With the number reduced by two relative to the parent cell. Cell division through meiosis occurs 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 cellular chromosomes are duplicated, but are in the form. The two pairs formed from the replication of one pair are located outside the nucleus. At the end of 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.
• Synapsis occurs (pairwise convergence of homologous chromosomes) and a tetrad is formed. Each tetrad consists of four chromatids.
• Genetic recombination may occur.
• Chromosomes condense and detach from the nuclear envelope.
• Likewise, the 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. Similar to 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 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 with half the number of chromosomes of the original parent cell.
• Depending on the type of cell, various processes may 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 in this phase.
• Chromosomes begin to migrate to the metaphase plate II (on the cell equator).

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

Metaphase II

In metaphase II of meiosis, the following changes occur:

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

At the end of metaphase II, 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. Spindle fibers that are not associated with chromatids are stretched and elongate 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, 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 two fewer chromosomes than the parent. During meiosis, only sex cells are produced. Others divide by mitosis. When the genitals unite during fertilization, they become. Diploid cells have a complete set of homologous chromosomes.

), each of which contains half of the original somatic chromosome set. During meiosis, genetic recombination occurs between homologous chromosomes.

Thus, meiosis is a type of cell division in which there is a decrease (reduction) in the number of chromosomes by half: from diploid (2n) to haploid (1n). At the same time, it is thanks to meiosis that new combinations of genetic material are created through various combinations of maternal and paternal genes. It must be remembered that the genome of each cell consists of half paternal, half maternal chromosomes: 46 chromosomes of each person are combined into 23 pairs of homologous chromosomes, each of which has one paternal chromosome, the other. maternal. Homologous chromosomes in a pair are the same in size, shape, in the same areas contain genes that determine the same characteristics of the organism, but the specific forms of these genes (alleles) may be different. The interaction of allelic genes determines the manifestation of signs.

Meiosis proceeds similarly in almost all organisms. It consists of two consecutive divisions: the first and the second, and DNA replication precedes only the first division. In meiosis, as well as in mitosis, chromosomes consisting of two sister chromatids enter. However, chromosomes after premeiotic interphase are slightly different from chromosomes entering mitosis. The difference lies mainly in the fact that chromosomal proteins are not fully synthesized and DNA replication is also not completely completed: in some parts of the chromosomes, DNA remained underreplicated. Such DNA is not much, just a few thousandths. These differences in the composition of chromosomes are sufficient for their behavior in the first prophase of meiosis to differ from that in the mitotic prophase (Fig. 71).

The understanding of the fact that germ cells are haploid and therefore must be formed using a special mechanism of cell division came as a result of observations, which, moreover, almost for the first time suggested that chromosomes contain genetic information. In 1883, it was discovered that the nucleus of an egg and the sperm of a certain type of worm contain only two chromosomes each, while there are already four in a fertilized egg. The chromosomal theory of heredity could thus explain the long-standing paradox that the role of father and mother in determining the traits of the offspring often seems to be the same, despite the huge difference in the size of the egg and sperm.

Another important implication of this discovery was that that germ cells must be formed as a result of a special type of nuclear division, in which the entire set of chromosomes is divided exactly in half. This type of division is called meiosis (a word of Greek origin meaning "reduction". The name of another type of cell division - mitosis - comes from the Greek word meaning "thread", this choice of name is based on the thread-like appearance of chromosomes during their condensation during nuclear division - this process occurs both during mitosis, and during meiosis.) The behavior of chromosomes during meiosis, when their number is reduced, turned out to be more complex than previously thought. Therefore, the most important features of meiotic division could only be established by the beginning of the 1930s as a result of a huge number of thorough studies that combined cytology and genetics.

In the first division of meiosis, each daughter cell inherits two copies of one of the two homologues and therefore contains a diploid amount of DNA.

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, this 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 different unicellular eukaryotes, meiosis can be observed at different stages of the life cycle. To restore diploidy in a 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(i.e. "equal"). The ploidy does not change as a result of the second division. During this division, as in mitosis, sister chromatids (copies of DNA) diverge. Between two divisions of meiosis, there is no DNA replication (since the "goal" of meiosis is to reduce the ploidy of the cell, there is no need to increase the amount of DNA here).

