Biological significance 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 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. V male cages 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. In that fundamental difference from a 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:

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 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 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. Other important meiosis is to provide extreme diversity in the genetic composition of gametes, both as a result of crossing over and as a result various combinations 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) directional bodies is due to the need to preserve all spare ones in one cell 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 structure, chemical composition, basic manifestations of vital 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 fusion different varieties 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. Appearance more 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, arising 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.

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 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

Basic concepts and definitions

Meiosis is a special method of dividing eukaryotic cells, in which the original number of chromosomes is reduced by 2 times (from the ancient Greek “ meyon" - less - and from " meiosis" - decrease). A decrease in the number of chromosomes is often called reduction.

Initial number of chromosomes in meiocytes(cells entering meiosis) is called diploid chromosome number (2n) The number of chromosomes in cells formed as a result of meiosis is called haploid chromosome number (n).

The minimum number of chromosomes in a cell is called the basic number ( x). The basic number of chromosomes in a cell corresponds to the minimum amount of genetic information (minimum amount of DNA), which is called a gene O m. Number of genes O mov in a cell is called a gene O many numbers (Ω). In most multicellular animals, in all gymnosperms and many angiosperms, the concept of haploidy-diploidy and the concept of gene O a lot of numbers match. For example, in a person n=x=23 and 2 n=2x=46.

Main feature meiosis is conjugation(pairing) homologous chromosomes with their subsequent divergence into different cells. The meiotic distribution of chromosomes to daughter cells is called chromosome segregation.

Short story discovery of meiosis

The individual phases of meiosis in animals were described by V. Flemming (1882), and in plants by E. Strasburger (1888), and then by the Russian scientist V.I. Belyaev. At the same time (1887), A. Weissman theoretically substantiated the need for meiosis as a mechanism for maintaining a constant number of chromosomes. First detailed description meiosis in rabbit oocytes was given by Winiworth (1900). The study of meiosis is still ongoing.

General course of meiosis

Typical meiosis consists of two successive cell division, which are called accordingly meiosis I And meiosis II. In the first division, the number of chromosomes decreases by half, therefore the first meiotic division is called reductionist, less often – heterotypic. In the second division, the number of chromosomes does not change; this division is called equational(equalizing), less often - homeotypic. The expressions “meiosis” and “reduction division” are often used interchangeably.

Interphase

Pre-meiotic interphase differs from ordinary interphase in that the DNA replication process does not reach completion: approximately 0.2...0.4% of DNA remains unduplicated. Thus, cell division begins at the synthetic stage of the cell cycle. Therefore, meiosis is figuratively called premature mitosis. However, in general, we can assume that in a diploid cell (2 n) DNA content is 4 With.

In the presence of centrioles, they are doubled in such a way that the cell has two diplosomes, each of which contains a pair of centrioles.

First division of meiosis (reduction division, or meiosis I)

The essence of reduction division is to reduce the number of chromosomes by half: from the original diploid cell, two haploid cells with bichromatid chromosomes are formed (each chromosome includes 2 chromatids).

Prophase 1(prophase of the first division) consists of a number of stages:

Leptotene(stage of thin threads). Chromosomes are visible in a light microscope in the form of a ball of thin threads. Early leptotene, when the strands of chromosomes are still very poorly visible, is called proleptotene.

Zygotene(stage of merging threads). Happening conjugation of homologous chromosomes(from lat. conjugation– connection, pairing, temporary merger). Homologous chromosomes (or homologues) are chromosomes that are similar to each other in morphological and genetic terms. In normal diploid organisms, homologous chromosomes are paired: the diploid organism receives one chromosome from the pair from the mother, and the other from the father. During conjugation, they are formed bivalents. Each bivalent is a relatively stable complex of one pair of homologous chromosomes. Homologues are held together by protein proteins. synaptonemal complexes. One synaptonemal complex can connect only two chromatids at one point. The number of bivalents is equal to the haploid number of chromosomes. Otherwise, bivalents are called tetrads, since each bivalent includes 4 chromatids.

Pachytena(thick filament stage). The chromosomes are spiralized, and their longitudinal heterogeneity is clearly visible. DNA replication is completed (a special pachytene DNA). Ends crossing over- chromosome crossing, as a result of which they exchange sections of chromatids.

Diplotena(double thread stage). Homologous chromosomes in bivalents repel each other. They are connected at separate points called chiasmata(from the ancient Greek letter χ - “chi”).

Diakinesis(stage of bivalent divergence). Individual bivalents are located on the periphery of the nucleus.

Metaphase I(first division metaphase)

IN prometaphase I the nuclear membrane is destroyed (fragmented). The fission spindle is formed. Next, metakinesis occurs - the bivalents move to the equatorial plane of the cell.

Anaphase I(anaphase of the first division)

The homologous chromosomes that make up each bivalent are separated, and each chromosome moves towards the nearest pole of the cell. The separation of chromosomes into chromatids does not occur. The process of distributing chromosomes to daughter cells is called chromosome segregation.

Telophase I(telophase of the first division)

Homologous bichromatid chromosomes completely diverge to the cell poles. Normally, each daughter cell receives one homologous chromosome from each pair of homologs. Two are formed haploid nuclei that contain half as many chromosomes as the nucleus of the original diploid cell. Each haploid nucleus contains only one chromosome set, that is, each chromosome is represented by only one homologue. The DNA content in daughter cells is 2 With.

In most cases (but not always), telophase I is accompanied by cytokinesis .

Interkinesis

Interkinesis is the short interval between two meiotic divisions. It differs from interphase in that DNA replication, chromosome duplication and centriole duplication do not occur: these processes occurred in pre-meiotic interphase and, partially, in prophase I.

Second division of meiosis (equation division, or meiosis II)

During the second division of meiosis, the number of chromosomes does not decrease. The essence of equational division is the formation of four haploid cells with single-chromatid chromosomes (each chromosome contains one chromatid).

Prophase II(prophase of the second division)

Does not differ significantly from prophase of mitosis. Chromosomes are visible under a light microscope as thin threads. A division spindle is formed in each of the daughter cells.

Metaphase II(second division metaphase)

Chromosomes are located in the equatorial planes of haploid cells independently of each other. These equatorial planes may lie in the same plane, may be parallel to each other, or mutually perpendicular.

Anaphase II(anaphase of the second division)

Chromosomes are separated into chromatids (as in mitosis). The resulting single-chromatid chromosomes, as part of anaphase groups, move to the poles of the cells.

Telophase II(telophase of the second division)

Single-chromatid chromosomes have completely moved to the poles of the cell, and nuclei are formed. The DNA content in each cell becomes minimal and amounts to 1 With.

Types of meiosis and its biological significance

In general, meiosis produces four haploid cells from one diploid cell. At gametic meiosis gametes are formed from the resulting haploid cells. This type of meiosis is characteristic of animals. Gametic meiosis is closely related to gametogenesis And fertilization. At zygotic And spore meiosis the resulting haploid cells give rise to spores or zoospores. These types of meiosis are characteristic of lower eukaryotes, fungi and plants. Spore meiosis is closely related to sporogenesis. Thus, meiosis is the cytological basis of sexual and asexual (spore) reproduction.

Biological significance of meiosis is to maintain a constant number of chromosomes in the presence of the sexual process. In addition, due to crossing over, recombination– the emergence of new combinations of hereditary inclinations in chromosomes. Meiosis also provides combinative variability– the emergence of new combinations of hereditary inclinations during further fertilization.

The course of meiosis is controlled by the genotype of the organism, under the control of sex hormones (in animals), phytohormones (in plants) and many other factors (for example, temperature).