Meiosis formulas. Brief description of the stages and scheme of cell division through meiosis

Meiosis is a method of indirect division of primary germ cells (2p2s), in as a result of which haploid cells (lnlc) are formed, most often sex cells.

Unlike mitosis, meiosis consists of two successive cell divisions, each of which is preceded by 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 like the interphase of mitosis. Meiosis I is divided into four phases: prophase I, metaphase I, anaphase I, and telophase I. B prophase I two important processes take place - conjugation and crossing over. Conjugation is the process of fusion of homologous (paired) chromosomes along their entire length. The pairs of chromosomes formed during conjugation are retained until the end of metaphase I.

Crossover- mutual exchange of homologous regions of homologous chromosomes (Fig. 2.54). As a result of crossing over, the chromosomes received by the body 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 to the poles of the cell, and the nuclear envelope disintegrates.

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

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

IN telophase I around the clusters of chromosomes at the poles of the cell, nuclear membranes are formed, nucleoli are formed.

Cytokinesis I provides separation of the cytoplasm of daughter cells.

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

Interphase II very short, since there is no DNA doubling 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. IN prophase II the same processes proceed as in prophase I, with the exception of conjugation and crossing over.

IN metaphase II chromosomes are located along the equator of the cell.

IN anaphase II chromosomes are split in centromeres and chromatids are already stretched to the poles.

IN telophase II nuclear membranes and nucleoli are formed around the 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 the chromosomes of the maternal and paternal organisms in the daughter cells.

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

Meiosis consists of 2 consecutive divisions:
Division I is called reduction or diminutive.
Division II is called equational or equalizing, i.e. goes by 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 such division, sex cells (gametes) in animals and spores in plants are formed.

The phases are also called as in mitosis, and before the onset of meiosis, the cell also undergoes an interphase.

Prophase I is the longest phase and is conventionally divided into 5 stages:
1) Leptonema (leptotene)- or the stage of fine filaments. There is a spiralization of chromosomes, a chromosome consists of 2 chromatids, thickenings or clumps of chromatin, which are called chromomeres, are visible on even thin filaments of chromatids.
2) Zigonema (zygotene, Greek merging threads) - the stage of paired threads. At this stage, homologous chromosomes (of the same size) converge in pairs, they are attracted and attached to each other along the entire length, i.e. conjugated in the region of chromomeres. It looks like a zipper lock. A pair of homologous chromosomes are called bivalents. The number of bivalents is equal to the haploid set of chromosomes.
3) Pachinema (pachytene, Greek. thick) - the stage of thick threads. Further spiralization of chromosomes is in progress. Then each homologous chromosome is split 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 (diplotene)- the stage of double strands. Homologous chromosomes begin to repel, move away from each other, but maintain interconnection with the help of bridges - chiasm, these are the places where crossing over occurs. In each chromatid compound (i.e. chiasma), chromatid regions are exchanged. Chromosomes spiralize and shorten.
5) Diakinesis- the stage of detached double strands. At this stage, the chromosomes are completely condensed and intensely stained. The nuclear membrane and nucleoli are destroyed. The centrioles move to the poles of the cell and form the filaments of the fission spindle. The chromosome set of prophase I is - 2n4c.
Thus, in prophase I:
1. conjugation of homologous chromosomes;
2. the 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 - chromosome spiralization reaches its maximum. Bivalents line up along the cell's equator, forming a metaphase plate. Fission spindle threads are attached to centromeres of homologous chromosomes. Bivalents are connected to different poles of the cell.
The chromosome set of metaphase I is - 2n4c.

Anaphase I - chromosome centromeres do not divide, the phase begins with the division of the chiasm. 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. they are randomly redistributed. At each pole, it turns out, the set of chromosomes is 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 is formed 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. chromosomes are already doubled and consist of sister chromatids, therefore interphase II is called interkinesis - i.e. there is a movement between two divisions.

Prophase II is very short and proceeds without significant changes, if a nuclear envelope does not form in telophase I, then fission spindle threads are immediately formed.

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

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

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

Meiotic division scheme

With a decrease in the number of chromosomes by half. It occurs in two stages (reduction and equational stages of meiosis). Meiosis should not be confused with gametogenesis - the formation of specialized germ cells, or gametes, from undifferentiated stem cells. With a decrease in the number of chromosomes as a result of meiosis in life cycle there is a transition from the diploid phase to the haploid phase.

The restoration of ploidy (the transition from the haploid phase to the diploid) 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 polyploids (tetra-, hexaploid, etc. cells).

Meiosis can also occur in odd polyploids (tri-, pentaploid, etc. cells), but in them, due to the impossibility of providing pairwise fusion of chromosomes in prophase I, chromosomes diverge with disorders that threaten the viability of the cell or the developing from it a multicellular haploid organism. The same mechanism underlies the sterility of interspecific hybrids.

Since interspecific hybrids in the nucleus of cells combine the chromosomes of parents belonging to different types, chromosomes usually cannot enter into conjugation. This leads to disturbances in chromosome separation during meiosis and, ultimately, to non-viability of germ cells, or gametes. Certain restrictions on the conjugation of chromosomes are also imposed by chromosomal mutations (large-scale deletions, duplications, inversions or translocations).

