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

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

The sex cells of the parents have a haploid set ( n) chromosomes, and in the 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 cells as a result of a special cell division - meiosis.

Meiosis - a kind of mitosis, as a result of which diploid (2n) somatic cells of the germ cellslez formed haploid gametes (1n). During fertilization, the gamete nuclei fuse and the diploid set of chromosomes is restored. Thus, meiosis ensures the preservation of a constant set of chromosomes and the amount of DNA for each species.

Meiosis is a continuous process consisting of two successive divisions called meiosis I and meiosis II. Each division is divided into prophase, metaphase, anaphase and telophase. As a result of meiosis I, the number of chromosomes is halved ( reduction division): during meiosis II, haploid cells are preserved (equational division). Cells entering meiosis contain the 2n2xp genetic information (Fig. 1).

In prophase I of meiosis, chromatin gradually coils to form chromosomes. Homologous chromosomes approach each other, forming 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 the homologous chromosomes, and the chromosomes first separate in the centromere region, remaining connected in the shoulder region, and form decussations (chiasmata). The divergence of chromatids gradually increases, and the decussations are displaced towards their ends. In the process of conjugation between some chromatids of homologous chromosomes, an exchange of sites can occur - crossing over, leading to a recombination of genetic material. By the end of prophase, the nuclear envelope and nucleoli dissolve, and the achromatin spindle is formed. The content of the genetic material remains the same (2n2хр).

In metaphase meiosis I chromosome bivalents are located in the equatorial plane of the cell. At this moment, their spiralization reaches a maximum. The content of the genetic material does not change (2n2xp).

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

in telophase the formation of nuclei and the division of the cytoplasm occur - two daughter cells are formed. Daughter cells contain a haploid set of chromosomes, each chromosome has two chromatids (1n2xp).

Interkinesis- a short interval between the first and second meiotic divisions. At this time, DNA replication does not occur, and two daughter cells quickly enter meiosis II, proceeding according to the type of mitosis.

Rice. 1. Diagram of meiosis (one pair of homologous chromosomes 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 meiosis II, the same processes occur as in the prophase of mitosis. In metaphase, the chromosomes are located in the equatorial plane. There are no changes in the content of genetic material (1n2хр). In the anaphase of meiosis II, the chromatids of each chromosome move to opposite poles of the cell, and the content of the 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 the prophase of meiosis I, a recombination of genetic material (crossing over) occurs, and in anaphase I and II, a random departure of chromosomes and chromatids to one or the other pole. These processes are the cause of combinative variability.

The 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 parent and to each other.

Also, the biological significance of meiosis lies in the fact that a decrease in the number of chromosomes is necessary for the formation of germ cells, since the gamete nuclei merge during fertilization. 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 constancy of the number of chromosomes. Due to meiosis, germ cells are haploid, and during fertilization in the zygote, a diploid set of chromosomes is restored (Fig. 2 and 3).

Rice. 2. Scheme of gametogenesis: ? - spermatogenesis; ? - ovogenesis

Rice. 3.Scheme illustrating the mechanism for maintaining a diploid set of chromosomes during sexual reproduction

Meiosis- This is a special way of cell division, as a result of which there is a reduction (reduction) in the number of chromosomes by half. It was first described by W. Flemming in 1882 in animals and by E. Sgrasburger in 1888 in plants. Meiosis produces spores and gametes. 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. In the course of 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 in a number of 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 rapidly following one after the other division. Before meiosis begins, each chromosome replicates (doubles in the S-period of interphase). For some time, its two formed 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 divisions).

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 packing of hereditary material (chromosome spiralization) is carried out. Simultaneously, there is a convergence of homologous (paired) chromosomes 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 threads. When the chromosomes are in a conjugated state, their further spiralization continues. In this case, individual chromatids of homologous chromosomes intertwine, intersect each other. Subsequently, homologous chromosomes repel each other somewhat. As a result of this, chromatid entanglements can break, and as a result, in the process of reunion of chromatid breaks, homologous chromosomes exchange the corresponding sections. As a result, the chromosome that came to this organism from the father includes a portion of the maternal chromosome, and vice versa. The crossing of homologous chromosomes, accompanied by the exchange of the corresponding sections between their chromatids, is called crossing over. After crossing over, the altered chromosomes further diverge, that is, with a different combination of genes. Being a natural process, crossing over each time leads to the exchange of regions of different size and thus ensures efficient recombination of chromosome material in gametes.

