Meiosis and mitosis - difference, phases. Meiosis and its phases

Mitosis- the main method of division of eukaryotic cells, in which doubling first occurs, and then a uniform distribution of hereditary material between daughter cells.

Mitosis is a continuous process in which there are four phases: prophase, metaphase, anaphase, and telophase. Before mitosis, the cell prepares for division, or interphase. The period of cell preparation for mitosis and mitosis itself together make up mitotic cycle. Below is a brief description of the phases of the cycle.

Interphase consists of three periods: presynthetic, or postmitotic, - G 1, synthetic - S, postsynthetic, or premitotic, - G 2.

Presynthetic period (2n 2c, where n- the number of chromosomes, with- the number of DNA molecules) - cell growth, activation of biological synthesis processes, preparation for the next period.

Synthetic period (2n 4c) is DNA replication.

Postsynthetic period (2n 4c) - preparation of the cell for mitosis, synthesis and accumulation of proteins and energy for the upcoming division, an increase in the number of organelles, doubling of centrioles.

Prophase (2n 4c) - the dismantling of nuclear membranes, the divergence of centrioles to different poles of the cell, the formation of fission spindle threads, the "disappearance" of the nucleoli, the condensation of two-chromatid chromosomes.

metaphase (2n 4c) - alignment of the most condensed two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes.

Anaphase (4n 4c) - the division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes).

Telophase (2n 2c in each daughter cell) - decondensation of chromosomes, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy). Cytotomy in animal cells occurs due to the fission furrow, in plant cells - due to the cell plate.

1 - prophase; 2 - metaphase; 3 - anaphase; 4 - telophase.

The biological significance of mitosis. The daughter cells formed as a result of this method of division are genetically identical to the mother. Mitosis ensures the constancy of the chromosome set in a number of cell generations. Underlies such processes as growth, regeneration, asexual reproduction, etc.

- This is a special way of dividing eukaryotic cells, as a result of which the transition of cells from a diploid state to a haploid one occurs. Meiosis consists of two consecutive divisions preceded by a single DNA replication.

First meiotic division (meiosis 1) called reduction, because it is during this division that the number of chromosomes is halved: from one diploid cell (2 n 4c) form two haploid (1 n 2c).

Interphase 1(at the beginning - 2 n 2c, at the end - 2 n 4c) - the synthesis and accumulation of substances and energy necessary for the implementation of both divisions, an increase in cell size and the number of organelles, doubling of centrioles, DNA replication, which ends in prophase 1.

Prophase 1 (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of fission spindle filaments, "disappearance" of nucleoli, condensation of two-chromatid chromosomes, conjugation of homologous chromosomes and crossing over. Conjugation- the process of convergence and interlacing of homologous chromosomes. A pair of conjugating homologous chromosomes is called bivalent. Crossing over is the process of exchanging homologous regions between homologous chromosomes.

Prophase 1 is divided into stages: leptotene(completion of DNA replication), zygotene(conjugation of homologous chromosomes, formation of bivalents), pachytene(crossing over, recombination of genes), diplotene(detection of chiasmata, 1 block of human oogenesis), diakinesis(terminalization of chiasma).

1 - leptotene; 2 - zygotene; 3 - pachytene; 4 - diplotene; 5 - diakinesis; 6 - metaphase 1; 7 - anaphase 1; 8 - telophase 1;
9 - prophase 2; 10 - metaphase 2; 11 - anaphase 2; 12 - telophase 2.

Metaphase 1 (2n 4c) - alignment of bivalents in the equatorial plane of the cell, attachment of the fission spindle threads at one end to the centrioles, the other - to the centromeres of the chromosomes.

Anaphase 1 (2n 4c) - random independent divergence of two-chromatid chromosomes to opposite poles of the cell (from each pair of homologous chromosomes, one chromosome moves to one pole, the other to the other), recombination of chromosomes.

Telophase 1 (1n 2c in each cell) - the formation of nuclear membranes around groups of two-chromatid chromosomes, the division of the cytoplasm. In many plants, a cell from anaphase 1 immediately transitions to prophase 2.

Second meiotic division (meiosis 2) called equational.

Interphase 2, or interkinesis (1n 2c), is a short break between the first and second meiotic divisions during which DNA replication does not occur. characteristic of animal cells.

Prophase 2 (1n 2c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle fibers.

Metaphase 2 (1n 2c) - alignment of two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes; 2 block of oogenesis in humans.

Anaphase 2 (2n 2with) - the division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes), recombination of chromosomes.

Telophase 2 (1n 1c in each cell) - decondensation of chromosomes, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy) with the formation of four haploid cells as a result.

The biological significance of meiosis. Meiosis is the central event of gametogenesis in animals and sporogenesis in plants. Being the basis of combinative variability, meiosis ensures the genetic diversity of gametes.

Amitosis

Amitosis- direct division of the interphase nucleus by constriction without the formation of chromosomes, outside the mitotic cycle. Described for aging, pathologically altered and doomed to death cells. After amitosis, the cell is unable to return to the normal mitotic cycle.

cell cycle

cell cycle- the life of a cell from the moment of its appearance to division or death. An obligatory component of the cell cycle is the mitotic cycle, which includes a period of preparation for division and mitosis proper. In addition, there are periods of rest in the life cycle, during which the cell performs its own functions and chooses its further fate: death or return to the mitotic cycle.

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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.

AT 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.

AT 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 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.

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:

*Stage*	*sign*
Leptotena	Chromosomes shorten, DNA condenses and thin filaments form.
Zygoten	Homologous chromosomes pair up.
Pachytene	By duration, the longest phase, during which homologous chromosomes are tightly attached to each other. As a result, there is an exchange of some sections between them.
Diploten	Chromosomes partially decondense, part of the genome begins to perform its functions. RNA is formed, protein is synthesized, while the chromosomes are still interconnected.
diakinesis	DNA condensation occurs again, the formation processes stop, the nuclear membrane disappears, the centrioles are located in opposite poles, but the chromosomes are interconnected.

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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Meiosis is a method of cell division in eukaryotes, in which haploid cells are formed. Meiosis is different from mitosis, which produces diploid cells.

In addition, meiosis proceeds in two successive divisions, which are called respectively the first (meiosis I) and the second (meiosis II). Already after the first division, the cells contain a single, i.e. haploid, set of chromosomes. Therefore, the first division is often called reduction. Although sometimes the term "reduction division" is used in relation to the entire meiosis.

The second division is called equational and similar in mechanism to mitosis. In meiosis II, sister chromatids diverge to the poles of the cell.

Meiosis, like mitosis, is preceded in interphase by DNA synthesis - replication, after which each chromosome already consists of two chromatids, which are called sister chromatids. Between the first and second divisions, DNA synthesis does not occur.

If as a result of mitosis two cells are formed, then as a result of meiosis - 4. However, if the body produces eggs, then only one cell remains, which has concentrated nutrients in itself.

The amount of DNA before the first division is usually denoted as 2n 4c. Here n denotes chromosomes, c denotes chromatids. This means that each chromosome has a homologous pair (2n), at the same time, each chromosome consists of two chromatids. Given the presence of a homologous chromosome, four chromatids are obtained (4c).

After the first and before the second division, the amount of DNA in each of the two daughter cells is reduced to 1n 2c. That is, homologous chromosomes diverge into different cells, but continue to consist of two chromatids.

After the second division, four cells are formed with a set of 1n 1c, i.e., each contains only one chromosome from a pair of homologous ones and it consists of only one chromatid.

The following is a detailed description of the first and second meiotic divisions. The designation of the phases is the same as in mitosis: prophase, metaphase, anaphase, telophase. However, the processes occurring in these phases, especially in prophase I, are somewhat different.

Meiosis I

Prophase I

This is usually the longest and most complex phase of meiosis. It takes much longer than with mitosis. This is due to the fact that at this time homologous chromosomes approach each other and exchange DNA segments (conjugation and crossing over occur).

Conjugation- the process of linking homologous chromosomes. Crossing over- exchange of identical regions between homologous chromosomes. Nonsister chromatids of homologous chromosomes can exchange equivalent regions. In places where such an exchange occurs, the so-called chiasma.

Paired homologous chromosomes are called bivalents, or tetrads. Communication is maintained until anaphase I and is provided by centromeres between sister chromatids and chiasmata between nonsister chromatids.

In prophase, chromosomes spiralize, so that by the end of the phase, the chromosomes acquire their characteristic shape and size.

In the later stages of prophase I, the nuclear envelope breaks up into vesicles and the nucleoli disappear. The meiotic spindle begins to form. Three types of spindle microtubules are formed. Some are attached to kinetochores, others - to tubules growing from the opposite pole (the structure acts as spacers). Still others form a stellate structure and are attached to the membrane skeleton, performing the function of a support.

Centrosomes with centrioles diverge towards the poles. Microtubules are introduced into the region of the former nucleus, attached to kinetochores located in the centromere region of chromosomes. In this case, the kinetochores of sister chromatids merge and act as a single whole, which allows the chromatids of one chromosome not to separate and subsequently move together to one of the poles of the cell.

Metaphase I

The fission spindle is finally formed. Pairs of homologous chromosomes are located in the plane of the equator. They line up opposite each other along the equator of the cell so that the equatorial plane is between pairs of homologous chromosomes.

Anaphase I

Homologous chromosomes separate and diverge to different poles of the cell. Due to the crossing over that occurred during prophase, their chromatids are no longer identical to each other.

Telophase I

The nuclei are restored. Chromosomes despiralize into thin chromatin. The cell is divided in two. In animals, by invagination of the membrane. Plants have a cell wall.

Meiosis II

The interphase between two meiotic divisions is called interkinesis, it is very short. Unlike interphase, DNA duplication does not occur. In fact, it is already doubled, just each of the two cells contains one of the homologous chromosomes. Meiosis II occurs simultaneously in two cells formed after meiosis I. The diagram below shows the division of only one cell out of two.

Prophase II

Short. The nuclei and nucleoli disappear again, and the chromatids spiralize. The spindle begins to form.

Metaphase II

Two spindle strands are attached to each chromosome, which consists of two chromatids. One thread from one pole, the other from the other. The centromeres are composed of two separate kinetochores. The metaphase plate is formed in a plane perpendicular to the equator of metaphase I. That is, if the parent cell in meiosis I divided along, now two cells will divide across.

Anaphase II

The protein that binds the sister chromatids separates, and they diverge to different poles. Sister chromatids are now called sister chromosomes.

Telophase II

Similar to telophase I. Despiralization of chromosomes occurs, the fission spindle disappears, the formation of nuclei and nucleoli, cytokinesis.

The meaning of meiosis

In a multicellular organism, only germ cells divide by meiosis. Therefore, the main meaning of meiosis is securitymechanismasexual reproduction,which maintains the constancy of the number of chromosomes in the species.

Another meaning of meiosis is the recombination of genetic information that occurs in prophase I, i.e. combinative variability. New combinations of alleles are created in two cases. 1. When crossing over occurs, i.e., non-sister chromatids of homologous chromosomes exchange sites. 2. With independent divergence of chromosomes to the poles in both meiotic divisions. In other words, each chromosome can be in the same cell in any combination with other non-homologous chromosomes.

Already after meiosis I, cells contain different genetic information. After the second division, all four cells differ from each other. This is an important difference between meiosis and mitosis, in which genetically identical cells are formed.

Crossing over and random segregation of chromosomes and chromatids in anaphases I and II create new combinations of genes and are oneof the causes of hereditary variability of organisms which makes possible the evolution of living organisms.

Meiosis- this is a method of indirect division of primary germ cells (2p2s), in which results in the formation of haploid cells (lnlc), most often sex.

Unlike mitosis, meiosis consists of two successive cell divisions, each preceded by an interphase (Fig. 2.53). The first division of meiosis (meiosis I) is called reduction, since in this case the number of chromosomes is halved, and the second division (meiosis II)-equational, since in its process the number of chromosomes is preserved (see Table 2.5).

Interphase I proceeds similarly to the interphase of mitosis. Meiosis I is divided into four phases: prophase I, metaphase I, anaphase I and telophase I. prophase I two major processes occur - conjugation and crossing over. Conjugation- this is the process of fusion of homologous (paired) chromosomes along the entire length. The pairs of chromosomes formed during conjugation are retained until the end of metaphase I.

Crossing over- mutual exchange of homologous regions of homologous chromosomes (Fig. 2.54). As a result of crossing over, the chromosomes received by the organism from both parents acquire new combinations of genes, which leads to the appearance of genetically diverse offspring. At the end of prophase I, as in the prophase of mitosis, the nucleolus disappears, the centrioles diverge towards the poles of the cell, and the nuclear membrane disintegrates.

ATmetaphase I pairs of chromosomes line up along the equator of the cell, spindle microtubules are attached to their centromeres.

AT anaphase I whole homologous chromosomes consisting of two chromatids diverge to the poles.

AT telophase I around clusters of chromosomes at the poles of the cell, nuclear membranes form, nucleoli form.

Cytokinesis I provides division of cytoplasms of daughter cells.

The daughter cells formed as a result of meiosis I (1n2c) are genetically heterogeneous, since their chromosomes, randomly dispersed to the poles of the cell, contain unequal genes.

Interphase II very short, since DNA doubling does not occur in it, that is, there is no S-period.

Meiosis II also divided into four phases: prophase II, metaphase II, anaphase II and telophase II. AT prophase II the same processes occur as in prophase I, with the exception of conjugation and crossing over.

AT metaphase II Chromosomes are located along the equator of the cell.

AT anaphase II Chromosomes split at the centromere and the chromatids stretch towards the poles.

AT telophase II nuclear membranes and nucleoli form around clusters of daughter chromosomes.

After cytokinesis II the genetic formula of all four daughter cells - 1n1c, however, they all have a different set of genes, which is the result of crossing over and a random combination of maternal and paternal chromosomes in daughter cells.