In prophase I of meiosis division, the most important process related to genetic recombination occurs - crossing over, that is, the exchange of sections of homologous chromosomes. As a result of this process, new combinations of genes are created in the offspring. Chromosomes as a whole are not passed 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 vary in size. Maternal and paternal chromosome sets are highlighted in blue and red.

The course of meiosis, as a rule, is disturbed in the cells of hybrid organisms, since pairwise fusion (conjugation) must occur in prophase I 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 combine the chromosomes of parents belonging to different species in the cell nucleus, the chromosomes usually cannot conjugate. This leads to disturbances in the divergence of chromosomes during meiosis and, ultimately, to the non-viability of gametes, and, consequently, to sterility (infertility) of hybrids.

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

The meaning of meiosis

The sex cells of the 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 of its cells 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 segregation of homologous chromosomes (anaphase I and II).

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

prophase I of meiotic division

Prophase I of meiosis division is peculiar, includes many processes and is divided into stages:

Leptotena

Zygoten

Pachytene

Diploten

diakinesis

Meiosis- This is a type of division of germ cells, in which 4 haploid cells are formed from one diploid cell. In the interphase preceding meiosis, incomplete DNA replication occurs (thus, sections of single-stranded Z-DNA remain) and histone proteins.

Meiosis includes two divisions: 1 - reduction (reduction) and 2 - equational (equalizing).

Reduction division starts with prophase I fundamentally different from the prophase of mitosis. Prophase I consists of stages: leptotene, zygoten, pachytene, diploten, diakinesis.

Leptotena(thin threads) - chromosomes consist of two chromatids, they are weakly spiralized, their number is equal to diploid - 2n4s).

Zygoten(stage conjugating strands) - homologous chromosomes are attracted to each other - conjugate, forming bivalents. The number of bivalents is equal to haploid (n4c) (i.e., there are 4 chromatids in each bivalent). They are connected to each other like a zipper. Conjugation mechanism: weak spiralization (little lysine-rich histones), the presence of Z-DNA, which are attracted according to the principle of complementarity, highly repetitive DNA sequences. Such association of homologous chromosomes is carried out due to the unique structure inherent in meiosis - the synaptonemal complex, which provides close contact between homologous chromatid segments.

Pachytene(stage of thick filaments) - there is a thickening and shortening of chromosomes due to spiral twisting. The bivalent looks like a tetrad of chromatids.

Diploten- Homologous chromosomes begin to repel from the centromere region. The chromosomes seem to unwind. The places where chromosomes cross over are called chiasmata. In each notebook, maybe. 2 to 5 chiasmus. In this stage, there is an exchange between the homologous regions of non-sister (paternal and maternal) chromatids - crossing over.

The process of moving chiasmata from the centromere to the ends of chromosomes is called chiasma terminalization.

diakinesis(stage of divergence). Contact between chromatids is maintained at one or both ends. The nucleoli and the nuclear membrane disappear.

IN metaphase I bivalents are located along the equator, they are attached in the centromere region to the spindle threads. Homologous chromosomes are connected to each other by chiasmata that have moved to the ends of the chromosomes.

IN anaphase I homologous chromosomes from each bivalent move to the poles.

Telophase I- very short, in the process of it is the formation of new nuclei. Chromosomes decondense and despiralize. There was a reduction in the number of chromosomes (in each nucleus - n2c). This reduced haploid set necessarily includes one homologous chromosome from each bivalent. An independent combination of homologous chromosomes (paternal + maternal) occurs - the number of possible options is 2 23. / 2 - more than 4 million. This is the fundamental difference between meiosis and mitosis. Thus ends the reduction division.

cytokinesis in many organisms, it does not occur immediately after nuclear fission, so that in one cell there are two nuclei smaller than the original one.

Then comes the stage interkinesis, which differs from interphase in that DNA replication does not occur in it. Interkinesis is an intermediate stage between the reduction and equational divisions of meiosis.

