Types of meiosis and its biological significance. Brief description of the stages and scheme of cell division through meiosis Who first described the phases of meiosis

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 "meion" - less - and from "meiosis" - reduction).

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).

Although meiosis was discovered more than 100 years ago, the study of meiosis continues to this day. Interest in meiosis increased dramatically in the late 1960s, when it became clear that the same gene-controlled enzymes could be involved in many DNA-related processes. Recently, a number of biologists have been developing an original idea: meiosis in higher organisms serves as a guarantor of the stability of the genetic material, because during meiosis, when pairs of homologous chromosomes are in close contact, DNA strands are checked for accuracy and damage is repaired that affects both strands at once. The study of meiosis linked the methods and interests of two sciences: cytology and genetics. This led to the birth of a new branch of knowledge - cytogenetics, which is now in close contact with molecular biology and genetic engineering.

The biological significance of meiosis lies in the following processes:

1. Due to the reduction in the number of chromosomes as a result of meiosis in a series of generations during sexual reproduction, the constancy of the number of chromosomes is ensured.

2. Independent distribution of chromosomes in the anaphase of the first division ensures the recombination of genes belonging to different linkage groups (located on different chromosomes). The meiotic distribution of chromosomes among daughter cells is called chromosome segregation.

3. Crossing over in prophase I of meiosis ensures the recombination of genes belonging to the same linkage group (located on the same chromosome).

4. The random combination of gametes during fertilization, together with the above processes, contributes to genetic variability.

5. In the process of meiosis, another significant phenomenon occurs. This is the process of activation of RNA synthesis (or transcriptional activity of chromosomes) during prophase (diplotenes), associated with the formation of lampbrush chromosomes (found in animals and some plants).

This reversion of prophase to the interphase state (during mitosis, mRNA synthesis occurs only in interphase) is a specific characteristic of meiosis as a special type of cell division.

It should be noted that in protozoa, a significant variety of meiotic processes is observed.

In accordance with the position in the life cycle, three types of meiosis are distinguished:

Zygote th (initial) meiosis occurs in the zygote, i.e. immediately after fertilization. It is characteristic of organisms whose life cycle is dominated by the haploid phase (ascomycetes, bisidiomycetes, some algae, sporozoans, etc.).

Gametic(terminal) meiosis occurs during the formation of gametes. It is observed in multicellular animals (including humans), as well as among protozoa and some lower plants, in the life cycle of which the diploid phase predominates.

Intermediate(spore) meiosis occurs during spore formation in higher plants, including between the stages of sporophyte (plant) and gametophyte (pollen, embryo sac).

Thus, meiosis is a form of nuclear division, accompanied by a decrease in the number of chromosomes from diploid to haploid and a change in the genetic material. The result of meiosis is the formation of cells with a haploid set of chromosomes (sex cells).

The duration of meiosis may differ depending on the type of plants and animals (Table 1).

Table 1. Duration of meiosis in various plant species

A typical meiosis consists of two consecutive 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.

The initial number of chromosomes in meiocytes (cells entering meiosis) is called the diploid chromosome number (2n). The number of chromosomes in cells formed as a result of meiosis is called the haploid chromosome number (n). The minimum number of chromosomes in a cell is called the base 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 the gene.

The number of genomes in a cell is called the genomic number (n). In most multicellular animals, in all gymnosperms and in many angiosperms, the concept of haploidy-diploidy and the concept of genomic number coincide. For example, in humans n=x=23 and 2n=2x=46.

Morphology of meiosis - characteristics of phases

Interphase

The 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 (2n) the DNA content is 4c.

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.

first division of meiosis

The DNA has been replicated. Prophase I is the longest stage of meiosis.

The prophase I stage is subdivided into the following stages:

leptotena - the stage of thin threads;

zygotene - stage of double threads;

pachytene - the stage of thick threads;

diplotena - crossing over;

diakinesis - the disappearance of the nuclear membrane and nucleolus.

In early prophase (leptoten), preparation for conjugation of chromosomes takes place. The chromosomes are already doubled, but the sister chromatids in them are still indistinguishable. Chromosomes begin to pack (spiralize).

In contrast to the prophase of mitosis, where the chromosomes are located along the nuclear membrane end to end and, being packed, are attracted to the membrane, the leptotene chromosomes with their telomeric regions (ends) are located in one of the poles of the nucleus, forming a “bouquet” figure in animals and squeezing into a ball. synesis" - in plants. Such an arrangement or orientation in the nucleus allows chromosomes to quickly and easily conjugate homologous chromosome loci (Fig. 1).

The central event is the mysterious process of recognition of homologous chromosomes and their pairwise approach to each other occurs in the prophase I zygotene. When conjugation (approach) of homologous chromosomes, pairs are formed - bivalents and the chromosomes are noticeably shortened. From this moment, the formation of the synaptonemal complex (SC) begins. The formation of the synaptonemal complex and the synopsis of chromosomes are synonyms.

