Mitosis- the main method of division of eukaryotic cells, in which the doubling first occurs, and then the hereditary material is evenly distributed between the daughter cells.

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

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

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

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

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

Prophase (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments, “disappearance” of nucleoli, condensation of biromatid chromosomes.

Metaphase (2n 4c) - alignment of maximally condensed bichromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of spindle threads at one end to the centrioles, the other to the centromeres of the chromosomes.

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

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

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

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

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

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

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

Prophase 1 (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments, “disappearance” of nucleoli, condensation of biromatid chromosomes, conjugation of homologous chromosomes and crossing over. Conjugation- the process of bringing together and intertwining homologous chromosomes. A pair of conjugating homologous chromosomes is called bivalent. Crossing over is the process of exchange of homologous regions between homologous chromosomes.

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

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

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

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

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

Second meiotic division (meiosis 2) called equational.

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

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

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

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

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

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

Amitosis

Amitosis — direct division interphase nucleus by constriction without the formation of chromosomes, outside the mitotic cycle. Described for aging, pathologically altered and doomed cells. After amitosis, the cell is not able to return to the normal mitotic cycle.

Cell cycle

Cell cycle- the life of a cell from the moment of its appearance until division or death. Required component The cell cycle is the mitotic cycle, which includes the period of preparation for division and mitosis itself. In addition, in the life cycle there are periods of rest, during which the cell performs its inherent functions and selects future fate: death or return to the mitotic cycle.

Go to lectures No. 12"Photosynthesis. Chemosynthesis"

Go to lectures No. 14"Reproduction of Organisms"

In the last two years, in variants test tasks The Unified State Exam in biology began to appear more and more questions on the methods of reproduction of organisms, methods of cell division, differences between the different stages of mitosis and meiosis, sets of chromosomes (n) and DNA content (c) in various stages cell life.

I agree with the authors of the assignments. To thoroughly understand the essence of the processes of mitosis and meiosis, you need to not only understand how they differ from each other, but also know how the set of chromosomes changes ( n), and, most importantly, their quality ( With), at various stages of these processes.

We remember, of course, that mitosis and meiosis are various ways divisions kernels cells rather than the division of the cells themselves (cytokinesis).

We also remember that thanks to mitosis, diploid (2n) somatic cells multiply and asexual reproduction is ensured, and meiosis ensures the formation of haploid (n) germ cells (gametes) in animals or haploid (n) spores in plants.

For ease of perception of information

In the figure below, mitosis and meiosis are depicted together. As we see, this diagram does not include , it does not contain full description what happens in cells during mitosis or meiosis. The purpose of this article and this figure is to draw your attention only to those changes that occur with the chromosomes themselves on different stages mitosis and meiosis. This is precisely what the emphasis is placed on in the new USE test tasks.

In order not to overload the figures, the diploid karyotype in cell nuclei is represented by only two pairs homologous chromosomes (i.e. n = 2). The first pair are larger chromosomes ( red And orange). The second pair are smaller ones ( blue And green). If we were to specifically depict, for example, a human karyotype (n = 23), we would have to draw 46 chromosomes.

So what was the set of chromosomes and their quality before the start of division in the interphase cell during the period G1? Of course he was 2n2c. We do not see cells with such a set of chromosomes in this figure. Since after S During the interphase period (after DNA replication), the number of chromosomes, although remains the same (2n), but since each chromosome now consists of two sister chromatids, the cell karyotype formula will be written like this : 2n4c. And these are the cells with such double chromosomes, ready to begin mitosis or meiosis, that are shown in the figure.

This figure allows us to answer the following questions test tasks

— How does prophase of mitosis differ from prophase I of meiosis? In prophase I of meiosis, chromosomes are not freely distributed throughout the entire volume of the former cell nucleus (the nuclear membrane dissolves in prophase), as in prophase of mitosis, but homologues unite and conjugate (intertwine) with each other. This can lead to crossover : exchange of some identical regions of sister chromatids among homologues.

— How does metaphase of mitosis differ from metaphase I of meiosis? In metaphase I of meiosis, cells are not lined up along the equator bichromatid chromosomes as in metaphase of mitosis, in bivalents(two homologues together) or tetrads(tetra - four, according to the number of sister chromatids involved in conjugation).

— How does anaphase of mitosis differ from anaphase I of meiosis? During anaphase of mitosis, the filaments of the spindle are pulled apart towards the poles of the cell. sister chromatids(which at this time should already be called single chromatid chromosomes). Please note that at this time, since two single-chromatid chromosomes were formed from each bichromatid chromosome, and two new nuclei have not yet been formed, the chromosomal formula of such cells will be 4n4c. In anaphase I of meiosis, dichromatid homologues are pulled apart by spindle filaments towards the cell poles. By the way, in the figure at anaphase I we see that one of the sister chromatids of the orange chromosome has sections from the red chromatid (and, accordingly, vice versa), and one of the sister chromatids of the green chromosome has sections from the blue chromatid (and, accordingly, vice versa). Therefore, we can assert that during prophase I of meiosis, not only conjugation, but also crossing over occurred between homologous chromosomes.

— How does telophase of mitosis differ from telophase I of meiosis? During the telophase of mitosis, the two newly formed nuclei (there are not two cells yet, they are formed as a result of cytokinesis) will contain diploid set of single chromatid chromosomes - 2n2c. In telophase I of meiosis, the two resulting nuclei will contain haploid set of bichromatid chromosomes - 1n2c. Thus, we see that meiosis I has already provided reduction division (the number of chromosomes has halved).

— What ensures meiosis II? Meiosis II is called equational(equalizing) division, as a result of which the four resulting cells will contain a haploid set of normal single-chromatid chromosomes - 1n1c.

— How does prophase I differ from prophase II? In prophase II, cell nuclei do not contain homologous chromosomes, as in prophase I, so homologues do not unite.

— How does metaphase of mitosis differ from metaphase II of meiosis? A very “insidious” question, since from any textbook you will remember that meiosis II generally proceeds as mitosis. But, pay attention, during the metaphase of mitosis, the cells line up along the equator dichromatid chromosomes and each chromosome has its homologue. In metaphase II of meiosis, they also line up along the equator dichromatid chromosomes, but no homologous ones . In a color drawing, as in this article above, this is clearly visible, but in the exam the drawings are black and white. This black and white drawing of one of the test tasks depicts metaphase of mitosis, since there are homologous chromosomes (large black and large white are one pair; small black and small white are the other pair).

— There may be a similar question regarding anaphase of mitosis and anaphase II of meiosis .

— How does telophase I of meiosis differ from telophase II? Although the set of chromosomes in both cases is haploid, during telophase I the chromosomes are bichromatid, and during telophase II they are single-chromatid.

When I wrote such an article on this blog, I never thought that the content of tests would change so much in three years. Obviously, due to the difficulties of creating more and more new tests, relying on school curriculum in biology, the author-compilers no longer have the opportunity to “dig broadly” (everything has long been “dug up”) and they are forced to “dig deep.”

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Who has questions about the article to Biology tutor via Skype, please contact me in the comments.

With the number reduced by two relative to the parent cell. Cell division through meiosis occurs in two main stages: meiosis I and meiosis II. At the end of the meiotic process, four are formed. Before a dividing cell enters meiosis, it goes through a period called interphase.

Interphase

• Phase G1: stage of cell development before DNA synthesis. At this stage, the cell, preparing for division, increases in mass.
• S-phase: the period during which DNA is synthesized. For most cells, this phase takes a short period of time.
• Phase G2: the period after DNA synthesis but before the onset of prophase. The cell continues to synthesize additional proteins and increase in size.

IN last phase During interphase, the cell still has nucleoli. surrounded by a nuclear membrane and cellular chromosomes duplicated, but in the form . The two pairs, formed from replication of one pair, are located outside the nucleus. At the end of interphase, the cell enters the first stage of meiosis.

Meiosis I:

Prophase I

In prophase I of meiosis the following changes occur:

• Chromosomes condense and attach to the nuclear envelope.
• Synapsis occurs (pairwise bringing together of homologous chromosomes) and a tetrad is formed. Each tetrad consists of four chromatids.
• Genetic recombination may occur.
• Chromosomes condense and detach from the nuclear membrane.
• Similarly, centrioles migrate away from each other, and the nuclear envelope and nucleoli are destroyed.
• Chromosomes begin migration to the metaphase (equatorial) plate.

At the end of prophase I, the cell enters metaphase I.

Metaphase I

In metaphase I of meiosis, the following changes occur:

• The tetrads are aligned on the metaphase plate.
• homologous chromosomes are oriented to opposite poles of the cell.

At the end of metaphase I, the cell enters anaphase I.

Anaphase I

In anaphase I of meiosis, the following changes occur:

• Chromosomes move to opposite ends of the cell. Similar to mitosis, kinetochores interact with microtubules to move chromosomes to the poles of the cell.
• Unlike mitosis, they remain together after moving to opposite poles.

At the end of anaphase I, the cell enters telophase I.

Telophase I

In telophase I of meiosis, the following changes occur:

• The spindle fibers continue to move homologous chromosomes to the poles.
• Once movement is complete, each pole of the cell has a haploid number of chromosomes.
• In most cases, cytokinesis (division) occurs simultaneously with telophase I.
• At the end of telophase I and cytokinesis, two daughter cells are produced, each with half the number of chromosomes of the original parent cell.
• Depending on the cell type, different processes may occur in preparation for meiosis II. However, the genetic material is not replicated again.

At the end of telophase I, the cell enters prophase II.

Meiosis II:

Prophase II

In prophase II of meiosis, the following changes occur:

• The nucleus and nuclei are destroyed while the fission spindle appears.
• Chromosomes no longer replicate in this phase.
• Chromosomes begin to migrate to metaphase plate II (at the equator of the cells).

At the end of prophase II, cells enter metaphase II.

Metaphase II

In metaphase II of meiosis, the following changes occur:

• Chromosomes line up on metaphase plate II in the center of the cells.
• The kinetochore strands of sister chromatids diverge to opposite poles.

At the end of metaphase II, cells enter anaphase II.

Anaphase II

In anaphase II of meiosis, the following changes occur:

• Sister chromatids separate and begin to move to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids elongate and lengthen cells.
• Once paired sister chromatids are separated from each other, each is considered a complete chromosome, called a chromosome.
• In preparation for the next stage of meiosis, the two cell poles also move away from each other during anaphase II. At the end of anaphase II, each pole contains a complete compilation of chromosomes.

After anaphase II, cells enter telophase II.

Telophase II

In telophase II of meiosis, the following changes occur:

• Separate nuclei are formed at opposite poles.
• Cytokinesis occurs (cytoplasm division and formation of new cells).
• At the end of meiosis II, four daughter cells are produced. Each cell has half the number of chromosomes of the original parent cell.

Result of meiosis

The end result of meiosis is the production of four daughter cells. These cells have half as many chromosomes as the parent. During meiosis, only sexual parts are produced. Others divide through mitosis. When the sexes unite during fertilization, they become . Diploid cells have a full set of homologous chromosomes.

Main Event maturation stages- meiosis, a method of formation of germ cells, which consists of two successive divisions occurring quickly one after another - reductionist And equational.

Meiosis (Fig. 6.4) solves two important tasks. First, cells (gametes) are formed with haploid set chromosomes. This result is achieved due to the fact that two meiotic divisions occur during a single DNA replication. Until now, it is not completely clear which stage of gametogenesis this replication should be attributed to: whether it occurs in the final phase of the growth stage or at the very beginning of the maturation stage, immediately before prophase 1 of the meiotic division or even during prophase. On the one hand, there is an opinion that the first-order ovo(oo) cyt, having completed the cytoplasmic transformations of the growth stage, immediately enters the prophase of the first division of the maturation stage. On the other hand, a number of embryologists attribute pre-meiotic replication

Rice. 6.4. Meiosis (diagram)

cation of DNA to the beginning of prophase of the first division of meiosis. It cannot be excluded that DNA replication, having begun at the growth stage, is completed at the beginning of the maturation stage. Secondly, in prophase and anaphase of the first division of meiosis, mechanisms of genotypic combinative variability are laid down, which makes gametes genotypically different from the precursor cells of germ cells, as well as in general from the somatic cells of both parents.