The biological significance of crossing over is extremely large, since genetic recombination allows you to create new combinations of genes that did not exist before and increases the survival of organisms in the process of evolution.

IN metaphaseI completion of the fission spindle. Its threads are attached to the kinetochores of chromosomes combined into bivalents. As a result, the strands associated with the kinetochores of the homologous chromosomes establish bivalents in the equatorial plane of the fission spindle.

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

IN telophase I at the poles of the spindle, 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 duration of 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 that were previously combined 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. The nucleoli and nuclear membrane are destroyed, and the chromosomes are shortened and thickened. Centrioles, if present, move to opposite poles of the cell, spindle fibers appear. IN metaphase II Chromosomes line up in the equatorial plane. IN anaphase II as a result of the movement of the fission spindle threads, the division of chromosomes into chromatids occurs, since their bonds in the centromere region are destroyed. Each chromatid becomes an independent chromosome. With the help of spindle threads, chromosomes are stretched to the poles of the cell. Telophase II characterized by the disappearance of the fission spindle filaments, the isolation of nuclei and cytokinesis, culminating in the formation of four haploid cells from two haploid cells. In general, after meiosis (I and II), 4 cells with a haploid set of chromosomes are formed from one diploid cell.

Reduction division is, in fact, 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, meiosis maintains a certain and constant number of chromosomes in all generations of any kind of plants, animals and fungi. Another important meaning of meiosis is to ensure the extreme diversity of the genetic composition of gametes both as a result of crossing over and as a result of a different combination of paternal and maternal chromosomes during their independent divergence in anaphase I of meiosis, which ensures the appearance of diverse and heterogeneous offspring during sexual reproduction of organisms.

Meiosis is a special way of cell division, which results in a reduction (reduction) 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. In the course of 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 in a number of 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 I of meiosis, the nucleoli dissolve, the nuclear envelope disintegrates, and the fission spindle begins to form. Chromatin spiralizes with the formation of two-chromatid chromosomes (in a diploid cell - a set of 2p4c). Homologous chromosomes come together in pairs, this process is called chromosome conjugation. During conjugation, the chromatids of homologous chromosomes cross in some places. Between some chromatids of homologous chromosomes, an exchange of the corresponding sections can occur - crossing over.

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

In anaphase I, homologous chromosomes (and not sister chromatids, as in mitosis) move away from each other and are stretched by spindle threads to opposite poles of the cell. Consequently, from each pair of homologous chromosomes, only one will get into 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 halved, but the chromosomes still remain two-chromatid.

In telophase I, the fission spindle is destroyed, the formation of two nuclei and the division of the cytoplasm occur. Two daughter cells are formed containing a haploid set of chromosomes, each chromosome consists 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 two daughter cells quickly enter the second division of meiosis, proceeding according to the type of 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, the 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 consists of one chromatid (lnlc).

Thus, meiosis is two consecutive divisions of the nucleus and cytoplasm, before which replication occurs only once. The energy and substances needed for both divisions of meiosis are accumulated during and in phase I.

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

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 a new generation receives a diploid set of chromosomes, which means that it retains the karyotype inherent in this type of organism. Therefore, meiosis prevents the increase in the number of chromosomes during sexual reproduction. Without such a division mechanism, chromosome sets would double with each successive generation.

In plants, fungi, and some protists, spores are produced by meiosis. The processes that occur 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 kind of plants, animals and fungi. Another important meaning of meiosis is to ensure the extreme diversity of the genetic composition of gametes both as a result of crossing over and as a result of a different combination of paternal and maternal chromosomes during their independent divergence in anaphase I of meiosis, which ensures the appearance of diverse and heterogeneous 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 the stage during which new combinations of genes are created by combining different maternal and paternal chromosomes. The recombination of hereditary inclinations arises, in addition, as a result of the exchange of regions between homologous chromosomes, which occurs in meiosis. Meiosis includes two successive divisions following one after another with virtually no interruption. As in mitosis, there are four stages in each meiotic division: prophase, metaphase, anaphase, and telophase. The second meiotic division - the essence of the maturation period is that in germ cells, by means of a double meiotic division, the number of chromosomes is halved, and the amount of DNA is halved. 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 converted into gametes - spermatozoa. In females, due to uneven meiosis, only one cell produces a viable egg. Three other daughter cells are much smaller, they turn into the so-called directional, or reduction, little bodies, which soon die. The biological meaning of the formation of only one egg and the death of three full-fledged (from a genetic point of view) directional 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 forms of life - viruses, but they show their properties only in the cells of living organisms. Cellular forms are divided into prokaryotes and eukaryotes.

The opening of the cell belongs to the English scientist R. Hooke, who, looking through a thin section of cork under a microscope, saw structures similar to honeycombs and called them cells. Later, unicellular organisms were studied by the Dutch scientist Anthony van Leeuwenhoek. The cell theory was formulated by the German scientists M. Schleiden and T. Schwann in 1839. The modern cell theory has been significantly supplemented by R. Birzhev and others.

The main provisions of modern cell theory:

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

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

cell reproduction occurs by dividing, 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 the 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 boundary of its divisibility, endowed with life and all the main features of an organism. As an elementary living system, it underlies the structure and development of all living organisms. At the cell level, such properties of life as the ability to exchange substances and energy, autoregulation, reproduction, growth and development, and irritability are manifested.

50. Patterns of inheritance established by G. Mendel .

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

The first and second laws of Mendel are based on monohybrid crosses, and the third - on di and polyhybrid. A monohybrid cross uses one pair of alternative traits, a dihybrid cross uses two pairs, and a polyhybrid cross uses more than two. The success of Mendel is due to the peculiarities of the applied hybridological method:

The analysis begins with crossing pure lines: homozygous individuals.

Separate alternative mutually exclusive signs are analyzed.

Accurate quantitative accounting of descendants with different combinations of traits

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

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

When crossing homozygous individuals analyzed for one pair of alternative traits, hybrids of the 1st generation show 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).

2nd Mendel's Law: "The Law of Splitting"

When crossing heterozygous hybrids of the 1st generation, analyzed by one pair of alternative traits, the hybrids of the second generation show splitting according to the phenotype 3:1, and according to the genotype 1:2:1

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

3rd Mendel's Law: "The Law of Independent Combination of Features"

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

To study the patterns of inheritance of plants that differed in one pair of alternative traits, Mendel used monohybrid crossing. He then 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 (b) and green (a), in the first generation all plants were with 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 varieties of gametes. To facilitate the calculation of their combination options, the English geneticist Bennet proposed a record in the form of a lattice - a table with the number of rows and columns according to the number of types of gametes formed by crossing individuals. Analyzing cross

Since individuals with a dominant trait in the phenotype may have a different genotype (Aa and AA), Mendel proposed to cross this organism with a recessive homozygote.

I have 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 it becomes very inconvenient to read such long "footcloths" 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 the dialogues in the comments."

What is interesting about the topic of this article? After all, it is 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 should not forget that in animal organisms in specialized organs (gonads) from diploid somatic cells (2n) meiosis are formed haploid sex cells gametes (n).

We also remember that all plants live with : sporophyte, which produces spores; and gametophyte, which produces gametes. meiosis in plants proceeds at the stage of maturation of haploid spores (n). A gametophyte develops from spores, all cells of which are haploid (n). Therefore, in gametophytes, mitoses form haploid male and female germ cells gametes (n).

Now let's see the materials of the comments on the article what are the tests for the exam on the issue on the biological significance of meiosis.

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

I analyzed 2 USE benefits Kalinov G.S. and here's what I found.

1 question.


2. The formation of cells with a double 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 appearance of a new nucleotide sequence;
2. Formation of cells with a diploid set of chromosomes;
3. The formation of cells with a haploid set of chromosomes;
4. Formation of a circular DNA molecule;
5. The emergence of new combinations of genes;
6. An increase in the number of germ layers.
The official answer is 1,3,5.

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

Hello Svetlana! There is a science of biology, set out in high school textbooks. There is the discipline of biology, set out (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 reprinted 12 times with the same errors).

Svetlana, what can we say about the test tasks, which have already been “composed” by tens of thousands (of course, there are outright mistakes in them, and all sorts of incorrectness associated with a double interpretation of questions and answers).

Yes, you are right, it comes to sheer absurdity when the same answer in different tasks, even by one author, is evaluated by him as correct and as incorrect. And such, to put it mildly, "confusion", very, very much.

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 emergence of a new combination of genes or, which is the same as a “new sequence of nucleotides”. In that is also one of the biological meanings of meiosis, so answer 1 is undeniably correct.

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

The 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 main correct answer offered to the question about the biological significance of meiosis, and answers 1 and 5 are actually the same.

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

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Who will have questions about the article to biology tutor via skype, contact in the comments.

The 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 sex cells have a haploid set of chromosomes. somatic cells are diploid. For example, human somatic cells contain 46 chromosomes. Mature gametes have 23 chromosomes.

2. In germ cells, the nuclear-cytoplasmic ratio has been 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 inverse pattern.

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

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

Gametogenesis

spermatogenesis- development of male germ cells. diploid cells of the convoluted tubules of the testes turn into haploid spermatozoa (Fig. 1). Spermatogenesis includes 4 periods: reproduction, growth, maturation, formation.

1. Reproduction . The starting material for the development of spermatozoa is spermatogonia. rounded cells with a large, well stained nucleus. contains a diploid set of chromosomes. Spermatogonia multiply rapidly by mitotic division.

2. Growth . Spermatogonia form spermatocytes of the first order.

3. Ripening. Two meiotic divisions occur in the maturation zone. 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 haploid. formed by 2 spermatids . Therefore, 4 haploid spermatids are formed from one first-order diploid spermatocyte.

4. Shaping. Spermatids gradually turn into mature spermatozoa . In men, the release of spermatozoa into the cavity of the seminiferous tubules begins after the onset of puberty. It continues until the activity of the gonads ceases.

Ovogenesis- development of female germ cells. ovarian cells - ovogonia turn into eggs (Fig. 2).

Ovogenesis includes three periods: reproduction, growth and maturation.

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

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

Rice. 2. Spermatogenesis and oogenesis (schemes).

3. Ripening. as in spermatogenesis, two meiotic divisions follow each other. After the first division, two cells are formed, different in size. One big - second order oocyte and smaller - the 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 guide body. Thus, four haploid cells are formed from one first-order diploid oocyte. 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. where DNA synthesis takes place. However, the prophase I of meiotic division differs from the prophase of mitosis. It consists of five stages: leptotene, zygoten, pachytene, diploten and diakinesis.

In the leptoneme, the nucleus enlarges and filiform weakly spiralized chromosomes are revealed in it.

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

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

Diplonema - the beginning of repulsion of homologous chromosomes. The divergence begins in the centromere region, however, in the places of crossing over, the connection is preserved.

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

IN anaphase I there is a divergence to the poles of the cell of homologous chromosomes from each pair, and not chromatids. This is the fundamental difference from the analogous stage of mitosis.

Telophase I. There is a formation of two cells with a haploid set of chromosomes (for example, in humans - 23 chromosomes). however, the amount of DNA is kept equal to the diploid set.

Second meiotic division (equatorial). First comes a short interphase. it lacks DNA synthesis. This is followed by prophase II and metaphase II. In anaphase II, not homologous chromosomes diverge, but only their chromatids. Therefore, daughter cells remain haploid. DNA in gametes is half that in somatic cells.

The biological significance of meiosis: