Meiosis

Basic concepts and definitions

Meiosis is a special way of dividing eukaryotic cells, in which the initial number of chromosomes is reduced by 2 times (from the ancient Greek. " mayon" - less - and from " meiosis" - decrease). Often a decrease in the number of chromosomes is called reduction.

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

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

Main Feature meiosis is conjugation(pairing) homologous chromosomes followed by their divergence in different cells. The meiotic distribution of chromosomes among daughter cells is called chromosome segregation.

Short story meiosis discovery

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

General course of meiosis

A typical meiosis consists of two successive cell divisions, respectively called meiosis I And meiosis II. In the first division, the number of chromosomes is halved, so the first meiotic division is called reduction, less often heterotypic. In the second division, the number of chromosomes does not change; this division is called equational(equalizing), less often - homeotypic. The expressions "meiosis" and "reduction division" are often used interchangeably.

Interphase

Premeiotic interphase differs from the usual interphase in that the process of DNA replication does not reach the end: approximately 0.2 ... 0.4% of the DNA remains undoubled. Thus, cell division begins at the synthetic stage of the cell cycle. Therefore, meiosis is figuratively called premature mitosis. However, in general, it can be considered that in a diploid cell (2 n) DNA content is 4 With.

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

The first division of meiosis (reduction division, or meiosis I)

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

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

Leptotena(stage of thin threads). Chromosomes are visible under a light microscope as a ball of thin filaments. Early leptotene, when the strands of chromosomes are still very poorly visible, is called proleptothene.

Zygoten(stage of merging threads). going on conjugation of homologous chromosomes(from lat. conjugation- connection, pairing, temporary merging). Homologous chromosomes (or homologues) are chromosomes that are morphologically and genetically similar to each other. In normal diploid organisms, homologous chromosomes are paired: a diploid organism receives one chromosome from a pair from the mother, and the other from the father. When conjugated, they form bivalents. Each bivalent is a relatively stable complex of one pair of homologous chromosomes. Homologues are held together by protein synaptonemal complexes. One synaptonemal complex can only bind two chromatids at one point. The number of bivalents is equal to the haploid number of chromosomes. Otherwise, bivalents are called tetrads, since each bivalent contains 4 chromatids.

Pachytene(stage of thick filaments). Chromosomes spiralize, their longitudinal heterogeneity is clearly visible. DNA replication is completed (a special pachytene DNA). ending crossing over Crossover of chromosomes, as a result of which they exchange sections of chromatids.

Diploten(double strand stage). Homologous chromosomes in bivalents repel each other. They are connected at separate points, which are called chiasma(from the ancient Greek letters χ - "chi").

diakinesis(stage of divergence of bivalents). Separate bivalents are located on the periphery of the nucleus.

Metaphase I(metaphase of the first division)

IN prometaphase I the nuclear envelope breaks down (fragments). The spindle is formed. Next, metakinesis occurs - the bivalents move to the equatorial plane of the cell.

Anaphase I(anaphase of the first division)

The homologous chromosomes that make up each bivalent separate, and each chromosome moves towards the nearest pole of the cell. Separation of chromosomes into chromatids does not occur. The process by which chromosomes are distributed among daughter cells is called chromosome segregation.

Telophase I(telophase of the first division)

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

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

Interkinesis

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

The second division of meiosis (equatorial division, or meiosis II)

During the second division of meiosis, there is no decrease in the number of chromosomes. The essence of equational division is the formation of four haploid cells with single chromatid chromosomes (each chromosome includes one chromatid).

Prophase II(prophase of the second division)

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

Metaphase II(metaphase of the second division)

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

Anaphase II(anaphase of the second division)

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

Telophase II(telophase of the second division)

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

Types of meiosis and its biological significance

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

The biological significance of meiosis It consists in maintaining the constancy of the number of chromosomes in the presence of the sexual process. In addition, due to crossing over, recombination- the emergence of new combinations of hereditary inclinations in the chromosomes. Meiosis also provides combinative variability- the emergence of new combinations of hereditary inclinations during further fertilization.

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

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 reasons hereditary variability organisms which makes possible the evolution of living organisms.

Meioms (from other Greek meYashchuit - 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 gametogenesis, the formation of specialized germ cells, or gametes, from undifferentiated stem cells.

With a decrease in the number of chromosomes as a result of meiosis in life cycle there is a transition from the diploid phase to the haploid phase. Restoration of ploidy (transition from haploid to diploid phase) occurs as a result of the sexual process.

Due to the fact that in the prophase of the first, reduction, stage, pairwise fusion (conjugation) of homologous chromosomes occurs, the correct course of meiosis is possible only in diploid cells or in even polyploid (tetra-, hexaploid, etc. cells). Meiosis can also occur in odd polyploids (tri-, pentaploid, etc. cells), but in them, due to the inability to ensure pairwise fusion of chromosomes in prophase I, chromosome divergence occurs with disturbances that threaten the viability of the cell or the developing from it a multicellular haploid organism.

The same mechanism underlies the sterility of interspecific hybrids. Since interspecific hybrids in the nucleus of cells combine the chromosomes of parents belonging to various types, chromosomes usually cannot conjugate. This leads to disturbances in the divergence of chromosomes during meiosis and, ultimately, to the non-viability of germ cells, or gametes (the main means of combating this problem is the use of polyploid chromosome sets, since in this case each chromosome conjugates with the corresponding chromosome of its set). Certain restrictions on the conjugation of chromosomes are also imposed by chromosomal rearrangements (large-scale deletions, duplications, inversions or translocations).

During meiosis, not only the number of chromosomes is reduced to their haploid number, but an extremely important genetic process occurs - the exchange of sites between homologous chromosomes, a process called crossing over.

There are several types of meiosis. With a zygote (characteristic of ascomycetes, basymycetes, some algae, sporozoans, etc.), for which the haploid phase predominates in the life cycle, two gamete cells merge, forming a zygote with a double (diploid) set of chromosomes. In this form, the diploid zygote (resting spore) proceeds to meiosis, divides twice, and four haploid cells are formed, which continue to multiply.

The spore type of meiosis occurs in higher plants, the cells of which have a diploid set of chromosomes. In this case, in the reproductive organs of plants, the haploid cells formed after meiosis divide several more times. Another type of meiosis, gametic, occurs during the maturation of gametes - the precursors of mature germ cells. It is found in multicellular animals, among some lower plants.

In the case of gametic meiosis, it is characteristic during the development of the organism that clones of germ cells are secreted, which will subsequently differentiate into germ cells. And only the cells of these clones will undergo meiosis during maturation and turn into germ cells. Consequently, all cells of developing multicellular animal organisms can be divided into two groups: somatic - from which cells of all tissues and organs will be formed, and germinal, which will give rise to germ cells.

This isolation of germ cells (gonocytes) usually occurs in the early stages of embryonic development. Thus, the determination of gonocytes in the cyclops crustacean occurs already at the first division of the zygote: one of the two cells gives rise to germ cells. In ascaris, germ cells or cells of the "germ line" (A. Weisman) are isolated at the stage of 16 blastomeres, in Drosophila - at the blastocyst stage, in humans - primary germ cells (gonoblasts) appear at the 3rd week of embryonic development in the wall of the yolk sac in caudal part of the embryo.

Phases of meiosis

Meiosis consists of 2 consecutive divisions with a short interphase between them.

Prophase I - the prophase of the first division is very complex and consists of 5 stages:
Leptothena or leptonema - packing of chromosomes, condensation of DNA with the formation of chromosomes in the form of thin threads (chromosomes shorten).
Zygoten or zygonem - conjugation occurs - the connection of homologous chromosomes with the formation of structures consisting of two connected chromosomes, called tetrads or bivalents and their further compaction.
Pachytene or pachinema - (the longest stage) - in some places, homologous chromosomes are tightly connected, forming chiasma. Crossing over occurs in them - the exchange of sites between homologous chromosomes.
Diploten or diplonema - partial decondensation of chromosomes occurs, while part of the genome can work, transcription processes (RNA formation), translation (protein synthesis) occur; homologous chromosomes remain connected to each other. In some animals, chromosomes in oocytes at this stage of prophase of meiosis acquire the characteristic shape of lampbrush chromosomes.
Diakinesis - DNA condenses as much as possible again, synthetic processes stop, the nuclear envelope dissolves; centrioles diverge towards the poles; homologous chromosomes remain connected to each other.

By the end of Prophase I, centrioles migrate to the poles of the cell, spindle fibers are formed, the nuclear membrane and nucleoli are destroyed.

· Metaphase I - bivalent chromosomes line up along the equator of the cell.
· Anaphase I - microtubules contract, bivalents divide, and chromosomes diverge towards the poles. It is important to note that, due to the conjugation of chromosomes in the zygotene, whole chromosomes consisting of two chromatids each diverge towards the poles, and not individual chromatids, as in mitosis.
· Telophase I - chromosomes despiralize and the nuclear membrane appears.

The second division of meiosis follows immediately after the first, without a pronounced interphase: there is no S-period, since no DNA replication occurs before the second division.

Prophase II - condensation of chromosomes occurs, the cell center divides and the products of its division diverge to the poles of the nucleus, the nuclear envelope is destroyed, a division spindle is formed, perpendicular to the first spindle.
Metaphase II - univalent chromosomes (consisting of two chromatids each) are located on the "equator" (at an equal distance from the "poles" of the nucleus) in the same plane, forming the so-called metaphase plate.
Anaphase II - univalents divide and chromatids diverge towards the poles.
· Telophase II - chromosomes despiralize and the nuclear membrane appears.

As a result, four haploid cells are formed from one diploid cell. In those cases where meiosis is associated with gametogenesis (for example, in multicellular animals), the first and second divisions of meiosis are sharply uneven during the development of eggs. As a result, one haploid egg and three so-called reduction bodies (abortive derivatives of the first and second divisions) are formed.

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

The biological significance of meiosis

The biological significance of meiosis is to maintain a constant number of chromosomes in the presence of the sexual process. In addition, as a result of crossing over, recombination occurs - the appearance of new combinations of hereditary inclinations in chromosomes. Meiosis also provides combinative variability - the emergence of new combinations of hereditary inclinations during further fertilization.

Meiosis (in higher plants) takes place on the eve of flowering and leads to the formation of a haploid gametophyte, in which gametes are later formed.

Twice. It occurs in two stages (reduction and equational stages of meiosis). Meiosis should not be confused with gametogenesis - the formation of specialized germ cells, or gametes, from undifferentiated stem cells.

With a decrease in the number of chromosomes as a result of meiosis, a transition from the diploid phase to the haploid phase occurs in the life cycle. Restoration of ploidy (transition from haploid to diploid phase) occurs as a result of the sexual process.

The same 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 germ cells, or gametes (the main means of combating this problem is the use of polyploid chromosome sets, since in this case each chromosome conjugates with the corresponding chromosome of its set). Chromosomal rearrangements (large-scale deletions, duplications, inversions, or translocations) also impose certain restrictions on chromosome conjugation.

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Meiosis consists of 2 consecutive divisions with a short interphase between them.
- Prophase I- the prophase of the first division is very complex and consists of 5 stages:
- Leptotena, or leptonema- packing of chromosomes, condensation of DNA with the formation of chromosomes in the form of thin threads (chromosomes shorten).
- Zygoten, or zygonema- conjugation occurs - the connection of homologous chromosomes with the formation of structures consisting of two connected chromosomes, called tetrads or bivalents, and their further compaction.
- Pachytene, or pachinema- (the longest stage) - in some places, homologous chromosomes are tightly connected, forming chiasmata. Crossing over occurs in them - the exchange of sites between homologous chromosomes.
- Diploten, or diplonema- partial decondensation of chromosomes occurs, while part of the genome can work, transcription processes (RNA formation), translation (protein synthesis) occur; homologous chromosomes remain connected to each other. In some animals, chromosomes in oocytes at this stage of meiotic prophase acquire a characteristic shape of lamp-brush chromosomes.
- diakinesis- DNA again condenses as much as possible, synthetic processes stop, the nuclear envelope dissolves; centrioles diverge towards the poles; homologous chromosomes remain connected to each other.
By the end of prophase I, centrioles migrate to the poles of the cell, spindle fibers are formed, and the nuclear membrane and nucleoli are destroyed.
- Metaphase I- bivalent chromosomes line up along the equator of the cell.
- Anaphase I- microtubules contract, bivalents divide, and chromosomes diverge towards the poles. It is important to note that, due to the conjugation of chromosomes in the zygotene, whole chromosomes consisting of two chromatids each diverge towards the poles, and not individual chromatids, as in mitosis.
- Telophase I
The second division of meiosis follows immediately after the first, without a pronounced interphase: there is no S-period, since no DNA replication occurs before the second division.
- Prophase II- condensation of chromosomes occurs, the cell center divides and the products of its division diverge to the poles of the nucleus, the nuclear envelope is destroyed, a division spindle is formed, perpendicular to the first spindle.
- Metaphase II- univalent chromosomes (consisting of two chromatids each) are located on the "equator" (at an equal distance from the "poles" of the nucleus) in the same plane, forming the so-called metaphase plate.
- Anaphase II- univalents divide and chromatids diverge towards the poles.
- Telophase II Chromosomes despiralize and the nuclear membrane appears.
As a result, four haploid cells are formed from one diploid cell. In cases where meiosis is associated with gametogenesis (for example, in multicellular animals), during the development