Entering first division (reduction) stage of maturation,

cells have a diploid set of chromosomes, but the amount of DNA is doubled - 2n4c.

Just like in normal mitosis, in prophase During this division, compaction (spiralization) of the chromosome material occurs. At the same time, unlike ordinary mitosis, pairwise rapprochement is observed in it (conjugation) homologous chromosomes that are in close contact with each other through mutually corresponding (homologous) regions. The result of conjugation is the formation of pairs of chromosomes or bivalents, the number of which is n. Since each chromosome entering meiosis consists of two chromatids, the bivalent is represented by four chromatids - n4c. In prophase I of meiosis, the formation of the division spindle is noted. Towards the end of prophase, the degree of chromosome spiralization in bivalents increases and they shorten. The prophase of the first division of meiosis takes longer than the prophase of ordinary mitosis. There are several stages in it.

Leptotene- chromosomes begin the process of spiralization and become visible under a microscope as thin and fairly long filamentous structures.

Zygotene- corresponds to the beginning of conjugation of homologous chromosomes, united into bivalents by special structures - synapse-tonemal complexes(Fig. 6.5). If not all homologous chromosomes conjugate and unpaired chromosomes remain outside the bivalents, the cell dies by apoptosis.

Pachytena- against the background of ongoing spiralization of chromosomes and their shortening, homologous chromosomes carry out crossing over or cross, consisting in the exchange of mutually corresponding (homologous) regions. Crossing over ensures recombination of paternal and maternal alleles in linkage groups (homologous

Rice. 6.5. Formation of bivalents by conjugating homologous chromosomes in zygotene of prophase I of meiosis: 1 - centromere

chromosomes). The crossing of chromosomes can occur in different places on the chromosomes, and therefore crossing over in each specific case leads to an exchange different areas genetic material. The formation of several crossovers between two chromatids is possible (Fig. 6.6) or the exchange of mutually corresponding fragments occurs between more than two chromatids of a bivalent (Fig. 6.7). All this increases the efficiency of crossing over as a mechanism of genotypic combinative variability.

Diplotena- homologous chromosomes begin to move away from each other, primarily in the centromere region, but remain connected in the places where crossing over has occurred - chiasmata. We can talk about longitudinal splitting of conjugated homologous chromosomes along their entire length. As a result, each pair of chromosomes is perceived as a complex of four chromatid structures (daughter chromosomes) - tetrad(Fig. 6.8).

Diakinesis- completes the prophase of the first meiotic division; homologous chromosomes remain part of the bivalents, but their connection is limited only to individual points of the chiasmata (Fig. 6.9). The bivalents themselves take the form of rings, eights, and crosses.

Rice. 6.6. Multiple crossing over between homologous chromosomes (scheme): A-E, a-e: chromosome loci

Rice. 6.7. Multiple exchange of regions between four chromatids in pachytene of prophase I of meiosis (scheme): all four chromatids of a bivalent can participate in crossing over; in Latin letters mutant alleles are indicated; wild-type (normal) alleles are indicated by “+”

During the period of diakinesis, the progression of gamete precursor cells through reduction division is suspended (according to earlier ideas, this occurs already in diplotene), and therefore this period is called stationary. Division resumes and

Rice. 6.8. Diplotene in prophase I of grasshopper meiosis

Rice. 6.9. Diakinesis in prophase I of human meiosis: arrows indicate chiasmata

ends in the event of ovulation of the egg (see here, below) and its fertilization. Despite the characterization of the period of diakinesis as stationary, synthetic processes actively occur in it. These processes belong to progenesis (pre-embryonic period of ontogenesis), since the results of these processes in the form of synthesized

molecules and formed structures are necessary mainly for early stages embryo development. Firstly, we're talking about about DNA amplification (see also clause 2.4.3.4-a), which consists in the formation of numerous copies of ribosomal RNA genes - small (18S) and large (28S) subunits. The copies, having become independent, are transformed morphologically into nucleoli numbering up to several thousand. In such nucleoli, ribosomal subunits are formed, which are used to organize protein biosynthesis by embryonic cells. Upon completion of their function, these nucleoli move into the cytoplasm and are destroyed there. In diakinesis, genes are amplified 5S ribosomal RNA and tRNA. These RNAs are produced in the necessary (i.e., large) quantities “for future use” for protein synthesis, also in embryogenesis. Thanks to gene amplification, the amount of “production” required for the early stages of embryogenesis, for example, of ribosomes in the African clawed frog (Xenopus laevis) is reduced from 500 years to 3 months. Secondly, during the period of diakinesis of prophase I of meiosis, chromosomes take on the appearance of “lamp brushes” (see section 2.4.3.4-a), which ensures the formation of a certain set of i(m)RNA “for future use” for the needs of the embryo. The described processes have been most fully studied in tailless amphibians (frogs), which are characterized by relatively late (gastrula stage) activation of their own genome. In mammals, for example, complete bioinformational support of embryogenesis processes due to the functional genetic activity (transcription) of their own genes is noted starting from the stage of 8 blastomeres.

IN metaphase The first division of meiosis completes the formation of the spindle. The threads of this spindle, associated, in particular, with the centromeres of homologous chromosomes, are directed to different poles. This position of the filaments ensures the regular orientation of the bivalents in the equatorial plane of the fission spindle.

IN anaphase During the first division of meiosis, due to the weakening of the bonds between homologous chromosomes in bivalents and the regular orientation of bivalents in the metaphase plate, homologs of each bivalent diverge to different poles of the cell. In this case, the homologous chromosomes of paternal and maternal origin of each pair diverge independently of each other. As a result, “random” associations of homologous chromosomes of paternal and maternal origin gather at the cell poles at the completion of anaphase I of the stage of meiotic maturation. Independent divergence to the poles in anaphase of the reduction division of chromosomes of paternal and maternal origin

of different bivalents represents, along with crossing over, another efficient mechanism genotypic combinative variability. In this case, recombination of entire linkage groups occurs, with a set of alleles already changed in comparison with the parent chromosomes due to the crossing over.

Due to the characteristics of anaphase, as a result telophases The first division of meiosis produces haploid cells. However, chromosomes in such cells are represented by two chromatids, i.e. contain two DNA bi-helices - p2s.

The second (equational) division of the stage of meiosis maturation passes without DNA replication and gives rise to cells with a haploid set of chromosomes (individual chromatids diverge to the poles), each of which contains one DNA double helix - nc.

The peculiarity of the maturation stage of ovo(oo)genesis in comparison with the stage of spermatogenesis of the same name is the asymmetric nature of both meiotic divisions. As a result, in ovo(oo)genesis, from one ovo(oo)cyt of the first order, one functionally complete egg and three so-called reduction or polar bodies are formed (one due to the asymmetric division of the egg and two due to the symmetric division of the reduction body that arose during the first division stage of maturation). These are small cells that die (but: see paragraph 6.2). Upon completion of the first meiotic division and separation of the first polar body, the cell that will give rise to a mature egg acquires the name ovo(oo)cyt II order(secondary oocyte).

The asymmetry of divisions contributes to the preservation in one female gamete of the entire supply of nutrients and other substances necessary for the development of a new organism.

Upon completion of the maturation stage of spermatogenesis, four cells are formed, each of which will produce a full-fledged sperm - ps.

The maturation stage of spermatogenesis ends with the formation of cells called spermatids. Spermatids to become functionally mature spermatozoa, pass formation stage. At this stage, the chromatin is compacted, the shape and size of the nucleus changes, and the apparatus for active cell movement is formed - flagellum, is formed acrosome(in representatives of some species), the mitochondrial apparatus of the cell is reconstructed, it loses some part of the cytoplasm.