Rice. 1. Prophase stage

During the next stage of prophase I - pachytene between homologous chromosomes, close contact is strengthened, which is called synapsis (from the Greek synopsis - connection, connection). Chromosomes at this stage are highly spiralized, which makes it possible to observe them under a microscope.

During synapsis, homologues intertwine, i.e. conjugate. The conjugating bivalents are linked by chiasmata. Each bivalent consists of two chromosomes and four chromatids, where each chromosome comes from its parent. During the formation of synapsis (SC), there is an exchange of sites between homologous chromatids. This process, called crossing over, causes the chromatids to now have a different gene composition.

The synaptonemal complex (SC) in pachytene reaches its maximum development and during this period is a ribbon-like structure located in the space between parallel homologous chromosomes. The SC consists of two parallel lateral elements formed by densely packed proteins and a less dense central element extending between them (Fig. 2).

Rice. 2. Scheme of the synaptonemal complex

Each lateral element is formed by a pair of sister chromatids in the form of a longitudinal axis of the leptoten chromosome and, before becoming part of the SC, is called the axial element. Lateral loops of chromatin lie outside the SC, surrounding it from all sides.

SC development during meiosis:

the leptotene structure of the chromosomes that have entered the leptothene immediately turns out to be unusual: in each homologue, a longitudinal strand is observed along the axis of the chromosomes along its entire length;

zygotene - at this stage, the axial strands of the homologues approach each other, while the ends of the axial strands attached to the nuclear membrane seem to slide along its inner surface towards each other;

pachytene. The SC reaches its greatest development in pachytene, when all its elements acquire maximum density, and chromatin looks like a dense continuous “fur coat” around it.

SC functions:

1. A fully developed synaptonemal complex is necessary for the normal retention of homologues in the bivalent for as long as it is necessary for crossing over and chiasm formation. Chromosomes are connected using the synaptonemal complex for some time (from 2 hours in yeast to 2–3 days in humans), during which homologous DNA regions are exchanged between homologous chromosomes - crossing over (from English, crossing over - cross formation).

2. Prevention of too strong connection of homologues and keeping them at a certain distance, preserving their individuality, creating an opportunity to push off in diplotene and disperse in anaphase.

The process of crossing over is associated with the work of certain enzymes, which, when chiasmata are formed between sister chromatids, “cut” them at the point of intersection, followed by the reunification of the formed fragments. In most cases, these processes do not lead to any disturbances in the genetic structure of homologous chromosomes; there is a correct connection of fragments of chromatids and the restoration of their original structure.

However, another (more rare) variant of events is also possible, which is associated with an erroneous reunion of fragments of cut structures. In this case, there is a mutual exchange of sections of genetic material between conjugating chromatids (genetic recombination).

On fig. Figure 3 shows a simplified diagram of some possible variants of a single or double crossing over involving two chromatids from a pair of homologous chromosomes. It should be emphasized that crossing over is a random event that, with one or another probability, can occur in any region (or in two or more regions) of homologous chromosomes. Consequently, at the stage of maturation of the gametes of a eukaryotic organism in the prophase of the first division of meiosis, the universal principle of random (free) combination (recombination) of the genetic material of homologous chromosomes operates.

In cytological studies of synapsis over the past two decades, an important role has been played by the method of spreading prophase meiotic cells of animals and plants under the action of a hypotonic solution. The method entered cytogenetics after the work of Moses and played the same role that the method of preparing "squashed" preparations for the study of metaphase chromosomes played in its time, saving cytogeneticists from microtome sections.

The Moses method and its modifications have become more convenient than the analysis of SC on ultrathin sections. This method became the basis of meiosis research and gradually covered the issues of gene control of meiosis in animals and plants.

Rice. 3. Separate variants of single and double crossing over involving two chromatids: 1 initial chromatids and a variant without crossing over; 2 single crossing-over in the region A B and crossover chromatids; 3 single crossing over in the B-C region and crossover chromatids; 4 double crossing over and crossover chromatids of several different sites based on the homology of the genetic material of these sites. It is believed that either one of the two sister chromatids of the corresponding chromosome or both chromatids can participate in the conjugation process on each side.

In a dippoten, homologous chromosomes begin to repel each other after mating and crossing over. The process of repulsion begins with the centromere. The divergence of homologues is prevented by chiasma - the junction of non-sister chromatids resulting from the crossing. As the chromatids separate, some of the chiasmata move towards the end of the chromosome arm. Usually there are several crossovers, and the longer the chromosomes, the more there are, therefore, in a diplotene, as a rule, there are several chiasmata in one bivalent.

