Meiosis morphology of the main phases of meiosis. Meiosis as the basis of sexual reproduction

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 envelope 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 are formed, nucleoli are formed.

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.

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.

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 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. Chromosomal mutations (large-scale deletions, duplications, inversions, or translocations) also impose certain restrictions on chromosome conjugation.

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:

• 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) crossing over (crossover), exchange of sites between homologous chromosomes; homologous chromosomes remain connected to each other.
• 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 the characteristic shape of lampbrush 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, 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 fission spindle is formed.
• 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.

Meaning

• In sexually reproducing organisms, the doubling of the number of chromosomes in each generation is prevented, since during the formation of germ cells by meiosis, a reduction in the number of chromosomes occurs.
• Meiosis creates an opportunity for the emergence of new combinations of genes (combinative variability), since the formation of genetically different gametes occurs.
• The reduction in the number of chromosomes leads to the formation of "pure gametes" carrying only one allele of the corresponding locus.
• The location of the bivalents of the equatorial plate of the spindle in metaphase 1 and of the chromosomes in metaphase 2 is determined randomly. The subsequent divergence of chromosomes in anaphase leads to the formation of new combinations of alleles in gametes. Independent segregation of chromosomes is at the heart of Mendel's third law.

Notes

Literature

• Babynin E. V. Molecular mechanism of homologous recombination in meiosis: origin and biological significance. Cytology, 2007, 49, N 3, 182-193.
• Alexander Markov. On the way to unraveling the mystery of meiosis. According to the article: Yu. F. Bogdanov. Evolution of meiosis in unicellular and multicellular eukaryotes. Aromorphosis at the cellular level. Journal of General Biology, Vol. 69, 2008. No. 2, March-April. Page 102-117
• "Variation and evolution of meiosis" - Yu. F. Bogdanov, 2003
• Biology: Allowances for applicants to universities: In 2 volumes. T.1.-B63 2nd ed., Corrected. and additional - M .: RIA "New Wave": Publisher Umerenkov, 2011.-500s.

Wikimedia Foundation. 2010 .

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.

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

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.

Meiosis, the most important process of cell division that occurs on the eve of the formation of germ cells and was discovered at the end of the 19th century, has long been the subject of close attention of a very narrow circle of cytologists. It came to the attention of molecular biologists only in the 1990s. The rapid development of research in this area was facilitated by work on the molecular genetics of model objects, as well as the emergence of new immunocytochemical methods, which gave researchers a convenient way to study proteins involved in meiosis.

In all eukaryotes, during meiosis, a submicroscopic structure is formed, called synaptonemal complex(from Greek synaptos - connected, peta - thread). The study of the molecular organization of this complex and its role in meiosis showed that it is needed for the recombination of chromosomes and the reduction of their number. This will be discussed in this article.

But first, let us recall the basic information about meiosis, which consists of two divisions: meiosis I and meiosis II. As a result of reduction division (meiosis I), the number of chromosomes in daughter cells is halved compared to the set of chromosomes in the parent cell. This is because the amount of DNA in chromosomes doubles only once before meiosis I (Fig. 1). A twofold reduction in the number of chromosomes during the formation of germ cells makes it possible to restore the initial (diploid) number of chromosomes during fertilization and maintain its constancy. This requires a strict separation of pairs of homologous chromosomes between germ cells. With errors, aneuploidy occurs - a lack or excess of chromosomes, and this imbalance leads to the death of the embryo or severe developmental anomalies (in humans, to the so-called chromosomal diseases).

Structure and function of the synaptonemal complex

The synaptonemal complex consists of two protein axes of homologous chromosomes connected by a protein zipper (Fig. 2). Zipper prongs are rod-shaped dimers of parallel-laid and identically oriented protein molecules with a long α-helix in the middle of the molecule. Yeast S. cerevisiae - it is the Zip1 protein, in mammals and humans it is SCP1 (SYCP1). These proteins are attached with their C-termini to the chromosome axes (lateral elements of the complex), while their N-termini are directed towards each other, inside the central space (Fig. 3). At the N-terminus of the molecules there are charged "spurs" - alternating peaks in the densities of positive and negative charges of amino acids (Fig. 4), the complementary interaction of which provides a strong electrostatic connection of the teeth.