Gametogenesis is a highly productive process. During sexual activity, a man produces about 500 billion sperm. At the 5th month of intrauterine development in the gonad of the female organ-

There are 6-7 million egg precursor cells. Back to top reproductive period(postnatal ontogenesis) there are approximately 100,000 first-order ovo(oo)cytes in the ovaries. From the moment of puberty female body Before the cessation of gametogenesis (menopause), 400-500 egg precursor cells mature in the ovaries, ready for fertilization. During the reproductive period of postnatal ontogenesis in the ovaries of a woman, under the influence of the luteinizing hormone of the pituitary gland, as a rule, one female gamete leaves the ovary every month (ovulation- rupture of a mature Graafian vesicle; the egg first enters the free abdominal cavity, and then in fallopian tube, where fertilization can occur) and, once fertilized, resumes meiosis.

Species that reproduce sexually are characterized by a typical structure life cycle, in which the alternation of haploid and diploid phases occurs (see paragraph 4.3.7.1 and Fig. 4.47).

It is known about living organisms that they breathe, feed, reproduce and die, this is their biological function. But why does all this happen? Due to the bricks - cells that also breathe, feed, die and reproduce. But how does this happen?

About the structure of cells

The house is made of bricks, blocks or logs. Likewise, an organism can be divided into elementary units - cells. The entire diversity of living beings consists of them; the difference lies only in their quantity and types. They make up muscles bone tissue, skin, everything internal organs- they differ so much in their purpose. But regardless of what functions a particular cell performs, they are all structured approximately the same. First of all, any “brick” has a shell and cytoplasm with organelles located in it. Some cells do not have a nucleus, they are called prokaryotic, but all more or less developed organisms consist of eukaryotes, which have a nucleus in which genetic information is stored.

Organelles located in the cytoplasm are diverse and interesting; they perform important functions. Cells of animal origin include the endoplasmic reticulum, ribosomes, mitochondria, Golgi complex, centrioles, lysosomes and motor elements. With their help, all the processes that ensure the functioning of the body take place.

Cell activity

As already mentioned, all living things eat, breathe, reproduce and die. This statement is true both for whole organisms, that is, people, animals, plants, etc., and for cells. This is surprising, but each “brick” has its own own life. Due to its organelles, it receives and processes nutrients, oxygen, removes everything unnecessary outside. The cytoplasm itself and the endoplasmic reticulum perform transport function Mitochondria are also responsible for respiration and providing energy. The Golgi complex is responsible for the accumulation and removal of cell waste products. Other organelles also participate in complex processes. And at a certain stage, it begins to divide, that is, the process of reproduction occurs. It is worth considering in more detail.

Cell division process

Reproduction is one of the stages of development of a living organism. The same applies to cells. At a certain stage in their life cycle, they enter a state where they are ready to reproduce. they simply divide in two, lengthening, and then forming a partition. This process is simple and almost completely studied using the example of rod-shaped bacteria.

Things are a little more complicated. They reproduce in three in different ways which are called amitosis, mitosis and meiosis. Each of these paths has its own characteristics, it is inherent a certain type cells. Amitosis

considered the simplest, it is also called direct binary fission. When it occurs, the DNA molecule doubles. However, a fission spindle is not formed, so this method is the most energy-efficient. Amitosis occurs in unicellular organisms, while tissues of multicellular organisms reproduce using other mechanisms. However, it is sometimes observed where mitotic activity is reduced, for example, in mature tissues.

Direct fission is sometimes distinguished as a type of mitosis, but some scientists consider it a separate mechanism. This process occurs quite rarely even in old cells. Next, meiosis and its phases, the process of mitosis, as well as the similarities and differences of these methods will be considered. Compared to simple division, they are more complex and perfect. This is especially true for reduction division, so the characteristics of the phases of meiosis will be the most detailed.

An important role in cell division is played by centrioles - special organelles, usually located next to the Golgi complex. Each such structure consists of 27 microtubules, grouped in groups of three. The entire structure is cylindrical in shape. Centrioles are directly involved in the formation of the cell division spindle during the process of indirect division, about which we'll talk further.