In the stage of diakinesis, the number of chiasmata decreases. Bivalents are located on the periphery of the nucleus. The nucleolus dissolves, the membrane collapses, and the transition to metaphase I begins. The nucleolus and nuclear membrane are preserved throughout the entire prophase. Before prophase, during the synthetic period of interphase, DNA replication and chromosome reproduction occur. However, this synthesis does not end completely: DNA is synthesized by 99.8%, and proteins - by 75%. DNA synthesis ends in pachytene, proteins - in diplotene.

In metaphase I, the spindle-shaped structure formed by microtubules becomes noticeable. During meiosis, individual microtubules are attached to the centromeres of the chromosomes of each bivalent. Then pairs of chromosomes move to the equatorial plane of the cell, where they line up in a random order. The centromeres of homologous chromosomes are located on opposite sides of the equatorial plane; in the metaphase of mitosis, on the contrary, the centromeres of individual chromosomes are located in the equatorial plane.

In metaphase I, bivalents are located in the center of the cell, in the zone of the equatorial plate (Fig. 4).

Rice. 4. Stages of meiosis: prophase I - metaphase I

Anaphase begins with the separation of homologous chromosomes and their movement towards the poles. In chromosomes without a centromere, attachment cannot exist. In anaphase of mitosis, centromeres divide and identical chromatids separate. In anaphase I of meiosis, the centromeres do not divide, the chromatids remain together, but the homologous chromosomes separate. However, due to the exchange of fragments as a result of crossing over, the chromatids are not identical, as at the beginning of meiosis. In anaphase I, the conjugating homologues diverge towards the poles.

In daughter cells, the number of chromosomes is half as much (haploid set), while the DNA mass is also halved and the chromosomes remain dichromatid. The exact divergence of homologous pairs to opposite poles underlies the reduction of their number.

In telophase I, chromosomes are concentrated at the poles, some of them decondense, due to which the spiralization of chromosomes weakens, they lengthen and again become indistinguishable (Fig. 5). As the telophase gradually passes into interphase, the nuclear envelope (including fragments of the parent cell nucleus envelope) and the cell septum arise from the endoplasmic reticulum. Finally, the nucleolus re-forms and protein synthesis resumes.

Rice. 5. Stages of meiosis: anaphase I - telophase I

In interkinesis, nuclei are formed, each of which contains n dichromatid chromosomes.

The peculiarity of the second division of meiosis is, first of all, that chromatin doubling does not occur in interphase II, therefore, each cell entering prophase II retains the same n2c ratio.

Second division of meiosis

During the second division of meiosis, the sister chromatids of each chromosome diverge towards the poles. Since crossing over could occur in prophase I and sister chromatids could become non-identical, it is customary to say that the second division proceeds according to the type of mitosis, but this is not true mitosis, in which daughter cells normally contain chromosomes identical in shape and set of genes.

At the beginning of the second meiotic division, the chromatids are still connected by centromeres. This division is similar to mitosis: if the nuclear membrane formed in telophase I, now it is destroyed, and by the end of the short prophase II, the nucleolus disappears.

Rice. 6. Stages of meiosis: prophase II-metaphase II

In metaphase II, the spindle and chromosomes, consisting of two chromatids, can again be seen. Chromosomes are attached by centromeres to spindle threads and line up in the equatorial plane (Fig. 6). In anaphase II, the centromeres divide and separate, and sister chromatids, now chromosomes, move toward opposite poles. In telophase II, new nuclear membranes and nucleoli are formed, the contraction of chromosomes weakens, and they become invisible in the interphase nucleus (Fig. 7).

Rice. 7. Stages of meiosis: anaphase II - telophase II

Meiosis ends with the formation of haploid cells - gametes, tetrads of spores - descendants of the original cell with a doubled (haploid) set of chromosomes and haploid DNA mass (original cell 2n, 4c, - spores, gametes - n, c).

The general scheme for the distribution of chromosomes of a homologous pair and the two pairs of differing allelic genes contained in them during two divisions of meiosis is shown in Fig. 8. As can be seen from this scheme, two fundamentally different variants of such a distribution are possible. The first (more probable) variant is associated with the formation of two types of genetically different gametes with chromosomes that have not undergone crossing overs in the regions where the genes under consideration are localized. Such gametes are called non-crossover. In the second (less probable) variant, along with non-crossover gametes, crossover gametes also arise as a result of genetic exchange (genetic recombination) in regions of homologous chromosomes located between the loci of two non-allelic genes.

Rice. 8. Two variants of the distribution of chromosomes of a homologous pair and the non-allelic genes contained in them as a result of two divisions of meiosis

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.

The main feature of meiosis is conjugation(pairing) homologous chromosomes with their subsequent divergence into different cells. The meiotic distribution of chromosomes among daughter cells is called chromosome segregation.

A Brief History of the Discovery of Meiosis

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 (from Greek. meiosis- decrease) is a special type of division of eukaryotic cells, in which, after a single duplication of DNA, the cell divided twice , and 4 haploid cells are formed from one diploid cell. Consists of 2 consecutive divisions (denoted by II and II); each of them, like mitosis, includes 4 phases (prophase, metaphase, anaphase, telophase) and cytokinesis.

Phases of meiosis:

Prophase I , it is complex, divided into 5 stages:

1. Leptonema (from Greek. leptos- thin, nema- thread) - chromosomes spiralize and become visible as thin threads. Each homologous chromosome is already 99.9% replicated and consists of two sister chromatids connected to each other in the centromere region. The content of genetic material - 2 n 2 xp 4 c. Chromosomes with the help of protein clusters ( attachment discs ) are attached at both ends to the inner membrane of the nuclear envelope. The nuclear membrane is preserved, the nucleolus is visible.

2. Zigonema (from Greek. zygon - paired) - homologous diploid chromosomes rush to each other and connect first in the centromere region, and then along the entire length ( conjugation ). Are formed bivalents (from lat. bi - double, valens- strong), or tetrads chromatids. The number of bivalents corresponds to the haploid set of chromosomes, the content of genetic material can be written as 1 n 4 xp 8 c. Each chromosome in one bivalent comes from either the father or the mother. sex chromosomes located near the inner nuclear membrane. This area is called sexual vesicle.

Between homologous chromosomes in each bivalent, specialized synaptonemal complexes (from Greek. synapsis- bond, connection), which are protein structures. At high magnification, the complex shows two parallel protein filaments, each 10 nm thick, connected by thin transverse bands about 7 nm in size; chromosomes in the form of many loops lie on both sides of them.

In the center of the complex passes axial element 20–40 nm thick. The synaptonemal complex is compared to rope ladder whose sides are formed by homologous chromosomes. A more accurate comparison is zipper .

By the end of the zygonema, each pair of homologous chromosomes is interconnected by synaptonemal complexes. Only the sex chromosomes X and Y do not fully conjugate, since they are not completely homologous.

3. In pachinema (from Greek. pahys- thick) bivalents shorten and thicken. Between the chromatids of maternal and paternal origin, connections occur in several places - chiasma (from Greek c hiazma- cross). In the region of each chiasm, a complex of proteins is formed, which are involved in recombination (d ~ 90 nm), and there is an exchange of the corresponding sections of homologous chromosomes - from paternal to maternal and vice versa. This process is called crossing over (from English. Withrossing- over- crossroads). In each human bivalent, for example, crossing over occurs in two to three sites.

4. In diplonome (from Greek. diploos- double) synaptonemal complexes disintegrate, and the homologous chromosomes of each bivalent move away from each other, but the connection between them is preserved in the chiasma zones.

5. diakinesis (from Greek. diakinein- pass through). In diakinesis, the condensation of chromosomes is completed, they are separated from the nuclear envelope, but the homologous chromosomes continue to remain connected to each other by the end sections, and the sister chromatids of each chromosome are centromeres. Bivalents take on a bizarre shape rings, crosses, eights etc. At this time, the nuclear envelope and nucleoli are destroyed. Replicated centrioles are sent to the poles, spindle fibers are attached to the centromeres of chromosomes.

In general, the prophase of meiosis is very long. With the development of sperm, it can last several days, and with the development of eggs, for many years.

metaphase I resembles a similar stage of mitosis. Chromosomes are installed in the equatorial plane, forming a metaphase plate. Unlike mitosis, spindle microtubules are attached to the centromere of each chromosome on only one side (from the side of the pole), while the centromeres of homologous chromosomes are located on both sides of the equator. The connection between chromosomes with the help of chiasma continues to be preserved.

IN anaphase I chiasmata disintegrate, homologous chromosomes separate from each other and diverge towards the poles. Centromeres these chromosomes, however, unlike the anaphase of mitosis, not replicated, which means that sister chromatids do not diverge. The divergence of chromosomes is random character. The content of genetic information becomes 1 n 2 xp 4 c at each pole of the cell, but in general in the cell - 2(1 n 2 xp 4 c) .

IN telophase I , as in mitosis, nuclear membranes and nucleoli are formed, the fission furrow. Then comes cytokinesis . Unlike mitosis, chromosome despiralization does not occur.

As a result of meiosis I, 2 daughter cells are formed containing a haploid set of chromosomes; each chromosome has 2 genetically distinct (recombinant) chromatids: 1 n 2 xp 4 c. Therefore, as a result of meiosis I occurs reduction (halving) the number of chromosomes, hence the name of the first division - reduction .

After the end of meiosis I, there is a short period - interkinesis , during which there is no DNA replication and doubling of chromatids.

Prophase II is short-lived, and conjugation of chromosomes does not occur.

IN metaphase II chromosomes line up in the plane of the equator.

IN anaphase II DNA in the centromere replicates, as it happens in the anaphase of mitosis, the chromatids diverge towards the poles.

