes to each pole (each chromosome consists of two chromatids).

IN telophase I At the spindle poles, a single, haploid set of chromosomes is assembled, in which each type of chromosome is no longer represented by a pair, but by one chromosome, consisting of two chromatids. In the short-lasting telophase I, the nuclear envelope is restored, after which the mother cell divides into two daughter cells.

Thus, the formation of bivalents during the conjugation of homologous chromosomes in prophase I of meiosis creates conditions for the subsequent reduction in the number of chromosomes. The formation of a haploid set in gametes is ensured by the divergence in anaphase I not of chromatids, as in mitosis, but of homologous chromosomes, which were previously united into bivalents.

After Telophase I division is followed by a short interphase, in which DNA is not synthesized, and the cells proceed to the next division, which is similar to normal mitosis. ProphaseII short-lived. The nucleoli and nuclear membrane are destroyed, and the chromosomes are shortened and thickened. Centrioles, if present, move to opposite poles of the cell, and spindle filaments appear. IN metaphase II chromosomes line up in the equatorial plane. IN anaphase II As a result of the movement of the spindle threads, the chromosomes are divided into chromatids, as their connections in the centromere region are destroyed. Each chromatid becomes an independent chromosome. With the help of spindle threads, chromosomes are stretched towards the poles of the cell. Telophase II characterized by the disappearance of spindle filaments, separation of nuclei and cytokinesis, culminating in the formation of four haploid cells from two haploid cells. In general, after meiosis (I and II), one diploid cell produces 4 cells with a haploid set of chromosomes.

Reduction division is, in essence, a mechanism that prevents a continuous increase in the number of chromosomes during the fusion of gametes; without it, during sexual reproduction, the number of chromosomes would double in each new generation. In other words, Thanks to meiosis, a certain and constant number of chromosomes is maintained in all generations of any species of plants, animals and fungi. Another important significance of meiosis is to ensure extreme diversity in the genetic composition of gametes, both as a result of crossing over and as a result of different combinations of paternal and maternal chromosomes during their independent divergence in anaphase I of meiosis, which ensures the appearance of diverse and different-quality offspring during sexual reproduction of organisms.

Meiosis is a special method of cell division, which results in a reduction (decrease) in the number of chromosomes by half. .With the help of meiosis, spores and germ cells - gametes are formed. As a result of the reduction of the chromosome set, each haploid spore and gamete receives one chromosome from each pair of chromosomes present in a given diploid cell. During the further process of fertilization (fusion of gametes), the organism of the new generation will again receive a diploid set of chromosomes, i.e. The karyotype of organisms of a given species remains constant over generations. Thus, the most important significance of meiosis is to ensure the constancy of the karyotype in a number of generations of organisms of a given species during sexual reproduction.

In prophase of meiosis I, the nucleoli dissolve, the nuclear envelope disintegrates, and spindle formation begins. Chromatin spiralizes to form bichromatid chromosomes (in a diploid cell - set 2n4c). Homologous chromosomes come together in pairs, this process is called chromosome conjugation. During conjugation, the chromatids of homologous chromosomes intersect in some places. Between some chromatids of homologous chromosomes, an exchange of corresponding sections can occur - crossing over.

In metaphase I, pairs of homologous chromosomes are located in the equatorial plane of the cell. At this moment, chromosome spiralization reaches its maximum.

In anaphase I, homologous chromosomes (and not sister chromatids, as in mitosis) move away from each other and are stretched by spindle filaments to opposite poles of the cell. Consequently, from each pair of homologous chromosomes, only one will end up in the daughter cell. Thus, at the end of anaphase I, the set of chromosomes and chromatids at each pole of the dividing cell is \ti2c - it has already been halved, but the chromosomes still remain bichromatid.

In telophase I, the spindle is destroyed, two nuclei are formed and the cytoplasm is divided. Two daughter cells are formed containing a haploid set of chromosomes, each chromosome consisting of two chromatids (\n2c).