The so-called central space of the complex (the gap between the protein axes, filled with the teeth of the "fastener", about 100 nm wide), as well as the entire complex (its cross section is about 150-200 nm) are not visible in a conventional light microscope, since the entire complex is masked by chromatin. For the first time, the synaptonemal complex was seen on ultrathin (0.8 µm thick) sections of the testes of crayfish and mice using a transmission electron microscope. It was discovered in 1956 independently by two American researchers - M. Moses and D.V. Fossett.

Now, when studying the complex, the so-called microspreading method is used. Testis cells (or plant anthers) after hypotonic shock are placed on a plastic substrate deposited on a glass slide. The content of the burst cell is fixed with a weak solution of formaldehyde and contrasted with salts of heavy metals (best of all - AgNO 3). The glass is examined in a phase-contrast microscope and, by indirect signs, cells are selected that should contain the complex. A circle of film with the desired cell is picked up on a metal mesh and placed in an electron microscope (Fig. 5). If necessary, before contrasting cells are treated with antibodies to proteins of interest to the researcher. These antibodies are labeled with calibrated colloidal gold beads, which are clearly visible under an electron microscope.

During prophase I of meiosis, the synaptonemal complex retains parallel homologous chromosomes almost until they are built on the equator of the cell (metaphase I). 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. In crossing over, which occurs with a frequency of at least one event (more often two, less often three or four) per pair of homologous chromosomes, dozens of meiosis-specific enzyme proteins participate.

The molecular mechanism of crossing over and its genetic implications are two big topics that are beyond the scope of this story. We are interested in this process because, as a result of it, homologous chromosomes are firmly bound by crossed DNA molecules (chiasmata) and the need for pairwise retention of the chromosome with the help of the synaptonemal complex disappears (after crossing over, the complex disappears). Homologous chromosomes connected by chiasmata line up at the equator of the cell division spindle and diverge with the help of the cell division spindle threads into different cells. After meiosis is completed, the number of chromosomes in daughter cells is halved.

So, only on the eve of meiosis I, the structure of chromosomes changes radically. A very specific intranuclear and interchromosomal structure - the synaptonemal complex - occurs once in the life cycle of an organism for a short time for pairing of homologous chromosomes and crossing over, and then is dismantled. These and many other events during meiosis at the molecular and subcellular (ultrastructural) levels are provided by the work of numerous proteins that perform structural, catalytic, and kinetic (motor) functions.

Proteins of the synaptonemal complex

Back in the distant 1970s, we obtained indirect evidence that the synaptonemal complex is formed by self-assembly of its elements, which can occur even in the absence of chromosomes. The experiment was set by nature itself, and we managed to observe it. It turned out that in the cytoplasm of pig roundworm cells preparing for meiosis I, packets or “stacks” of absolutely correctly stacked morphological elements of the synaptonemal complex appear (although there are no chromosomes in the cytoplasm: they are in the nucleus). Since there is still no synaptonemal complex in the cell nuclei at the stage of cell preparation for meiosis, an assumption appeared that the control of the order of meiotic events in this primitive organism is imperfect. An excess of newly synthesized proteins in the cytoplasm leads to their polymerization and the appearance of a structure that does not differ from the synaptonemal complex. This hypothesis was confirmed only in 2005 thanks to the work of an international group of researchers working in Germany and Sweden. They showed that if the gene encoding the mammalian zipper prong protein (SCP1) is introduced into somatic cells growing on an artificial nutrient medium and activated, then inside the cultured cells a powerful network of SCP1 proteins, "zipped" between itself in the same way as in the central space of the complex. The formation of a layer of continuous protein "zippers" in cell culture means that the ability of the proteins of the complex to self-assemble, which we predicted, has been proven.

In 1989 and in 2001 Our laboratory staff O. L. Kolomiets and Yu. S. Fedotova investigated the natural “dismantling” of synaptonemal complexes at the final stages of their existence. This multi-step process has been best observed in pollen mother cells in rye anthers, where there is partial synchrony of meiosis. It turned out that the lateral elements of the complex are dismantled by gradual “unwinding” of the protein supercoil, which has three levels of packaging (Fig. 6).

The basis of the extended lateral elements is a complex of four cohesin proteins (from the English. cohesion- grip). On the eve of meiosis, a specific Rec8 cohesin protein appears in the chromosomes, which replaces the somatic cohesin Rad21. Then, three other cohesin proteins, which are also present in somatic cells, join it, but instead of the somatic cohesin SMC1, the meiosis-specific protein SMC1b appears (its N-terminus differs by 50% from the N-terminus of the somatic SMC1 protein). This cohesin complex sits within the chromosome between two sister chromatids, holding them together. Meiosis-specific proteins bind to the cohesin complex, which become major proteins of chromosome axes and turn them (these axes) into lateral elements of the synaptonemal complex. In mammals, the major proteins of the synaptonemal complex are SCP2 and SCP3, in yeast, the Hop1 and Red1 proteins, and the meiosis-specific protein, Rec8.

The evolutionary paradox of proteins

In mammals and yeast, the proteins of the synaptonemal complex have different amino acid sequences, but their secondary and tertiary structures are the same. Thus, the zipper protein SCP1 in mammals and the non-homologous protein Zip1 in yeast are built according to a single plan. They consist of three amino acid domains: the central one is an α-helix capable of forming a second-order helix (supercoiling), and two terminal domains are globules. The major proteins SCP2 and SCP3, which have no homology with yeast Hop1 and Red1 proteins and, apparently, with still insufficiently studied proteins of the complex in plants, also build morphologically and functionally identical structures of the synaptonemal complex. This means that the primary structure (sequence of amino acids) of these proteins is an evolutionarily neutral feature.

So, non-homologous proteins in evolutionarily distant organisms build the synaptonemal complex according to a single plan. Explaining this phenomenon, I will use the analogy with the construction of houses from different materials, but according to a single plan. It is important that such houses have walls, ceilings, a roof, and that building materials meet the conditions of strength. Similarly, the formation of the synaptonemal complex requires lateral elements ("walls"), transverse filaments ("zipper teeth") - "overlays" and a central space (room for the "kitchen"). “Kitchen robots” should fit there - complexes of recombination enzymes assembled into so-called “recombination knots”.

The width of the central space of the synaptonemal complex in yeast, maize, and humans is approximately 100 nm. This is due to the length of single-stranded DNA regions coated with the Rad51 recombination protein. This protein belongs to a group of enzymes (similar to the bacterial recombination protein RecA) that have retained homology since the advent of DNA recombination (about 3.5 billion years ago). The inevitability of the homology of recombination proteins in distant organisms is determined by their function: they interact with the DNA double helix (the same in bacteria and mammals), dividing it into single strands, cover them with a protein sheath, transfer one strand to the homologous chromosome, and there again restore the double helix. Naturally, most of the enzymes involved in these processes retain their homology for more than 3 billion years. In contrast, the synaptonemal complexes that appeared in eukaryotes after the onset of meiosis (about 850 million years ago) are built from non-homologous proteins ... but the scheme of their domain structure is the same. Where did this scheme come from?

A clue is the mentioned Rec8 protein, which begins the formation of chromosome axes in the meiotic cycle and which is present in all studied organisms. It can be assumed that the building material for the axes of meiotic chromosomes and the lateral elements of the synaptonemal complex can be any intermediate proteins that are able to form a fibrous structure (SCP2, Hop1, etc.), interact with the Rec8 cohesin and “settle” on it, like concrete on a metal fittings.

In recent years, experiencing difficulties in conducting experimental work due to insufficient funding, we began to actively use bioinformatics methods. We were interested in the zipper protein in Drosophila. Considering the similarity of the secondary and tertiary structures of yeast Zip1 proteins and human SCP1, we hypothesized that the Drosophila zipper protein has the same structure. We began work in 2001, when the Drosophila genome had already been sequenced and it became known that it contained approximately 13,000 potential genes. How to find the gene for the protein we are looking for?

Among the 125 meiosis genes known by that time in Drosophila, we foresaw only one candidate for this role. The fact is that the mutation of the gene c(3)G deprived the chromosomes of the ability to connect in pairs with the help of a "zipper" and enter into recombination. We hypothesized that the mutants have a defective protein that forms submicroscopic teeth of the “fastener”. The secondary structure and conformation of the desired protein should be similar to the Zip1 and SCP1 proteins.

Knowing that the gene c(3)G is located on chromosome 3 in Drosophila, we searched the database for this region (comprising 700 kb) for an open reading frame that could encode a similar protein. We understood that in the absence of homology in the primary structure of the desired protein and yeast, their size, organization (of three domains) and the ability of the central domain to form an α-helix of a certain length (about 40 nm) should be similar. This was evidenced by the similarity of the electron microscopic picture of the synaptonemal complex in meiosis in yeast and Drosophila.

Open reading frames were scanned for nearly 80 genes in the search area. Using computer programs that allow predicting the secondary structure of a virtual protein, its physicochemical properties and the distribution of electrostatic charges in molecules, T. M. Grishaeva found such a reading frame at the border of the gene localization zone c(3)G.(This was not very accurately predicted by Japanese geneticists on a microscopic map of chromosomes.) It turned out to be a gene CG1J604 according to the genomic map of the Celera company.

We concluded that this virtual gene must be a long-known gene c(3)G and encode a protein similar to the yeast Zip1 protein. In response to our message, we received an email from S. Hawley from the USA. He experimentally proved that c(3)G encodes a protein that forms a "zipper" between chromosomes during meiosis in Drosophila. The results of our work coincided, but the experimental work of Hawley's group took about seven years, and our computer work by three people took only about three months. The articles went out of print at the same time. In 2003 we published the method of our computer searches and provided examples of similar virtual proteins in other organisms. This work is now readily cited by foreign colleagues, and our method works successfully in their hands in combination with experimental verification. So, in 2005, a group of English biologists discovered the gene and protein of the zipper teeth in the plant Arabidopsis thaliana .

In conclusion, I will give an example of another finding in the field of molecular biology of meiosis, but we must start with mitosis. In order for the chromatids to separate in the anaphase of mitosis, it is necessary to destroy the cohesin that “glues” them together. Hydrolysis of cohesins during mitosis is a genetically programmed event. But in the metaphase of meiosis I, when the homologous chromosomes are lined up at the equator of the cell and the protein spindle is ready to pull them to the poles, the hydrolysis of cohesins is impossible. That is why both chromatids of each chromosome, glued together in the region of the kinetic center of chromosomes (kinetochore), go to the same pole (see Fig. 1). In the late 1990s, Japanese researchers studying meiosis in yeast found that in the kinetochore region, cohesins are protected by a protein they called shugoshin (the root of this term is taken from the samurai lexicon and means protection). Very quickly, the world community of meiosis researchers came to the conclusion that Drosophila, corn, and other objects have similar shugoshin proteins. At the same time, genes that "prohibit" the separation of chromatids in meiosis I in Drosophila were known 10 years before, but their protein product was not deciphered. And in 2005, a group of American researchers from the University of California at Berkeley, including our compatriot and my longtime colleague in the study of meiosis I.N. kinetochores, and it appears in this region only if there is already a Rec8 cohesin, which it protects from hydrolysis (but only in meiosis I). These results were obtained using fluorescent antibodies to proteins and a confocal microscope. It remains to add that Japanese researchers immediately reported that shugoshin protected Rec8 from hydrolysis if shugoshin was dephosphorylated. Phosphorylation and dephosphorylation, as well as acetylation and deacetylation, are important modifications that change the properties of protein molecules.

Applied aspect

Everything told is a beautiful fundamental science, but can this knowledge be used for practical purposes? Can. Back in the mid-1980s, British researchers and our laboratory proved using various experimental models that, using microspreads of synaptonemal complexes, it is possible to detect twice as many chromosomal rearrangements (deletions, translocations, inversions) as compared with the traditional method of analyzing chromosomes at the metaphase stage. (Fig. 7). The fact is that the synaptonemal complex is the skeletal structure of meiotic chromosomes in prophase. At this time, the chromosomes are about 10 times longer, which significantly increases the resolution of the analysis. However, it is practically impossible to study prophase chromosomes entangled into a coil, and the rigid skeletal structures of the synaptonemal complex are not afraid of spreading, and, in addition, an electron microscope is able to distinguish miniaberrations that are inaccessible to a light microscope.

We asked ourselves the question: is it possible to establish the cause of sterility in the offspring of irradiated mice by studying not the chromosomes, but the synaptonemal complex? It turned out that in sterile mice that inherited chromosomal translocations from their parents, these rearrangements are detected using the complex in 100% of the cells under study, and with the usual methods of "metaphase" analysis - only in 50% of the cells. A group of Spanish researchers examined more than 1 thousand men suffering from infertility. In a third of them, the cause of infertility could not be previously determined, and the study of the synaptonemal complex from the cells of the testes of these patients allowed half of them to make a diagnosis: the cause of infertility is the absence of the synaptonemal complex, which is why spermatocytes (spermatozoa progenitor cells) do not develop, i.e. they do not develop. e. there was an "arrest" of the process of meiosis and all spermatogenesis. Similar results were obtained by OL Kolomiets together with doctors from Kharkov. The study of the synaptonemal complex in combination with other methods of analysis increases the percentage of detection of the causes of infertility in the examined male patients from 17 to 30%. Some English clinics already in the 90s of the XX century. used these methods extensively. Such diagnostics, of course, requires high theoretical and practical skills of physicians and the use of electron microscopes. Russian laboratories have not yet reached such a level, with the exception of the Institute of General Genetics. N. I. Vavilov Institute of Cytology and Genetics of the Siberian Branch of the Russian Academy of Sciences (Novosibirsk).

One might think that intensive studies of meiosis mechanisms will inevitably lead to the application of the acquired knowledge in those areas of biology and medicine that are associated with the fertility of living organisms, including humans. However, the law of applying scientific achievements in practice is unchanged: it is useless to “implement” something by force. Practitioners themselves must follow the achievements of science and use them. This is the approach used by leading pharmaceutical and biotech firms.

About 70 years passed from the discovery of meiosis (1885) to the discovery of the synaptonemal complex (1956), and another 30 years from 1956 to the discovery of the proteins of the synaptonemal complex (1986). Over the next 20 years, we learned the structure of these proteins, their genes, interaction proteins in the construction and operation of synaptonemal complexes, in particular, their interaction with protein-enzymes of DNA recombination, etc., i.e. more than in the previous 30-year period of descriptive cytological studies. It is possible that it will take no more than two decades to decipher the main molecular mechanisms of meiosis. The history of science, as well as of the whole civilization, is characterized by the "compression of time", the increasing compaction of events and discoveries.

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

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

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

The phases are called the same as in mitosis, and before the start of meiosis, the cell also goes through interphase.

Prophase I is the longest phase and is conventionally divided into 5 stages:
1) Leptonema (leptoten)- or the stage of thin threads. There is a spiralization of chromosomes, the chromosome consists of 2 chromatids, thickenings or clumps of chromatin, which are called chromomeres, are visible on the still thin threads of chromatids.
2) Zygonema (zygoten, Greek merging threads) - the stage of paired threads. At this stage, homologous chromosomes approach each other in pairs (they are identical in shape and size), they are attracted and applied to each other along the entire length, i.e. conjugate in the region of chromomeres. It looks like a zipper lock. A pair of homologous chromosomes is called a bivalent. The number of bivalents is equal to the haploid set of chromosomes.
3) Pachinema (pachytene, Greek thick) - the stage of thick threads. There is further spiralization of chromosomes. Then each homologous chromosome splits in the longitudinal direction and it becomes clearly visible that each chromosome consists of two chromatids; such structures are called tetrads, i.e. 4 chromatids. At this time, there is a crossing-over, i.e. exchange of homologous regions of chromatids.
4) Diplonema (diploten)- stage of double strands. Homologous chromosomes begin to repel, move away from each other, but remain interconnected with the help of bridges - chiasm, these are the places where crossing over will occur. At each chromatid junction (i.e. chiasm), chromatid segments are exchanged. Chromosomes coil and shorten.
5) Diakinesis- the stage of isolated double strands. At this stage, the chromosomes are fully compacted and intensely stained. The nuclear envelope and nucleoli are destroyed. Centrioles move to the poles of the cell and form spindle fibers. The chromosome set of prophase I is - 2n4c.
Thus, in prophase I, the following occurs:
1. conjugation of homologous chromosomes;
2. formation of bivalents or tetrads;
3. crossing over.

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

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

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

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

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

Prophase II is very short and goes on without any special changes, if the nuclear envelope does not form in telophase I, then spindle fibers immediately form.

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

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

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

Diagram of meiotic division