After telophase II And cytokinesis II daughter cells are formed with the content of genetic material in each - 1 n 1 xp 2 c. In general, the second division is called equational (equalizing).

So, as a result of two consecutive divisions of meiosis, 4 cells are formed, each of which carries a haploid set of chromosomes.

Meiosis is carried out in the cells of organisms that reproduce sexually.

The biological meaning of the phenomenon is determined by a new set of traits in the descendants.

In this paper, we will consider the essence of this process and, for clarity, present it in the figure, see the sequence and duration of germ cell division, and also find out what are the similarities and differences between mitosis and meiosis.

What is meiosis

A process accompanied by the formation of four cells with a single chromosome set from one source.

The genetic information of each newly formed corresponds to half of the set of somatic cells.

Phases of meiosis

Meiotic division includes two stages, each consisting of four phases.

First division

Includes prophase I, metaphase I, anaphase I, and telophase I.

Prophase I

At this stage, two cells with a half set of genetic information are formed. The prophase of the first division includes several stages. It is preceded by premeiotic interphase, during which DNA replication takes place.

Then condensation occurs, forming long thin filaments with a protein axis during leptotene. This thread is attached to the nuclear membrane with the help of terminal extensions - attachment discs. The halves of the doubled chromosomes (chromatids) are not yet distinguishable. When examined, they look like monolithic structures.

Next comes the zygoten stage. Homologues merge to form bivalents, the number of which corresponds to a single number of chromosomes. The process of conjugation (connection) is carried out between paired, similar in genetic and morphological aspects. Moreover, the interaction begins from the ends, spreading along the bodies of the chromosomes. A complex of homologues linked by a protein component is a bivalent or tetrad.

Spiralization occurs during the stage of thick filaments - pachytene. Here, DNA duplication has already been completed, crossing over begins. This is an exchange of homologue sites. As a result, linked genes with new genetic information are formed. Transcription proceeds in parallel. Dense sections of DNA - chromomeres - are activated, which leads to a change in the structure of chromosomes like "lamp brushes".

Homologous chromosomes condense, shorten, diverge (except for the connection points - chiasma). This is a stage in the biology of diplotene or dictyoten. Chromosomes at this stage are rich in RNA, which is synthesized in the same areas. By properties, the latter is close to informational.

Finally, the bivalents diverge towards the periphery of the nucleus. The latter shorten, lose their nucleoli, become compact, not associated with the nuclear envelope. This process is called diakinesis (transition to cell division).

Metaphase I

Next, the bivalents move to the central axis of the cell. Spindles of division depart from each centromere, each centromere is equidistant from both poles. Small amplitude movements of the threads hold them in this position.

Anaphase I

Chromosomes built from two chromatids diverge. Recombination occurs with a decrease in genetic diversity (due to the absence in the set of genes located in loci (areas) of homologues).

Telophase I

The essence of the phase is the divergence of chromatids with their centromeres to opposite parts of the cell. In an animal cell, cytoplasmic division occurs, in a plant cell, the formation of a cell wall.

Second division

After the interphase of the first division, the cell is ready for the second stage.

Prophase II

The longer the telophase, the shorter the duration of the prophase. Chromatids line up along the cell, forming a right angle with their axes relative to the filaments of the first meiotic division. In this stage, they shorten and thicken, the nucleoli undergo disintegration.

Metaphase II

The centromeres are again located in the equatorial plane.

Anaphase II

Chromatids separate from each other, moving towards the poles. Now they are called chromosomes.

Telophase II

Despiralization, stretching of formed chromosomes, disappearance of the division spindle, doubling of centrioles. The haploid nucleus is surrounded by a nuclear membrane. Four new cells are formed.

Comparison table of mitosis and meiosis

Briefly and clearly, the features and differences are presented in the table.

Characteristics	meiotic division	Mitotic division
Number of divisions	carried out in two stages	carried out in one step
metaphase	after doubling, the chromosomes are arranged in pairs along the central axis of the cell	after doubling, the chromosomes are located singly along the central axis of the cell
merger	There is	No
Crossing over	There is	No
Interphase	no DNA duplication in interphase II	DNA doubles before division
division result	gametes	somatic
Localization	in mature gametes	in somatic cells
Playback path	sexual	asexual

The presented data is a diagram of the differences, and the similarities are reduced to the same phases, DNA replication and coiling before the start of the cell cycle.

The biological significance of meiosis

What is the role of meiosis:

Gives new combinations of genes due to crossing over.
Supports combinative variability. Meiosis is the source of new traits in a population.
Maintains a constant number of chromosomes.

Conclusion

Meiosis is a complex biological process in which four cells are formed, with new traits obtained as a result of crossing over.