The interval between meiosis I and meiosis II is very short. Interphase II is practically absent. At this time, DNA replication does not occur and the two daughter cells quickly enter the second meiotic division, which occurs as mitosis.

In prophase II, the same processes occur as in the prophase of mitosis: chromosomes are formed, they are randomly located in the cytoplasm of the cell. The spindle begins to form.

In metaphase II, chromosomes are located in the equatorial plane.

In anaphase II, the sister chromatids of each chromosome separate and move to opposite poles of the cell. At the end of anaphase II, the set of chromosomes and chromatids at each pole is \ti\c.

In telophase II, four haploid cells are formed, each chromosome consisting of one chromatid (lnlc).

Thus, meiosis consists of two successive divisions of the nucleus and cytoplasm, before which replication occurs only once. The energy and substances required for both divisions of meiosis are accumulated during inter phase I.

In prophase of meiosis I, crossing over occurs, which leads to recombination of hereditary material. In anaphase I, homologous chromosomes randomly disperse to different poles of the cell; in anaphase II, the same happens with sister chromatids. All these processes determine the combinative variability of living organisms, which will be discussed later.

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 the karyotype inherent to this type of organism. 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 by meiosis. The processes occurring during meiosis serve as the basis for the combinative variability of organisms.

Thanks to meiosis, a certain and constant number of chromosomes is maintained in all generations of any species of plants, animals and fungi. Another important significance of meiosis is to ensure extreme diversity in the genetic composition of gametes, both as a result of crossing over and as a result of different combinations of paternal and maternal chromosomes during their independent divergence in anaphase I of meiosis, which ensures the appearance of diverse and different-quality offspring during sexual reproduction of organisms.

The essence of meiosis is that each sex cell receives a single haploid set of chromosomes. However, 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 the exchange of sections between homologous chromosomes that occurs in meiosis. Meiosis includes two sequential divisions following each other almost without interruption. As with mitosis, each meiotic division has four stages: prophase, metaphase, anaphase and telophase. The second meiotic division - the essence of the maturation period is that in germ cells, through double meiotic division, the number of chromosomes is halved, and the amount of DNA is reduced fourfold. The biological meaning of the second meiotic division is that the amount of DNA is brought into line with the chromosome set. In males, all four haploid cells formed as a result of meiosis are subsequently transformed into gametes - sperm. In females, due to uneven meiosis, only one cell produces a viable egg. The other three daughter cells are much smaller; they turn into so-called guiding, or reducing, bodies, which soon die. The biological meaning of the formation of only one egg and the death of three full-fledged (from a genetic point of view) guide bodies is due to the need to preserve in one cell all the reserve nutrients for the development of the future embryo.

Cell theory.

A cell is an elementary unit of structure, functioning and development of living organisms. There are non-cellular life forms - viruses, but they manifest their properties only in the cells of living organisms. Cellular forms are divided into prokaryotes and eukaryotes.

The discovery of the cell belongs to the English scientist R. Hooke, who, looking at a thin section of cork under a microscope, saw structures similar to a honeycomb and called them cells. Later, single-celled organisms were studied by the Dutch scientist Antonie van Leeuwenhoek. The cell theory was formulated by the German scientists M. Schleiden and T. Schwann in 1839. The modern cell theory was significantly supplemented by R. Birzhev et al.

Basic provisions of modern cell theory:

cell is the basic unit of structure, functioning and development of all living organisms, the smallest living unit capable of self-reproduction, self-regulation and self-renewal;

the cells of all unicellular and multicellular organisms are similar (homologous) in their structure, chemical composition, basic manifestations of life activity and metabolism;

Cell reproduction occurs through cell division, each new cell is formed as a result of the division of the original (mother) cell;

in complex multicellular organisms, cells are specialized in the functions they perform and form tissues; tissues consist of organs that are closely interconnected and subject to nervous and humoral regulation.

These provisions prove the unity of origin of all living organisms, the unity of the entire organic world. Thanks to the cell theory, it became clear that the cell is the most important component of all living organisms.