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:

Phase leptotenes or leptonemes- packing of chromosomes.

- Zygotena or zigonema- conjugation (connection) of homologous chromosomes with the formation of structures consisting of two connected chromosomes, called tetrads or bivalents.

- Paquitena or pachynema- crossing over (cross), exchange of sites between homologous chromosomes; homologous chromosomes remain connected.

- Diplotena or diplonema- there is a partial decondensation of chromosomes, while a part of the genome can work, there are processes of transcription (RNA formation), translation (protein synthesis); homologous chromosomes remain connected to each other.

- Diakinesis- DNA again condenses as much as possible, synthetic processes stop, the nuclear shell dissolves; centrioles diverge to the poles; homologous chromosomes remain connected to each other.

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

The second division of meiosis immediately follows the first, without a pronounced interphase: the S-period is absent, since there is no DNA replication 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 (each consisting of two chromatids) are located at the "equator" (at an equal distance from the "poles" of the nucleus) in one plane, forming the so-called metaphase plate.
• Anaphase II- univalents divide and chromatids diverge to the poles.
• Telophase II- the chromosomes are despiralized and a nuclear envelope appears.

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

Crossingo? Ver(another name in biology crossroad) - the phenomenon of exchange of regions of homologous chromosomes during conjugation in meiosis. In addition to meiotic, mitotic crossing over has also been described. Since crossing over disturbs the picture of linked inheritance, it was used to map "linkage groups" (chromosomes).

The mapping possibility was based on the assumption that the more often crossing over between two genes is observed, the farther from each other these genes are located in the linkage group and the more often deviations from linked inheritance will be observed. The first chromosome maps were constructed in 1913 for the classic experimental fruit fly object Drosophila melanogaster Alfred Sturtevant, student and collaborator of Thomas Hunt Morgan.

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

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

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

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

Another important point of this discovery was. that germ cells should be formed as a result nuclear fission special type, in which the entire set of chromosomes is exactly halved. This type of division is called meiosis (a word of Greek origin, meaning "decrease." - this process occurs both during mitosis and during meiosis.) The behavior of chromosomes during meiosis, when their number is reduced, turned out to be more complex than previously thought. Therefore, it was possible to establish the most important features of meiotic division only by the beginning of the 30s as a result of a huge number of careful studies that combined cytology and genetics.

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

The development and growth of living organisms is impossible without the process of cell division. In nature, there are several types and methods of division. In this article, we will briefly and clearly tell you about mitosis and meiosis, explain the main meaning of these processes, introduce you to how they differ and how they are similar.

Mitosis

The process of indirect division, or mitosis, is most often found in nature. The division of all existing non-sex cells is based on it, namely muscle, nerve, epithelial and others.

Mitosis consists of four phases: prophase, metaphase, anaphase and telophase. The main role of this process is equal distribution genetic code from a parent cell to two children. Moreover, the cells of the new generation are one-to-one similar to those of the mother.

Fig. 1. Scheme of mitosis

The time between fission processes is called interphase ... Most often, the interphase is much longer than mitosis. This period is characterized by:

• protein synthesis and ATP molecules in a cage;
• doubling of chromosomes and the formation of two sister chromatids;
• an increase in the number of organelles in the cytoplasm.

Meiosis

The division of germ cells is called meiosis, it is accompanied by a decrease in the number of chromosomes by half. The peculiarity of this process is that it takes place in two stages, which continuously follow each other.

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The interphase between the two stages of meiotic division is so short-lived that it is practically invisible.

Fig. 2. Scheme of meiosis

The biological significance of meiosis is the formation of pure gametes that contain a haploid, in other words, single, set of chromosomes. Diploidy is restored after fertilization, that is, the fusion of the maternal and paternal cells. As a result of the fusion of two gametes, a zygote with a full set of chromosomes is formed.

A decrease in the number of chromosomes during meiosis is very important, since otherwise, with each division, the number of chromosomes would increase. Due to reduction division, a constant number of chromosomes is maintained.

Comparative characteristics

The difference between mitosis and meiosis is the duration of the phases and the processes occurring in them. Below we offer you the table "Mitosis and Meiosis", which shows the main differences between the two methods of division. The phases of meiosis are the same as in mitosis. You can learn more about the similarities and differences between the two processes in the comparative characteristic.

Fig. 3. Comparative scheme of mitosis and meiosis

What have we learned?

In nature, cell division differs depending on their purpose. So, for example, non-sex cells divide by mitosis, and sex cells - by meiosis. These processes have similar division patterns at some stages. The main difference is the presence of the number of chromosomes in the formed new generation of cells. So, during mitosis, the newly formed generation has a diploid set, and with meiosis, a haploid set of chromosomes. The duration of the fission phases is also different. Both methods of division play a huge role in the life of organisms. Without mitosis, not a single renewal of old cells, reproduction of tissues and organs takes place. Meiosis helps maintain a constant number of chromosomes in the newly formed organism during reproduction.

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