Meiosis- This is the division of diploid cells, as a result of which haploid cells are formed. That is, from each pair of homologous chromosomes of the mother cell, only one chromosome gets into the daughter cells. Meiosis underlies the formation of sex cells - gametes. As a result of the fusion of male and female gametes, the diploid set is restored. Thus, one of the important values ​​of meiosis is to ensure the constancy of the number of chromosomes in a species during sexual reproduction.

In the cell that starts meiotic division, the duplication (replication) of chromosomes has already occurred, just as it happens in the interphase of mitosis. So each chromosome consists of two chromatids, and the number of chromosomes is diploid. That is, in terms of the amount of genetic information, the cells entering into mitosis and meiosis are the same.

Unlike mitosis, meiosis proceeds in two divisions. As a result of the first division, the homologous chromosomes of each pair diverge into different daughter cells, and two cells with a haploid number of chromosomes are formed, but each chromosome consists of two chromatids. The second division proceeds in the same way as mitotic, since the chromatids of each chromosome are separated, and one chromatid of each chromosome gets into the daughter cells.

Thus, as a result of meiosis, four cells with a haploid set of chromosomes are formed. In males, all four become sperm. But in females, only one becomes an egg, others die off. This is due to the fact that the supply of nutrients is concentrated only in one cell.

Stages, or phases, of the first meiotic division:

1. Prophase I. Spiralization of chromosomes. Homologous chromosomes are located parallel to each other and exchange some homologous regions (conjugation of chromosomes and crossing over, resulting in recombination of genes). The nuclear envelope is destroyed, a fission spindle begins to form.
2. Metaphase I. Pairs of homologous chromosomes are located in the equatorial plane of the cell. A spindle thread is attached to the centromere of each chromosome. And to each only one in such a way that a thread is attached to one homologous chromosome from one pole of the cell, and to the other - from the other.
3. Anaphase I. Each chromosome from a pair of homologous ones goes to its own pole of the cell. Moreover, each chromosome continues to consist of two chromatids.
4. Telophase I. Two cells are formed containing a haploid set of doubled chromosomes.

Stages, or phases, of the second meiotic division:

1. Prophase II. Destruction of nuclear shells, formation of a fission spindle.
2. Metaphase II. Chromosomes are located in the equatorial plane, and spindle filaments are attached to them. Moreover, in such a way that two threads are attached to each centromere - one from one pole, the other from the other.
3. Anaphase II. The chromatids of each chromosome are divided in the centromere region, and each of the pair of sister chromatids goes to its own pole.
4. Telophase II. Formation of nuclei, unwinding of chromosomes, division of the cytoplasm.

The diagram shows the behavior during meiosis of only one pair of homologous chromosomes. There are more of them in real cells. So in human cells there are 23 pairs. The diagram shows that the daughter cells are genetically different from each other. This is an important difference between meiosis and mitosis.

Another important importance of meiosis should be noted (the first, as already indicated, is the provision of a mechanism for sexual reproduction). As a result of crossing over, new combinations of genes are created. They are also created as a result of an independent divergence of chromosomes during meiosis. Therefore, meiosis underlies the combinative variability of organisms, which in turn is one of the sources of natural selection, i.e., evolution.

With the number reduced by two relative to the parent cell. Cell division by meiosis takes place in two main stages: meiosis I and meiosis II. At the end of the meiotic process, four are formed. Before a dividing cell enters meiosis, it goes through a period called interphase.

Interphase

• Phase G1: stage of cell development before DNA synthesis. At this stage, the cell, preparing for division, increases in mass.
• S-phase: the period during which DNA is synthesized. For most cells, this phase takes a short period of time.
• Phase G2: the period after DNA synthesis, but before the onset of prophase. The cell continues to synthesize additional proteins and grow in size.

In the last phase of interphase, the cell still has nucleoli. surrounded by a nuclear membrane, and the cell chromosomes are duplicated, but in shape. The two pairs, formed from the replication of one pair, are located outside the core. At the end of the interphase, the cell enters the first stage of meiosis.

Meiosis I:

Prophase I

In prophase I of meiosis, the following changes occur:

• Chromosomes condense and attach to the nuclear envelope.
• A synapsis occurs (pairwise convergence of homologous chromosomes) and a tetrad is formed. Each tetrad consists of four chromatids.
• Genetic recombination can occur.
• Chromosomes thicken and detach from the nuclear envelope.
• Likewise, centrioles migrate away from each other, and the nuclear envelope and nucleoli are destroyed.
• Chromosomes begin to migrate to the metaphase (equatorial) plate.

At the end of prophase I, the cell enters metaphase I.

Metaphase I

In metaphase I of meiosis, the following changes occur:

• The tetrads are aligned on the metaphase plate.
• homologous chromosomes are oriented to opposite poles of the cell.

At the end of metaphase I, the cell enters anaphase I.

Anaphase I

In anaphase I of meiosis, the following changes occur:

• Chromosomes move to opposite ends of the cell. Like mitosis, kinetochores interact with microtubules to move chromosomes to the poles of the cell.
• Unlike mitosis, they stay together after they move to opposite poles.

At the end of anaphase I, the cell enters telophase I.

Telophase I

In telophase I of meiosis, the following changes occur:

• The spindle fibers continue to move homologous chromosomes to the poles.
• Once the movement is complete, each pole of the cell has a haploid number of chromosomes.
• In most cases, cytokinesis (division) occurs simultaneously with telophase I.
• At the end of telophase I and cytokinesis, two daughter cells are formed, each of which has half the number of chromosomes of the original parent cell.
• Depending on the type of cell, different processes can occur in preparation for meiosis II. However, the genetic material does not replicate again.

At the end of telophase I, the cell enters prophase II.

Meiosis II:

Prophase II

In prophase II of meiosis, the following changes occur:

• The nuclear and nuclei are destroyed until the fission spindle appears.
• Chromosomes no longer replicate during this phase.
• Chromosomes begin to migrate to metaphase plate II (at the equator of the cells).

At the end of prophase II, the cells enter metaphase II.

Metaphase II

In metaphase II of meiosis, the following changes occur:

• Chromosomes line up on metaphase plate II in the center of the cells.
• Kinetochoric filaments of sister chromatids diverge to opposite poles.

At the end of metaphase II, the cells enter anaphase II.

Anaphase II

In anaphase II of meiosis, the following changes occur:

• Sister chromatids separate and begin to move to opposite ends (poles) of the cell. Fibers of the spindle of division, not associated with chromatids, stretch and lengthen the cells.
• Once paired sister chromatids are separated from each other, each of them is considered a complete chromosome, called.
• In preparation for the next stage of meiosis, the two poles of the cells also move away from each other during anaphase II. At the end of anaphase II, each pole contains a complete compilation of chromosomes.

After anaphase II, the cells enter telophase II.

Telophase II

In telophase II of meiosis, the following changes occur:

• Separate nuclei are formed at opposite poles.
• Cytokinesis occurs (division of the cytoplasm and the formation of new cells).
• At the end of meiosis II, four daughter cells are produced. Each cell has half the number of chromosomes of the original parent cell.

Meiosis result

The end result of meiosis is the production of four daughter cells. These cells have half the number of chromosomes relative to the parent. With meiosis, only sex products are produced. Others divide through mitosis. When the genitals unite during fertilization, they become. Diploid cells have a complete set of homologous chromosomes.

Definition

Meiosis (reduction cell division)- division, during which 4 haploid (n) cells are obtained from one diploid (2n) cell.

Since in daughter cells there is a decrease (reduction) in the number of chromosomes from 2n to n, such a division is called reduction.

Meiosis scheme

Meiosis in animals is observed during the formation of gametes (gametogenesis). Meiosis in plants and fungi usually occurs with the formation of haploid spores. In various unicellular eukaryotes, meiosis can be observed on different stages life cycle... To restore diploidy in the cycle, the fusion of haploid cells (fertilization) is always necessary.

Meiosis consists of two divisions. The first of them is actually reduction, that is, it is during the first division that the ploidy of the cell decreases. The reason for this is the divergence of homologous chromosomes ("maternal" and "paternal") in two different daughter cells. The second division is similar to mitosis and is called equational(that is, "equal"). The ploidy does not change as a result of the second division. In the course of this division, as in mitosis, sister chromatids (DNA copies) diverge. There is no DNA replication between two divisions of meiosis (since the "goal" of meiosis is to reduce the ploidy of the cell, there is no need to increase the amount of DNA).

In the prophase of the I division of meiosis, the most important process related to genetic recombination occurs - crossing over, that is, the exchange of regions of homologous chromosomes. As a result of this process, new combinations of genes are created in the offspring. Chromosomes as a whole are not transmitted directly from grandparents to grandchildren, but are “reconstructed” in each generation in the process of crossing over.

The following table describes the phases of meiosis in a cell for which n = 2, 2n = 4. Each set has three chromosomes that differ in size. The maternal and paternal chromosome sets are highlighted in blue and red.

The course of meiosis, as a rule, is disrupted in the cells of hybrid organisms, since in prophase I pairwise fusion (conjugation) should occur homologous chromosomes, and in hybrids the set of maternal genes is not homologous to the paternal one.

This mechanism underlies the sterility of interspecific hybrids. Since interspecific hybrids in the nucleus of cells combine the chromosomes of parents belonging to different species, the chromosomes usually cannot enter into conjugation. This leads to disturbances in the separation of chromosomes during meiosis and, ultimately, to the nonviability of gametes, and, consequently, to sterility (sterility) of hybrids.

In breeding to overcome the sterility of hybrids, polyploidy (multiple increase) of chromosome sets is artificially caused. In this case, each chromosome conjugates to the corresponding chromosome of its own set.

The value of meiosis

Sex cells of parents formed by meiosis have a haploid set (n) of chromosomes. In a zygote, when two such sets are combined, the number of chromosomes becomes diploid (2n). The formation of a new organism occurs by mitotic divisions of the zygote, and each cell contains a diploid (2n) set of chromosomes. Each pair of homologous chromosomes contains one paternal and one maternal chromosome. Based on this:

Meiosis is the basis of combinative variability due to crossing over (prophase I) and independent divergence of homologous chromosomes (anaphase I and II).

Due to a decrease in the number of chromosomes in gametes, a constant diploid (2n) set of chromosomes is maintained in new organisms.

prophase of I division of meiosis

The prophase of the I division of meiosis is peculiar, includes many processes and is subdivided into stages:

Leptotene

Zygotena

Paquitena

Diplotena

Diakinesis

This article will help you learn about the type of cell division. We will tell you briefly and clearly about meiosis, about the phases that accompany this process, outline their main features, and 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 a decrease.

This process takes place in two stages:

• Reducing ;

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

• Equational ;

During the second division, the cells remain haploid.

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A feature of this process is that it occurs only in diploid, as well as in even polyploid cells. This is 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 in 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 complex stage of the entire process as a whole, it consists of five stages, which are included in the following table:

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

Metophase 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 are separated and chromosomes diverge to different poles.

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

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

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

Second division has the following signs:

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

The value of meiosis

During meiosis, at the prophase 1 stage, 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 stages of 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 fuse. Otherwise, the number of chromosomes in the zygote would be twice as large. Due to this division, the sex cells are haploid, and during fertilization, the diploidity of the chromosomes is restored.

What have we learned?

Meiosis is a type of 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 the formation of gametes, for the transfer of genetic information to future generations, and also carries out the recombination of genetic material.

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Meiosis(Greek meiosis - decrease, decrease) or reduction division. As a result of meiosis, a decrease in the number of chromosomes occurs, i.e. from the diploid set of chromosomes (2p), a haploid (n) is formed.

Meiosis consists of 2 consecutive divisions:
Division I is called reduction or diminutive.
Division II is called equational or equalizing, i.e. goes by the type of mitosis (which means the number of chromosomes in the mother and daughter cells remains the same).

The biological meaning of meiosis is that four haploid cells are formed from one mother cell with a diploid set of chromosomes, thus the number of chromosomes is halved, and the amount of DNA is four times. As a result of such division, sex cells (gametes) in animals and spores in plants are formed.

The phases are also called as in mitosis, and before the onset of meiosis, the cell also undergoes an interphase.

Prophase I is the longest phase and is conventionally divided into 5 stages:
1) Leptonema (leptotene)- or the stage of fine filaments. There is a spiralization of chromosomes, the chromosome consists of 2 chromatids, thickenings or clumps of chromatin, which are called chromomeres, are visible on even thin filaments of chromatids.
2) Zigonema (zygotene, Greek merging threads) - the stage of paired threads. At this stage, homologous chromosomes (of the same size) converge in pairs, they are attracted and attached to each other along the entire length, i.e. conjugated in the region of chromomeres. It looks like a zipper lock. A pair of homologous chromosomes are called bivalents. The number of bivalents is equal to the haploid set of chromosomes.
3) Pachinema (pachytene, Greek. thick) - the stage of thick threads. Further spiralization of chromosomes is in progress. Then each homologous chromosome is split in the longitudinal direction and it becomes clearly visible that each chromosome consists of two chromatids, such structures are called tetrads, i.e. 4 chromatids. At this time, there is a crossing over, i.e. exchange of homologous regions of chromatids.
4) Diplonema (diplotene)- the stage of double strands. Homologous chromosomes begin to repel, move away from each other, but maintain interconnection with the help of bridges - chiasm, these are the places where crossing over occurs. In each chromatid compound (i.e. chiasma), chromatid regions are exchanged. Chromosomes spiralize and shorten.
5) Diakinesis- the stage of detached double strands. At this stage, the chromosomes are completely compacted and intensely stained. The nuclear membrane and nucleoli are destroyed. The centrioles move to the poles of the cell and form the filaments of the fission spindle. The chromosome set of prophase I is - 2n4c.
Thus, in prophase I:
1. conjugation of homologous chromosomes;
2. the formation of bivalents or tetrads;
3. crossing over.

Depending on the conjugation of chromatids, there may be different kinds crossing over: 1 - correct or incorrect; 2 - equal or unequal; 3 - cytological or effective; 4 - single or multiple.

Metaphase I - chromosome spiralization reaches its maximum. Bivalents line up along the cell's equator, forming a metaphase plate. Fission spindle threads are attached to centromeres of homologous chromosomes. Bivalents are connected to different poles of the cell.
The chromosome set of metaphase I is - 2n4c.

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

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

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

Prophase II is very short and proceeds without significant changes, if a nuclear envelope does not form in telophase I, then fission spindle threads are immediately formed.

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

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

Telophase II - chromosomes are despiralized, stretched and then poorly distinguished. Nuclear membranes and nucleoli are formed. Telophase II ends with cytokinesis.
The chromosome set after telophase II is - nc.

Meiotic division scheme