A cell is the smallest unit of an organism, the limit of its divisibility, endowed with life and all the basic characteristics of the organism. As an elementary living system, it underlies the structure and development of all living organisms. At the cellular level, such properties of life as the ability to metabolize substances and energy, autoregulation, reproduction, growth and development, and irritability appear.

50. Patterns of inheritance established by G. Mendel .

The laws of inheritance were formulated in 1865 by Gregory Mendel. In his experiments, he crossed different varieties of peas.

Mendel's first and second laws are based on monohybrid crosses, and the third - on di and polyhybrid crosses. Monohybrid crossing involves one pair of alternative traits, dihybrid crossing involves two pairs, and polyhybrid crossing involves more than two. Mendel's success is due to the peculiarities of the hybridological method used:

The analysis begins with crossing pure lines: homozygous individuals.

Separate alternative mutually exclusive features are analyzed.

Accurate quantitative accounting of descendants with different combinations of traits

The inheritance of the analyzed traits can be traced over a number of generations.

Mendel's 1st law: "Law of uniformity of hybrids of the 1st generation"

When crossing homozygous individuals analyzed for one pair of alternative traits, the 1st generation hybrids exhibit only dominant traits and uniformity in phenotype and genotype is observed.

In his experiments, Mendel crossed pure lines of pea plants with yellow (AA) and green (aa) seeds. It turned out that all descendants in the first generation are identical in genotype (heterozygous) and phenotype (yellow).

Mendel's 2nd law: "Law of splitting"

When crossing heterozygous hybrids of the 1st generation, analyzed according to one pair of alternative characters, in the second generation hybrids a 3:1 cleavage is observed in the phenotype, and 1:2:1 in the genotype

In his experiments, Mendel crossed the hybrids (Aa) obtained in the first experiment with each other. It turned out that in the second generation the suppressed recessive trait reappeared. The data from this experiment indicate the elimination of the recessive trait: it is not lost, but appears again in the next generation.

Mendel's 3rd law: "The law of independent combination of characteristics"

When crossing homozygous organisms analyzed for two or more pairs of alternative traits, in hybrids of the 3rd generation (obtained by crossing hybrids of the 2nd generation) an independent combination of traits and the corresponding genes of different allelic pairs is observed.

To study the pattern of inheritance of plants that differed in one pair of alternative characters, Mendel used monohybrid crossing. Next, he moved on to experiments on crossing plants that differed in two pairs of alternative traits: dihybrid crossing, where he used homozygous pea plants that differed in color and seed shape. As a result of crossing smooth (B) and yellow (A) with wrinkled (c) and green (a), in the first generation all plants had yellow smooth seeds. Thus, the law of uniformity of the first generation manifests itself not only in mono, but also in polyhybrid crossing, if the parent individuals are homozygous.

During fertilization, a diploid zygote is formed due to the fusion of different types of gametes. To facilitate the calculation of variants of their combination, the English geneticist Bennett proposed a grid entry - a table with the number of rows and columns according to the number of types of gametes formed by crossing individuals. Analysis cross

Since individuals with a dominant trait in the phenotype can have different genotypes (Aa and AA), Mendel proposed crossing this organism with a recessive homozygote.

I've been blogging for almost three years now. biology tutor. Some topics are of particular interest and comments on articles become incredibly bloated. I understand that reading such long “foot wraps” becomes very inconvenient over time.
Therefore, I decided to post some of the readers’ questions and my answers to them, which may be of interest to many, in a separate blog section, which I called “From dialogues in the comments.”

Why is the topic of this article interesting? It's clear that main biological significance of meiosis : ensuring the constancy of the number of chromosomes in cells from generation to generation during sexual reproduction.

Moreover, we must not forget that in animal organisms in specialized organs (gonads) from diploid somatic cells (2n) are formed by meiosis haploid germ cells gametes (n).

We also remember that all plants live with : sporophyte, which produces spores; and gametophyte, which produces gametes. Meiosis in plants occurs at the stage of maturation of haploid spores (n). From the spores a gametophyte develops, all of whose cells are haploid (n). Therefore, in gametophytes, haploid male and female gamete germ cells (n) are formed by mitosis.