Nikolai Mushkambarov, Dr. biol. Sciences

Humanity is aging, and yet everyone wants to live not just long, but also without those diseases that come with age. Over the past half century, there have been many "revolutionary" theories of aging, almost every one of which offers a sure and reliable way to slow down or even stop time. Every year - new sensations, new discoveries and new statements, encouraging and promising. Peptide bioregulators, longevity elixir, life-giving ions, or SkQ antioxidant. Run to the pharmacy, pay and live, according to the enclosed instructions, up to 100-120 years! To what extent can sensational discoveries be trusted and what is the “truth about aging”?

Professor N. N. Mushkambarov. Photo by Andrey Afanasiev.

August Weismann (1834-1914) German zoologist and evolutionist. He created a theory according to which hereditary traits are preserved and transmitted through the ageless germ plasm.

Leonard Hayflick is an American microbiologist. In the 1960s, he discovered that under laboratory conditions, human and animal cells can divide only a limited number of times.

Alexey Matveyevich Olovnikov is a Russian biochemist. To explain Hayflick's experiments in 1971, he put forward a hypothesis about the shortening of the terminal sections of chromosomes (telomeres) with each cell division.

Science and life // Illustrations

Elizabeth Blackburn and Carol Greider are American biologists. In 1985, the enzyme telomerase was discovered. The mechanism of action of telomerase is the repeated coding of new nucleotide sequences at the terminal sections of telomeres and the restoration of their original length.

Benjamin Gompertz (1779-1865), British mathematician. Proposed a function that describes the statistics of human mortality depending on age. This feature has been used to evaluate risks in life insurance.

The book by M. M. Vilenchik “The Biological Foundations of Aging and Longevity”, published in 1976, was one of the first popular science books on the topic of aging and was a huge success.

Scheme of meiosis (on the example of a pair of homologous chromosomes). In the prophase of the first division of meiosis, the chromosomes double; then homologous chromosomes conjugate with each other and, retaining their activity, enter into crossing over.

The questions of the special correspondent of the journal "Science and Life" Natalia Leskova are answered by Doctor of Biological Sciences, Professor of the Department of Histology of the Moscow State Medical University. I. M. Sechenov Nikolai Mushkambarov.

Nikolai Nikolayevich, you sharply criticize many widely known provisions of modern gerontology. Please outline the objects of your criticism.

More than enough objects! For example, it is now fashionable to refer to Weismann almost as the ultimate truth. This is a famous biologist who, back in the 19th century, postulated that aging did not appear in evolution immediately, but only at some stage as an adaptive phenomenon. From this they concluded that there must be ageless species: first of all, the most primitive organisms. At the same time, they somehow forget that if they do not age, then they should have 100% DNA repair. This is the most something primitive! Somehow it doesn't fit with each other.

There is a myth associated with the name of another famous biologist - Leonard Hayflick. Since the sixties of the last century, the scientific world has been convinced that a limit of 50 divisions has been set for human somatic cells, and such a limit in biology is called the “Hayflick limit”. Twenty years ago, stem cells were isolated that were supposedly capable of an unlimited number of divisions. And this myth (50 for everyone and infinity for stem cells) remains in the minds to this day. In fact, stem cells, as it turns out, are aging (that is, infinity is canceled), and it is not at all clear where these 50 divisions should be counted from. It is so incomprehensible that, most likely, there is no one division limit universal for all dividing human cells.

- What about the telomeric theory of aging? Does she distrust you too?

This is the most popular myth. According to this theory, the whole mechanism of aging comes down to the fact that in dividing cells there is no telomerase enzyme, which lengthens the ends of chromosomes (these ends are called telomeres), and therefore, with each division, telomeres are shortened by 50-100 nucleotide pairs of DNA. The enzyme telomerase does exist, and its discovery was awarded the 2009 Nobel Prize. And the phenomenon of chromosome shortening in dividing cells devoid of telomerase is also beyond doubt (although it is due to a slightly different reason, which was pointed out by the author of the telomere theory Alexei Olovnikov). But reducing aging to this phenomenon is like replacing the most complex score of a symphony with the notes of a drumbeat. It is no coincidence that in 2003 A. Olovnikov publicly abandoned his theory, replacing it with the so-called redumeric theory (also, by the way, not indisputable). But until now, even in medical schools in the course of biology, they present the telomeric theory as the latest achievement of scientific thought. This, of course, is absurd.

Another example is from mortality statistics. The main formula for this statistic is the Gompertz equation, proposed in 1825, or, with a correction term, the Gompertz-Makem equation (1860). In these equations, respectively, there are two and three coefficients, and the values of the coefficients vary greatly for different populations of people. And so, it turns out that changes in the coefficients of each equation correlate with each other. On the basis of which global, worldwide patterns are formulated: the so-called Strehler-Mildvan correlation and the compensatory effect of mortality that replaced it at this post is the hypothesis of the Gavrilov spouses.

I made a small model for a conditional population of people and with its help made sure that all these patterns are most likely an artifact. The fact is that a small error in determining one coefficient creates a sharp deviation from the true value of another coefficient. And this is perceived (in semi-logarithmic coordinates) as a biologically significant correlation and serves as a message for thoughtful conclusions.

- Are you sure you're right about the artifact?

Of course not! It is generally harmful for scientists to be absolutely sure of something, although there are plenty of such examples. But I did my best to test the opposite: that correlations are not an artifact. And I haven't been able to verify this. So for the time being, on the basis of a personal, very modest in scale, analysis, I have more reason to believe that these correlations are still artificial. They reflect method errors, not biological patterns.

And how do you evaluate the statements that there are a huge number of ageless organisms in nature and their list is growing every year?

Alas, popular theories that there are both ageless cells and ageless organisms lack sufficient evidence. Indeed, every year the circle of "ageless" animals is inexorably expanding. At first, these were practically only unicellular, then lower multicellular organisms (hydras, mollusks, sea urchins, etc.) were added to them. And now hotheads have appeared that “discover” individual ageless species even among fish, reptiles and birds. So it will go - they will soon get to mammals and establish, for example, that elephants also do not age, but die simply because of excess body weight!

- Are you convinced that there are no ageless animals?

I am not convinced that there are no such animals (although I am inclined to this), but that there is not a single species of animal for which the absence of aging has been proven absolutely reliably. With regard to human cells (as well as cells and other representatives of the animal world), the degree of certainty is perhaps even higher: stem cells, germ cells, and even tumor cells, in principle, age. Stem cells were considered to be undeniably ageless, and now there are experimental works proving the opposite.

What is the basis for such confidence? Have you conducted the relevant experiments yourself?

Generally speaking, a very long time ago, in 1977-1980, I tried to approach the problem of aging in experiments on mice. But not very reliable results (although they seem to confirm the initial assumption) convinced that it is better to do not experiment, but analysis. And here is one of the results of this analysis - the concept of "Anerem", or the ameiotic theory of aging. It includes six theses (if you like, postulates), of which one (the very first) is purely my work, and the rest are formulated on the basis of ideas already available in the literature. And, of course, it is important that all these theses form a fairly clear picture as a whole.

So, it is the ameiotic concept, if adhered to, that excludes the possibility of the existence of both non-senescent cells in multicellular organisms and non-aging organisms (starting with unicellular ones). At the same time, of course, I am aware that all theses of the concept are still hypotheses. But they seem to be much more justified than other views.

So your concept is like a tester, with which you can evaluate, relatively speaking, the truth of certain assumptions? In that case, tell us more about it.

I'll try to make it as accessible as possible. The very name of the concept ("Anerem") is an abbreviation of the words autocatalysis, instability, reparation, meiosis. Thesis one. Do you remember that the definition of life according to Engels was very well known before: “Life is a way of existence of protein bodies”? I revised this definition and gave my own, which made up the first thesis: "Life is a way of autocatalytic multiplication of DNA (less often RNA) in nature." This means that the driving force of both the emergence of life and its subsequent evolution is the indomitable desire of nucleic acids for endless self-reproduction. In essence, any organism is a biomachine improved in evolution, designed to effectively preserve and multiply the genome contained in it, with the subsequent efficient distribution of its copies in the environment.

- It is unusual to feel like a biomachine ...

Nothing, the sensation will pass, but the function, excuse me, will remain. Thesis two: "Genome instability is a central element of aging." This is how most sane scientists in the West, and even here, understand aging. The fact is that, with all their remarkable abilities, nucleic acids are subject to the damaging effects of many factors - free radicals, reactive oxygen species, etc. And although evolution has created many protective systems (such as the antioxidant system), numerous damages constantly occur in DNA strands. To detect and correct them, there is another protective system - DNA repair (restoration). The next thesis, the third one, is a filter that filters out everything "ageless": "Genome repair in mitotic and postmitotic cells is not complete." That is, any repair system in these cells does not provide 100% correction of all emerging DNA defects. And this means the universal nature of aging.

- But if everything and everything is aging, then how is life sustained on Earth?

That's right, I became interested in this issue in 1977. And I found, as it seemed to me, my own, albeit lying on the surface, answer. And 25 years later, in 2002, going through my old books, I realized that this hypothesis was not mine at all, but I had read about it a year before in the book of M. M. Vilenchik, I safely forgot and then remembered, but perceived it as your own. Such are the quirks of memory. But, in the end, the essence of the matter is important, not the ambitions of the discoverer.

The essence is formulated by the fourth thesis: "Effective repair can be achieved only in meiosis (or in its simplified version - endomixis) - with the conjugation (fusion) of chromosomes." What is meiosis, everyone seemed to be taught at school, but, unfortunately, sometimes even our medical students do not know this. I remind you: meiosis is the last twofold division in the formation of germ cells - sperm and eggs. By the way, I’ll tell you a secret: women do not form eggs. In them, the second meiotic division (at the stage of oocyte II - the development of the female germ cell) cannot occur independently - without the help of a spermatozoon. Because the cell somewhere “lost” its centrioles (the bodies in the cell involved in division): they were just there (during the previous division), and now they have gone somewhere. And the fertilization of oocyte II is absolutely required for the spermatozoon to bring in its centrioles and save the situation. I consider this as typical "female things". So the second division of meiosis eventually occurs, but the resulting cell is no longer an ovum, but a zygote.

We got carried away with “female things” and did not clarify how complete DNA repair is achieved in meiosis.

The first division of meiosis is preceded by a very long prophase: in male gametogenesis, it lasts a whole month, and in female - up to several decades! At this time, homologous chromosomes approach each other and remain in this state almost all the time of prophase.

At the same time, enzymes that cut and sew DNA strands are sharply activated. It was believed that this was necessary only for crossing over - the exchange of chromosomes with their own sections, which increases the genetic variability of the species. Indeed, “father's” and “mother's” genes, which are still distributed in each pair of homologous (similar structurally) chromosomes over different chromosomes, after crossing over, turn out to be mixed.

But M. M. Vilenchik, and after him I, drew attention to the fact that crossover enzymes are very similar to DNA repair enzymes, in which, when cutting out damaged areas, it is also necessary to break and sew DNA strands. That is, DNA superrepair is probably taking place simultaneously with crossing over. It is possible to imagine other mechanisms of major “repair” of genes during meiosis. One way or another, in this case, a radical (more precisely, complete) “rejuvenation” of cells occurs, which is why mature germ cells start counting time, as it were, from scratch. If something did not work out, then self-monitoring sensors for the state of their own DNA are triggered in the cell and the process of apoptosis starts - self-
cell killing.

- So, in nature, rejuvenation occurs only in maturing germ cells?

Quite right. But this is quite enough to ensure the immortality of the species - against the background, alas, of the inevitable mortality of all individuals. After all, sex cells - and only they! - the only material substrate of parental organisms from which a new life is born - the life of offspring.

And the fact that this mechanism concerns only germ cells is discussed in the two remaining theses of the concept, which dot all i's. The fifth thesis: “Meiosis improves the state of the genome only for subsequent generations (several generations at once in simple organisms and only one in all others).” Sixth thesis: "Hence follows the inevitability of aging of individuals (species) and the relative immortality of the species as a whole."

- Does meiosis exist in all animal species?

It should be in all kinds of animals - according to the concept of "Anerem", if it turns out to be true. Indeed, the concept comes from the universality of not only aging, but also meiosis. I have carefully researched this issue in the literature. Of course, in sufficiently developed animals - in fish and "above" - there is only a sexual method of reproduction, which also implies the presence of meiosis. In addition, there are huge sectors and flora and fauna in which mixed types of reproduction are common. This means that they alternate between more or less prolonged acts of asexual reproduction (for example, mitotic divisions, sporulation, budding, fragmentation, etc.) and single acts of sexual or quasi-sexual reproduction. An essential feature of the quasi-sex process (the so-called endomixis) is that structurally identical chromosomes from the paternal and maternal set also join here (conjugation of homologous chromosomes), although it does not end with their divergence in different cells.

Thus, with mixed reproduction, several generations of organisms live, as if gradually aging (similar to how mitotically dividing cells grow old in more complex animals), and then the sexual process returns individual organisms to a “zero” age and
provides a comfortable life for several more generations. And finally, it is believed that a number of simple animals reproduce only asexually. But with regard to them, I still have some doubt: have these organisms, in a long series of asexual reproductions, seen something similar to meiosis or endomixis (self-fertilization)?

It turns out that the concept you are developing puts an end to all dreams of prolonging human life. After all, ordinary (non-sex) cells are doomed to grow old and grow old?

No, I don't give up. Firstly, because for us it is not the fact of aging that is much more important, but the speed of this process. And there are many ways to influence the rate of aging. Some of them are known, some (like the Skulachev ions) are under study, some will be discovered later.

Secondly, it is possible that over time it will be possible to initiate some meiotic processes in somatic cells, for example, in stem and non-dividing cells. I mean those processes that restore the state of the genome: this, apparently, is the conjugation of homologous chromosomes, crossing over, or something more subtle and still unknown. I see no reason why it would be impossible in principle. In the lines of germ cells, meiosis enters, in general, cells of the same structure as many others. Moreover, even after the conjugation of chromosomes, the activity of the corresponding genes is preserved in the latter. However, to implement this project, it is necessary to first completely determine the genes responsible for various aspects of meiosis, and establish ways to purposefully influence them. This is, of course, a very fantastic project. However, did not much of what we have today seemed fantastic yesterday?!