Following interkinesis comes second division of meiosis - equational . It proceeds according to the type of mitosis, only a cell enters it not with a diploid (2n4s), but with a haploid (n2s) number of chromosomes consisting of two chromatids (their doubling occurred even in interphase before meiosis 1). Equational division consists of the same phases as mitosis: rophase II, metaphase II, anaphase II(chromatids diverge towards the poles), telophase II(each nucleus has a haploid number of single-stranded chromosomes). Cytokinesis occurs in the cell, resulting in the formation of four haploid cells (nc).

So, a diploid cell with a double set of chromosomes enters meiosis I. Meiosis I produces two haploid cells with duplicated chromosomes. As a result of meiosis II, four haploid, genetically heterogeneous cells with single chromosomes are formed.

The difference between meiosis and mitosis (fig.3.6) .

1. The prophase I of meiosis division, in contrast to the prophase of mitosis, is very extended, important processes occur in it associated with the conjugation of homologous chromosomes and crossing over.

2. The functional unit of mitosis is the chromatid, while that of meiosis is the whole chromosome.

3. During two divisions of meiosis, only a single duplication of DNA takes place.

4. As a result of mitosis, cells are formed with a diploid set of chromosomes and DNA, and as a result of meiosis - with a haploid set of chromosomes and DNA.

The biological significance of meiosis.

1. Due to meiosis in all living organisms during sexual reproduction, the constancy of the number of chromosomes (karyotype) is maintained in generations of organisms.

2. - Meiosis is a powerful factor of combinative variability:

1) Thanks to crossing over, recombination occurs at the level of genes (paternal and maternal) and the formation of qualitatively new chromosomes.

2) Due to the independent divergence of paternal and maternal chromosomes in anaphase 1 division, recombination occurs at the level of whole chromosomes: 1 paternal, 22 maternal, or 2 from and 21 mat, etc.

Meiosis underlies the formation of germ cells during sexual reproduction of multicellular organisms.

This is an important process in evolutionary terms, which allows organisms to create diverse populations in response to environmental changes. Without understanding the significance of meiosis, further study of such sections of biology as selection, genetics, and ecology is impossible.

What is meiosis

This method of division is characteristic for the formation of gametes in animals, plants and fungi. Meiosis produces cells that have a haploid set of chromosomes, also called sex cells.

Unlike another variant of cell multiplication - mitosis, in which the number of chromosomes of daughter individuals is characteristic of the mother, during meiosis, the number of chromosomes is halved. This happens in two stages - meiosis 1 and meiosis 2. The first part of the process is similar to mitosis - DNA doubling occurs before it, an increase in the number of chromosomes. Next comes the reduction division. As a result, cells with a haploid (rather than diploid) set of chromosomes are formed.

Basic concepts

In order to understand what meiosis is, it is necessary to remember the definitions of some concepts so as not to return to them later.

• Chromosome - a structure in the nucleus of a cell, which has a nucleoprotein nature and has concentrated most of the hereditary information.
• Somatic and germ cells - cells of the body that have a different set of chromosomes. Normally (excluding polyploids) somatic cells are diploid (2n) and sex haploid (n). When two germ cells merge, a complete somatic cell is formed.
• Centromere is a section of the chromosome responsible for gene expression and connecting chromatids to each other.
• Telomeres - end sections of chromosomes, perform a protective function.
• Mitosis is a way of dividing somatic cells, creating copies identical to them in the process.
• Euchromatin and heterochromatin are sections of chromatin in the nucleus. The first retains the despiralized state, the second is spiralized.

Process steps

Meiosis of a cell consists of two consecutive divisions.

First division. During prophase 1, chromosomes can be seen even with a light microscope. The structure of a double chromosome consists of two chromatids and a centromere. Spiralization occurs and, as a result, shortening of the chromatids in the chromosome. Meiosis begins at metaphase 1. Homologous chromosomes are located in the equatorial plane of the cell. This is called the alignment of tetrads (bivalents) of chromatid to chromatid. At this point, the processes of conjugation and crossing over occur, they are described below. During these actions, telomeres often cross over and overlap each other. The shell of the nucleus begins to disintegrate, the nucleolus disappears and the fission spindle threads become visible. During anaphase 1, whole chromosomes, consisting of two chromatids, move to the poles, and in a random way.

As a result of the first division in the telophase 1 stage, two cells with a single set of DNA are formed (in contrast to mitosis, the daughter cells of which are diploid). Interphase is short because it does not require DNA duplication.

In the second division at the stage of metaphase 2, already one chromosome (from two chromatids) departs to the equatorial part of the cell, forming a metaphase plate. The centromere of each chromosome divides, the chromatids diverge towards the poles. At the telophase stage of this division, two cells are formed containing each haploid set of chromosomes. There is already a normal interphase.

conjugation and crossing over

Conjugation is the process of fusion of homologous chromosomes, and crossing over is the exchange of the corresponding sections of homologous chromosomes (begins in the prophase of the first division, ends in metaphase 1 or in anaphase 1 when the chromosomes diverge). These are two related processes that are involved in the additional recombination of genetic material. Thus, the chromosomes in haploid cells are not similar to those in the mother, but already exist with substitutions.

Variety of gametes

Gametes formed during meiosis are not homologous to each other. Chromosomes diverge into daughter cells independently of each other, so they can bring a variety of alleles to future offspring. Consider the simplest classical problem: determine the types of gametes formed in the parent organism according to two simple traits. Let us have a dark-eyed and dark-haired parent, heterozygous for these traits. The allele formula that characterizes it will look like AaBb. Sex cells will look like this: AB, Ab, aB, ab. That is four types. Naturally, the number of alleles in a living organism with many traits will be many times higher, which means that there will be many times more options for the diversity of gametes. These processes are enhanced by conjugation and crossing over occurring in the process of fission.

There are errors in replication and divergence of chromosomes. This leads to the formation of defective gametes. Normally, such cells should undergo apoptosis (cell death), but sometimes they merge with another germ cell, forming a new organism. For example, Down's disease is formed in a person in this way, associated with one extra chromosome.

It should be mentioned that the formed germ cells in different organisms undergo further development. For example, in a person, four equivalent spermatozoa are formed from one parental cell - as in classical meiosis, what an egg is - it is somewhat more difficult to find out. From four potentially identical cells, one egg and three reduction bodies are formed.

Meiosis: biological significance

Why in the process of meiosis the number of chromosomes in a cell decreases is understandable: if this mechanism did not exist, then when two germ cells merge, there would be a constant increase in the chromosome set. Due to reduction division, in the process of reproduction, a full-fledged diploid cell emerges from the fusion of two gametes. Thus, the constancy of the species, the stability of its chromosome set, is preserved.

Half of the DNA of the daughter organism will contain maternal and half paternal genetic information.

The mechanisms of meiosis underlie the sterility of interspecific hybrids. Due to the fact that the cells of such organisms contain chromosomes from two species, during metaphase 1 they cannot enter into conjugation and the process of formation of germ cells is disrupted. Fertile hybrids are possible only between closely related species. In the case of polyploid organisms (for example, many agricultural plants), in cells with an even set of chromosomes (octoploids, tetraploids), the chromosomes diverge as in classical meiosis. In the case of triploids, chromatids are formed unevenly, there is a high risk of getting defective gametes. These plants propagate vegetatively.

Thus, understanding what meiosis is is a fundamental question in biology. The processes of sexual reproduction, the accumulation of random mutations, and their transmission to offspring underlie hereditary variability and indefinite selection. Modern selection is formed on the basis of these mechanisms.

Meiosis variants

The considered variant of division in meiosis is characteristic mainly of multicellular organisms. In the simplest, the mechanism looks somewhat different. In the process of it, one meiotic division proceeds, the crossing-over phase, respectively, also shifts. Such a mechanism is considered more primitive. It served as the basis for the division of haploid cells of modern animals, plants, fungi, which proceeds in two phases and provides the best recombination of genetic material.

Differences between meiosis and mitosis

Summing up the differences between these two types of division, it is necessary to note the ploidy of the daughter cells. If during mitosis the amount of DNA, chromosomes in both generations is the same - diploid, then in meiosis haploid cells are formed. In this case, as a result of the first process, two are formed, and as a result of the second - four cells. There is no crossing over in mitosis. The biological significance of these divisions also varies. If the goal of meiosis is the formation of germ cells and their subsequent fusion in different organisms, that is, the recombination of genetic material in generations, then the goal of mitosis is to maintain tissue stability and the integrity of the body.