Now let's look at the comments to the article: what tests exist for the Unified State Exam on the question about the biological significance of meiosis.

Svetlana(biology teacher). Good afternoon, Boris Fagimovich!

I analyzed 2 Unified State Examination manuals by G.S. Kalinov. and this is what I discovered.

1 question.


2. Formation of cells with double the number of chromosomes;
3. Formation of haploid cells;
4. Recombination of sections of non-homologous chromosomes;
5. New combinations of genes;
6. The appearance of a larger number of somatic cells.
The official answer is 3,4,5.

Question 2 is similar, BUT!
The biological significance of meiosis is:
1. The emergence of a new nucleotide sequence;
2. Formation of cells with a diploid set of chromosomes;
3. Formation of cells with a haploid set of chromosomes;
4. Formation of a circular DNA molecule;
5. The emergence of new gene combinations;
6. Increase in the number of germ layers.
The official answer is 1,3,5.

What happens : in question 1, answer 1 is discarded, but in question 2 is it correct? But 1 is most likely the answer to the question of what ensures the mutation process; if - 4, then, in principle, this can also be correct, since in addition to homologous chromosomes, non-homologous ones also seem to be able to recombine? I'm more inclined towards answers 1,3,5.

Hello Svetlana! There is the science of biology, which is presented in university textbooks. There is the discipline of biology, which is presented (as accessible as possible) in school textbooks. Accessibility (and, in fact, the popularization of science) often results in all sorts of inaccuracies that school textbooks “sin” with (even those republished 12 times with the same errors).

Svetlana, what can we say about test tasks, which have already been “composed” by tens of thousands (of course, they contain outright errors and all sorts of incorrectness associated with double interpretation of questions and answers).

Yes, you are right, it reaches the point of obvious absurdity when the same answer in different tasks, even by the same author, is assessed by him as correct and incorrect. And there is a lot of such “confusion,” to put it mildly.

We teach schoolchildren that the conjugation of homologous chromosomes in prophase 1 of meiosis can lead to crossing over. Crossing over provides combinative variability - the appearance of a new combination of genes or, which is the same thing, a “new nucleotide sequence”. In that is also one of the biological meanings of meiosis, Therefore, answer 1 should undoubtedly be considered correct.

But I see the correctness of answer 4 regarding the recombination of sections of NON-HOMOLOGIC chromosomes a huge “sedition” in compiling such a test in general. During meiosis, HOMOLOGIC chromosomes are normally conjugated (this is the essence of meiosis, this is its biological significance). But there are chromosomal mutations that arise due to meiotic errors when non-homologous chromosomes are conjugated. Here in the answer to the question: “How do chromosomal mutations occur” - this answer would be correct.

Compilers sometimes apparently “do not see” the particle “not” before the word “homologous,” since I also came across other tests where, when asked about the biological significance of meiosis, I had to choose this answer as the correct one. Of course, applicants need to know that the correct answers here are 1,3,5.

As you can see, these two tests are also bad because they generally no basic correct answer offered to the question about the biological significance of meiosis, and answers 1 and 5 are actually the same thing.

Yes, Svetlana, these are “blunders” for which graduates and applicants pay for exams when passing the Unified State Exam. Therefore, the main thing is still, even for passing the Unified State Exam, teach your students mainly from textbooks, and not on test tasks. Textbooks provide comprehensive knowledge. Only such knowledge will help students answer any correctly composed tests.

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Who has questions about the article to Biology tutor via Skype, please contact us in the comments.

Biological significance of meiosis:

Characteristics of animal germ cells

Gametes - highly differentiated cells. They are designed to reproduce living organisms.

The main differences between gametes and somatic cells:

1. Mature germ cells have a haploid set of chromosomes. somatic cells have a diploid set. For example, human somatic cells contain 46 chromosomes. mature gametes have 23 chromosomes.

2. In germ cells, the nuclear-cytoplasmic ratio is changed. In female gametes, the volume of the cytoplasm is many times greater than the volume of the nucleus. in male cells there is an opposite pattern.

3. Gametes have a special metabolism. in mature germ cells the processes of assimilation and dissimilation are slowed down.

4. Gametes are different from each other and these differences are due to the mechanisms of meiosis.

Gametogenesis

Spermatogenesis- development of male reproductive cells. diploid cells of the convoluted tubules of the testes transform into haploid sperm (Fig. 1). Spermatogenesis includes 4 periods: reproduction, growth, maturation, formation.

1. Reproduction . The starting material for sperm development is spermatogonia. the cells are round in shape with a large, well-stained nucleus. contains a diploid set of chromosomes. Spermatogonia reproduce rapidly by mitotic division.

2. Growth . Spermatogonia form first order spermatocytes.

3. Maturation. In the maturation zone, two meiotic divisions occur. Cells after the first division of maturation are called second order spermatocytes . Then comes the second division of maturation. the diploid number of chromosomes is reduced to the haploid number. is formed by 2 spermatids . Consequently, from one first-order diploid spermatocyte, 4 haploid spermatids are formed.

4. Formation. Spermatids gradually turn into mature sperm . In men, the release of sperm into the cavity of the seminiferous tubules begins after puberty. It continues until the activity of the gonads subsides.

Oogenesis- development of female reproductive cells. ovarian cells - oogonia - turn into eggs (Fig. 2).

Oogenesis includes three periods: reproduction, growth and maturation.

1. Reproduction Oogonia, like spermatogonia, occurs by mitosis.

2. Growth . During growth, oogonia turn into first-order oocytes.

Rice. 2. Spermatogenesis and oogenesis (schemes).

3. Maturation. as in spermatogenesis, two meiotic divisions follow each other. After the first division, two cells are formed, different in size. One big one - second order oocyte and the smaller one - first directional (polar) body. As a result of the second division, two cells of unequal size are also formed from a second-order oocyte. Big - mature egg cell and small - second directional body. Thus, from one diploid oocyte of the first order, four haploid cells are formed. One mature egg and three polar bodies. This process takes place in the fallopian tube.

Meiosis

Meiosis - biological process during the maturation of germ cells. Meiosis includes first And second meiotic division .

First meiotic division (reduction). The first division is preceded by interphase. DNA synthesis occurs in it. However, prophase I of the meiotic division is different from prophase of mitosis. It consists of five stages: leptotene, zygotene, pachytene, diplotene and diakinesis.

In leptonema, the nucleus enlarges and filamentous, weakly spiraled chromosomes are revealed in it.

In the zygonema, pairwise union of homologous chromosomes occurs, in which the centromeres and arms precisely approach each other (the phenomenon of conjugation).

In the pachynema, progressive spiralization of chromosomes occurs and they are combined into pairs - bivalents. In chromosomes, chromatids are identified, resulting in the formation of tetrads. In this case, an exchange of chromosome sections occurs - crossing over.

Diplonema is the beginning of the repulsion of homologous chromosomes. The divergence begins in the centromere region, but the connection remains at the crossing-over sites.

In diakinesis, further divergence of chromosomes occurs, which, nevertheless, still remain connected in bivalents by their terminal sections. As a result, characteristic ring shapes appear. The nuclear membrane dissolves.

IN anaphase I homologous chromosomes from each pair, rather than chromatids, diverge to the poles of the cell. This is a fundamental difference from the similar stage of mitosis.

Telophase I. The formation of two cells with a haploid set of chromosomes occurs (for example, a person has 23 chromosomes). however, the amount of DNA is kept equal to the diploid set.

Second meiotic division (equational). First there is a short interphase. there is no DNA synthesis in it. This is followed by prophase II and metaphase II. In anaphase II, it is not homologous chromosomes that separate, but only their chromatids. Therefore, the daughter cells remain haploid. DNA in gametes is half that in somatic cells.

Biological significance of meiosis: