The number of chromosomes in a cell is reduced. What periods are distinguished in the development of germ cells? Describe how the period of maturation (meiosis) proceeds

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

What is meiosis?

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

Translated from the ancient Greek language, meiosis means reduction.

This process takes place in two stages:

• reduction ;

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

• equational ;

During the second division, the haploid cells are preserved.

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A feature of this process is that it occurs only in diploid, as well as in even polyploid cells. And all because as a result of the first division in prophase 1 in odd polyploids there is no way to ensure pairwise fusion of chromosomes.

Phases of meiosis

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

First division:

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

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

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

During anaphase 1 microtubules contract, bivalents separate, and chromosomes diverge to different poles.

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

At the stage telophase chromosomes despiralize and a new nuclear envelope is formed.

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

Second division has the following features:

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

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

As a result, four haploid cells are obtained from one diploid cell by such division. Based on this, we conclude that meiosis is a form of mitosis, as a result of which gametes are formed from the diploid cells of the sex glands.

The meaning of meiosis

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

In nature, meiosis is of great importance, namely:

• This is one of the main steps in gametogenesis;

Rice. 3. Scheme of gametogenesis

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

Meiosis is very important for the formation of germ cells, since as a result of fertilization of gametes, the nuclei merge. Otherwise, the number of chromosomes in the zygote would be twice as large. Due to this division, the germ cells are haploid, and during fertilization, the diploidy of the chromosomes is restored.

What have we learned?

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

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Cells of multicellular organisms usually have a double, or diploid (2 n), set of chromosomes, since one set of chromosomes enters the zygote (the egg from which the organism develops) as a result of fertilization from each parent. Therefore, all the chromosomes of the set are paired, homologous - one from the father, the other from the mother. In cells, this set remains constant due to mitosis.

Sex cells (gametes) - eggs and sperm (or sperm in plants) - have a single, or haploid, set of chromosomes (n). This set of gametes is obtained through meiosis (from the Greek word meiosis - reduction). In the process of meiosis, one duplication of chromosomes and two divisions occur. - reduction and equational (equal). Each of them consists of a number of phases: interphase, prophase, metaphase, anaphase and telophase (Fig. 1).

In interphase I (the first division), doubling - reduplication - of chromosomes occurs. Each chromosome then consists of two identical chromatids connected by a single centromere. In prophase I of meiosis, pairing (conjugation) of doubled homologous chromosomes occurs, which form bivalents consisting of four chromatids. At this time, spiralization, shortening and thickening of chromosomes occurs. In metaphase I, paired homologous chromosomes line up at the equator of the cell; in anaphase I, they diverge to its opposite poles; in telophase I, the cell divides. After the first division, only one doubled chromosome from each pair of homologous chromosomes enters each of the two cells, i.e., there is a decrease (reduction) in the number of chromosomes by half.

After the first division, a short interphase II (second division) passes in the cells without doubling the chromosomes. The second division proceeds like mitosis. In metaphase II, the chromosomes, consisting of two chromatids, line up at the equator of the cell. In anaphase II, the chromatids diverge towards the poles. In telophase II, both cells divide. It has been established that there is a direct relationship between the set of chromosomes in the nucleus (2 n or n) and the amount of DNA in it (denoted by the letter C). There is twice as much DNA in a diploid cell (2C) as in a haploid one (C). In interphase I of a diploid cell, before preparing it for division, DNA replication occurs, its number doubles and becomes equal to 4C. After the first division, the amount of DNA in daughter cells decreases to 2C, after the second division - to 1C, which corresponds to the haploid set of chromosomes.

The biological meaning of meiosis is as follows. First of all, in a number of generations, the set of chromosomes characteristic of this species is preserved, since during fertilization, haploid gametes merge and the diploid set of chromosomes is restored.

In addition, processes occur in meiosis that ensure the implementation of the basic laws of heredity: firstly, due to conjugation and the obligatory subsequent divergence of homologous chromosomes, the law of gamete purity is implemented - only one chromosome from a pair of homologues and, therefore, only one allele from a pair gets into each gamete - A or a, B or b.

Second, random segregation of nonhomologous chromosomes in the first division ensures independent inheritance of traits controlled by genes located on different chromosomes and leads to the formation of new combinations of chromosomes and genes (Fig. 2).

Thirdly, genes located on the same chromosome exhibit linked inheritance. However, they can combine and form new combinations of genes as a result of crossing over - the exchange of regions between homologous chromosomes, which occurs during their conjugation in the prophase of the first division (Fig. 3).

Thus, two mechanisms for the formation of new combinations (genetic recombination) in meiosis can be distinguished: random segregation of nonhomologous chromosomes and crossing over.

What periods are distinguished in the development of germ cells? Describe how the period of maturation (meiosis) proceeds.

In the process of gametogenesis (the formation of germ cells), four stages are distinguished.