Mitosis

The lifespan of cells varies. Some live for a couple of days, and some can be classified as long-livers, since their complete change occurs very rarely. And almost all of these cells reproduce through mitosis. For most of them, an average of 10-24 hours passes between division periods. Mitosis itself takes a short period of time - in animals approximately 0.5-1

hour, and for plants about 2-3. This mechanism ensures the growth of the cell population and the reproduction of units identical in their genetic content. This is how the continuity of generations is maintained at the elementary level. In this case, the number of chromosomes remains unchanged. This mechanism is the most common type of reproduction of eukaryotic cells.

The significance of this type of division is great - this process helps tissues grow and regenerate, due to which the development of the entire organism occurs. In addition, it is mitosis that underlies asexual reproduction. And one more function is the movement of cells and the replacement of already obsolete ones. Therefore, it is incorrect to assume that because the stages of meiosis are more complex, its role is much higher. Both of these processes perform different functions and are important and irreplaceable in their own way.

Mitosis consists of several phases that differ in their morphological features. The state in which the cell is ready for indirect division is called interphase, and the process itself is divided into 5 more stages, which need to be considered in more detail.

Phases of mitosis

While in interphase, the cell prepares to divide: DNA and proteins are synthesized. This stage is divided into several more, during which the growth of the entire structure and doubling of chromosomes occurs. The cell remains in this state for up to 90% of its entire life cycle.

The remaining 10% is occupied by division itself, which is divided into 5 stages. During mitosis of plant cells, preprophase is also released, which is absent in all other cases. New structures are formed, the nucleus moves to the center. A preprophase ribbon is formed, marking the expected site of future division.

In all other cells, the process of mitosis proceeds as follows:

Table 1

After the division of genetic information is completed, cytokinesis or cytotomy occurs. This term refers to the formation of daughter cell bodies from the mother’s body. In this case, the organelles, as a rule, are divided in half, although exceptions are possible; a septum is formed. Cytokinesis is not separated into a separate phase; as a rule, it is considered within the framework of telophase.

So, the most interesting processes involve chromosomes, which carry genetic information. What are they and why are they so important?

About chromosomes

Even without the slightest idea about genetics, people knew that many qualities of the offspring depend on the parents. With the development of biology, it became obvious that information about a particular organism is stored in every cell, and part of it is transmitted to future generations.

At the end of the 19th century, chromosomes were discovered - structures consisting of a long

DNA molecules. This became possible with the improvement of microscopes, and even now they can only be seen during the division period. Most often, the discovery is attributed to the German scientist W. Fleming, who not only streamlined everything that had been studied before him, but also made his own contribution: he was one of the first to study cellular structure, meiosis and its phases, and also introduced the term “mitosis.” The very concept of “chromosome” was proposed a little later by another scientist - the German histologist G. Waldeyer.

The structure of chromosomes when they are clearly visible is quite simple - they are two chromatids connected in the middle by a centromere. It is a specific sequence of nucleotides and plays important role during the process of cell reproduction. Ultimately, the chromosome in appearance in prophase and metaphase, when it can be best seen, resembles the letter X.

In 1900, principles describing the transmission of hereditary characteristics were discovered. Then it became finally clear that chromosomes are exactly what genetic information is transmitted through. Subsequently, scientists conducted a number of experiments proving this. And then the subject of study was the influence that cell division has on them.

Meiosis

Unlike mitosis, this mechanism ultimately leads to the formation of two cells with a set of chromosomes that is 2 times less than the original one. Thus, the process of meiosis serves as a transition from the diploid phase to the haploid phase, and primarily

We are talking about the division of the nucleus, and secondly, the division of the entire cell. The restoration of the full set of chromosomes occurs as a result of further fusion of gametes. Due to the reduction in the number of chromosomes, this method is also defined as reduction cell division.

Meiosis and its phases were studied by such famous scientists as V. Fleming, E. Strasburger, V. I. Belyaev and others. The study of this process in cells of both plants and animals is still ongoing - it is so complex. Initially, this process was considered a variant of mitosis, but almost immediately after its discovery it was identified as a separate mechanism. The characteristics of meiosis and its theoretical significance were first sufficiently described by August Weissmann back in 1887. Since then, the study of the process of reduction division has greatly advanced, but the conclusions drawn have not yet been refuted.

Meiosis should not be confused with gametogenesis, although both processes are closely related. Both mechanisms are involved in the formation of germ cells, but there are a number of serious differences between them. Meiosis occurs in two stages of division, each of which consists of 4 main phases, with a short break between them. The duration of the entire process depends on the amount of DNA in the nucleus and the structure of the chromosomal organization. In general, it is much longer compared to mitosis.

By the way, one of the main reasons for the significant species diversity- namely meiosis. As a result of reduction division, the set of chromosomes is split in two, so that new combinations of genes appear, primarily potentially increasing the adaptability and adaptability of organisms, which ultimately receive certain sets of characteristics and qualities.

Phases of meiosis

As already mentioned, reduction cell division is conventionally divided into two stages. Each of these stages is divided into 4 more. And the first phase of meiosis - prophase I, in turn, is divided into 5 more separate stages. As the study of this process continues, others may be identified in the future. Now the following phases of meiosis are distinguished:

Table 2

So, it is obvious that the division phases of meiosis are much more complex than the process of mitosis. But, as already mentioned, this does not detract biological role indirect division, since they perform different functions.

By the way, meiosis and its phases are also observed in some protozoa. However, as a rule, it includes only one division. It is assumed that this one-stage form later developed into the modern two-stage form.

Differences and similarities between mitosis and meiosis

At first glance, it seems that the differences between these two processes are obvious, because these are completely different mechanisms. However, upon deeper analysis, it turns out that the differences between mitosis and meiosis are not so global; in the end, they lead to the formation of new cells.

First of all, it’s worth talking about what these mechanisms have in common. In fact, there are only two coincidences: in the same sequence of phases, and also in the fact that

DNA replication occurs before both types of division. Although, as for meiosis, this process is not completely completed before the start of prophase I, ending at one of the first substages. And although the sequence of phases is similar, in essence, the events occurring in them do not completely coincide. So the similarities between mitosis and meiosis are not that many.

There are much more differences. First of all, mitosis occurs in while meiosis is closely related to the formation of germ cells and sporogenesis. In the phases themselves, the processes do not completely coincide. For example, crossing over in mitosis occurs during interphase, and not always. In the second case, this process involves anaphase of meiosis. V indirect division is usually not carried out, which means it does not play any role in the evolutionary development of the organism and the maintenance of intraspecific diversity. The number of cells resulting from mitosis is two, and they are genetically identical to the mother and have a diploid set of chromosomes. During reduction division everything is different. The result of meiosis is 4 different from the maternal one. In addition, both mechanisms differ significantly in duration, and this is due not only to the difference in the number of division stages, but also to the duration of each stage. For example, the first prophase of meiosis lasts much longer, because at this time chromosome conjugation and crossing over occur. That is why it is further divided into several stages.

In general, the similarities between mitosis and meiosis are quite minor compared to their differences from each other. It is almost impossible to confuse these processes. Therefore, it is now somewhat surprising that reduction division was previously considered a type of mitosis.

Consequences of meiosis

As already mentioned, after the end of the reduction division process, instead of the mother cell with a diploid set of chromosomes, four haploid ones are formed. And if we talk about the differences between mitosis and meiosis, this is the most significant. Recovery required quantity, when it comes to germ cells, occurs after fertilization. Thus, with each new generation the number of chromosomes does not double.

In addition, gene recombination occurs during meiosis. During the process of reproduction, this leads to the maintenance of intraspecific diversity. So the fact that even siblings are sometimes very different from each other is precisely the result of meiosis.

By the way, the sterility of some hybrids in the animal world is also a problem of reduction division. The fact is that the chromosomes of parents belonging to different types, cannot enter into conjugation, which means that the process of formation of full-fledged viable germ cells is impossible. Thus, it is meiosis that underlies the evolutionary development of animals, plants and other organisms.