1. The reproduction period is characterized by mitotic division of primary germ cells; while their number increases.

2. The period of growth is to increase the size of the cell. At the end of the period in interphase I, DNA replication occurs. The cell formula becomes 2n4c.

3. The period of maturation (meiosis). During meiosis, cells divide twice.

As a result of the I meiotic (reduction) division in daughter cells, a decrease (reduction) in the number of chromosomes by 2 times occurs.

Prophase I. Cell formula 2n4c. DNA coiling in progress. Chromosomes shorten and thicken, becoming visible as long thin threads. Conjugation of homologous chromosomes occurs. Conjugation is the process of exact and close approximation of homologous chromosomes, in which each point of one chromosome is combined with the corresponding point of another homologous chromosome. Homologous - these are paired chromosomes that are identical in structure, containing in the same loci allelic genes responsible for the same traits. Chromosomes are held together by a zipper-like connection. The connection is formed by protein filaments with a thickening at the free ends. As a result of conjugation, a bivalent (tetrad) is formed, consisting of four chromatids. In the future, crossing over can occur between homologous chromosomes - an exchange of homologous regions. The probability of crossing over for each chromosome is 50%. In this case, two adjacent, non-sister chromatids exchange sites. As a result of crossing over, each chromosome turns out to consist of one chromatid with an unchanged set of genes and the second one with recombined genes (all chromatids in the bivalent are different). Spiralization of chromosomes intensifies, repulsive forces arise between them. They remain connected at the sites of crossing over where chiasmata (crossovers) form. As the spiralization and repulsive force increase, the chiasmata shift to the ends of the chromosome arms, where terminal (terminal) chiasmata are formed.

Metaphase I. Spiralization of chromosomes reaches its maximum. The bivalents line up along the equator of the cell. In the plane of the equator, there are sections of terminal chiasmata, and the centromeres of homologous chromosomes face different poles of the cell, the spindle of division is attached to them.

Anaphase I. Sections of the terminal chiasmata are torn, and homologous chromosomes from the bivalent begin to move to different poles of the cell.

As a result of meiotic division I, each daughter cell contains one chromosome from each pair. Haploid cells with the formula 1n2c are formed.

Interphase II is short, DNA replication does not occur. There is a reparative DNA synthesis aimed at restoring possible damage to the DNA structure that has arisen in the process of crossing over.

II meiotic division - equational (equalizing). It consists in bringing the amount of DNA into line with the chromosome set and proceeds according to the type of mitosis. In anaphase II, sister chromatids, after dividing the centromere, become independent chromosomes and begin to move to different poles of the cell. As a result of meiotic division II, each haploid cell (1n2c) produces two daughter cells with the formula 1n1c.

4. The period of formation consists in the acquisition by the cell of the appropriate shape and size necessary to perform specific functions.

Reduction [number] of chromosomes gametic reduction- reduction of gametes, reduction of [number] of chromosomes.

Reducing the number of chromosomes by half against the somatic set; R.g.- an integral part of the reduction division (meiosis).

(Source: "English-Russian Explanatory Dictionary of Genetic Terms". Arefiev V.A., Lisovenko L.A., Moscow: VNIRO Publishing House, 1995)

See what "reduction [number] of chromosomes" is in other dictionaries:

Reduction (syn. haplosis obsolete) in genetics, halving the somatic number of chromosomes; in animals, as a rule, occurs during the formation of germ cells. Selective reduction (syn. selective maturation division) P., in which ... ... Wikipedia

gamete reduction- reduction [number] of chromosomes Reducing the number of chromosomes by half against the somatic set; R.g. an integral part of the reduction division (meiosis). [Arefiev V.A., Lisovenko L.A. English Russian explanatory dictionary of genetic terms 1995 407s.] ... ... Technical Translator's Handbook

gamete reduction. See reduction [number] of chromosomes. (Source: "English Russian Explanatory Dictionary of Genetic Terms". Arefiev V.A., Lisovenko L.A., Moscow: VNIRO Publishing House, 1995) ... Molecular biology and genetics. Dictionary.

I Reduction (Latin reductio retraction, return, restoration) in biology is a reduction in size, simplification of the structure or complete loss of an organ, tissue or cell in the course of historical development (phylogenesis). II Reduction in cytology regeneration ... Medical Encyclopedia

REDUCTION- 1. Reduction of organs or tissues (until they disappear) and often their loss of function in the process of ontogenesis or phylogenesis. 2. Reducing the number of chromosomes in cells as a result of meiosis ... Glossary of botanical terms

gametic reduction- ANIMAL EMBRYOLOGY GAMETIC REDUCTION - a halving of the number of chromosomes that occurs during meiosis, during the formation of germ cells - gametes ... General Embryology: Terminological Dictionary

- (from the Greek méiosis reduction) reduction division, division of maturation, a method of cell division, as a result of which there is a decrease (reduction) in the number of chromosomes by half and one diploid cell (containing two sets of chromosomes) ... ... Great Soviet Encyclopedia

- (from the Greek meiosis reduction), division of maturation, a special way of cell division, as a result of which there is a reduction (decrease) in the number of chromosomes and the transition of cells from a diploid state to a haploid one; main link of gametogenesis. M open B.… … Biological encyclopedic dictionary

- (from the Greek meiosis reduction) or reduction cell division division of the nucleus of a eukaryotic cell with a halving of the number of chromosomes. It occurs in two stages (reduction and equational stages of meiosis). Meiosis should not be confused with ... ... Wikipedia

Elementary unit of life. The cell is delimited from other cells or from the external environment by a special membrane and has a nucleus or its equivalent, in which the main part of the chemical information that controls heredity is concentrated. By studying… … Collier Encyclopedia

Process oocyte maturation the first order begins by the time it is released from the follicle. As in males, two divisions pass quickly here, but instead of four functioning gametes, females eventually form only one. With each division of maturation, two cells are also formed here. But one of them receives from the oocyte of the first order practically all the food reserves, while the other receives almost or nothing at all and soon dies.
Cell, which did not receive yolk material, was originally called the "polar body". This is an oocyte with a reduced amount of cytoplasm.

First division maturation usually takes place in the ovary just before the rupture of the follicle. In this division, a first-order oocyte divides into two second-order oocytes. One of them receives little cytoplasm and is called the first polar body. The second division of maturation does not occur until the egg is released from the ovary and (in mammals) a spermatozoon enters it. At the second division, the second-order oocyte, which has received all the food reserves, divides again. The bulk of the cytoplasm during this division also passes into one of the two resulting ootids, now called a mature egg.

Other ootida is the second polar body. Sometimes the first polar body also divides, which indicates the homology of maturation divisions in both sexes. Usually, however, it degenerates somewhat earlier. The second polar body similarly degenerates shortly after its appearance, leaving only one of the four potential ootids that is able to function normally.

Reduction in the number of chromosomes during maturation

At the same time with reviewed above phenomena during the maturation of male and female sex gametes, changes occur in their nuclear substance, which are also of great importance. Chromatin is an essential part of the nucleus. In a resting cell, chromatin is dispersed throughout the nucleus, forming small granules. In a dividing cell, these granules are combined into bodies of various lengths and shapes - chromosomes.

According to them behavior in cell division, in the maturation of germ cells, in parthenogenesis, and in connection with genetic data, we know that chromosomes play a crucial role in heredity, determining the path along which individual development should proceed.

With mitotic division chromosome cells are located in the equatorial plane of the spindle, split with mathematical accuracy along the length, and each daughter chromosome passes into one of the new cells. Then both the chromosomes and the cytoplasm grow until they are ready for the next division.

Fairly not only that every cell arises from a pre-existing cell, as Virchow stated about a hundred years ago in his famous phrase "Omnis cellula e cellula", but we now know that every chromosome also arises from a pre-existing chromosome. We also know that the daughter cell is similar to the mother cell because it has the same chromosomes.

It is known that any In an animal species, all body cells have the same number of chromosomes. In the horse roundworm (Ascaris megalocephala), their number is only four (except for the sex chromosomes), which is why this form has given us a lot of information about the chromosomes. Drosophila, the fruit fly, has only eight chromosomes; as these flies are easily bred by the thousands, they have contributed enormously to our knowledge of the nature of inheritance. Among mammals, the smallest number - 22 chromosomes - has the opossum, experiments on which helped Painter in his discovery of sex chromosomes in mammals.

Based this work Painter was able to determine the sex chromosomes in a person and establish that he has 48 of them.
If thoroughly study the chromosomes present in the cells of a species, it will become clear that each chromosome has its own properties. They are not at all the same, as is unfortunately shown in many simplified images of mitosis. Moreover, chromosomes exist in pairs, the members of which are the same in size and shape. The components of these pairs are not necessarily next to each other in the spindle of normal somatic mitosis, but methodical micromeasurements and comparisons have allowed cytologists to arrange cell chromosomes in similar pairs.

The meaning of this interesting fact will be discussed below in connection with maturation and fertilization.
genetics confirmed and extended the discovery of cytologists regarding the biological significance of chromosomes. Hereditary elements, or "genes", are seen as self-repairing bodies in chromosomes, with each gene defining a particular "single trait". The genes for various traits seem to be located at a specific location on the chromosome. This has been established by breeding animals in such a way that certain traits are changed. A microscopic study of germ cells in individuals that exhibit or have lost these characteristics revealed corresponding changes in the substance of the chromosomes.

Certainly, genes, like atoms, are ultramicroscopic in size. The biologist can judge their existence and arrangement only by observing the combinations and recombinations of substances in which he believes genes are present, just as the physicist judges the electronic structure of an atom, which he cannot see. Thus, from a variety of data, it became absolutely clear that chromosomes are the most important links in an endless chain of heredity. A certain number of pairs of chromosomes is constantly preserved due to mitosis in all cells of an individual and is transmitted with the help of gametes to organisms of the next generations.