Every day, changes occur in our body that are invisible to the human eye and consciousness: the cells of the body exchange substances with each other, synthesize proteins and fats, are destroyed, and new ones are created to replace them.

If a person accidentally cuts his hand while cooking, after a few days the wound will heal, and in its place only a whitish scar will remain; every few weeks our skin changes completely; after all, each of us was once one tiny cell and was formed by its repeated divisions.

The basis of all these most important processes, without which life itself would not be possible, is mitosis. It can be given a brief definition: mitosis (also called karyokinesis) is an indirect cell division that produces two cells that match the original one in genetic makeup.

Biological significance and role of mitosis

For mitosis, it is typical to copy information contained in the nucleus in the form of DNA molecules, and no changes are made to the genetic code, unlike meiosis, therefore, two daughter cells are formed from the mother cell, absolutely identical to it, having the same properties.

Thus, the biological meaning of mitosis lies in maintaining genetic immutability and constancy of cell properties.

Cells that have gone through mitotic division contain genetic information about the structure of the entire organism, so its development is quite possible from a single cell. This is the basis for vegetative propagation of plants: if you take a potato tuber or a leaf plucked from a violet and place it in suitable conditions, you will be able to grow a whole plant.

In agriculture, it is important to maintain constant yield, fertility, resistance to pests and environmental conditions, so it is understandable why the vegetative method of plant propagation is used whenever possible.

Also, with the help of mitosis, the process of regeneration occurs - the replacement of cells and tissues. When a part of the body is damaged or lost, cells begin to actively divide, replacing the lost ones.

Particularly impressive is the regeneration of the hydra, a small coelenterate animal that lives in fresh water.

The length of the hydra is several centimeters; at one end of the body it has a sole, with the help of which it attaches to the substrate, and at the other there are tentacles that serve to capture food.

If you cut the body into several parts, each of them will be able to restore the missing one, while maintaining proportions and shape.

Unfortunately, the more complex the organism is, the weaker its regeneration is, so more developed animals, including humans, may not even dream of such a thing.

Stages and scheme of mitosis

The entire life of a cell can be divided into six phases in the following sequence:

Click to enlarge

Moreover, the division process itself consists of the last five.

Briefly, mitosis can be described as follows: the cell creates and accumulates substances, DNA is doubled in the nucleus, chromosomes enter the cytoplasm, which is preceded by their spiralization, are placed at the equator of the cell and are pulled apart in the form of daughter chromosomes to the poles with the help of spindle threads.

After all the organelles of the mother cell are divided approximately in half, two daughter cells are formed. Their genetic makeup remains the same:

• 2n, if the original one was diploid;
• n, if the original one was haploid.

Worth noting: in the human body, all cells, excluding sex cells, contain a double set of chromosomes (they are called somatic), therefore mitosis occurs only in the diploid form.

Haploid mitosis is inherent in plant cells, in particular gametophytes, for example, a fern sprout in the form of a heart-shaped plate, and leafy plants in mosses.

The general scheme of mitosis can be depicted as follows:

Interphase

Mitosis itself is preceded by a long preparation (interphase), and that is why such division is called indirect.

During this phase, the actual life of the cell occurs. It synthesizes proteins, fats and ATP, stores them, grows, and increases the number of organelles for subsequent division.

Worth noting: Cells are in interphase for about 90% of their life.

It consists of three stages in the following order: presynthetic (or G1), synthetic (S) and postsynthetic (G2).

During the presynthetic period, the main growth of the cell and the accumulation of energy in ATP for future division occurs; the chromosome set is 2n2c (where n is the number of chromosomes, and c is the number of DNA molecules). The most important event of the synthetic period was the doubling (or replication, or reduplication) of DNA.

This happens as follows: the bonds between corresponding nitrogenous bases (adenine - thymine and guanine - cytosine) are broken with the help of a special enzyme, and then each of the single chains is completed to a double chain according to the rule of complementarity. This process is depicted in the following diagram:

Thus, the chromosome set becomes 2n4c, that is, pairs of two-chromatid chromosomes appear.

During the post-synthetic period of interphase, final preparation for mitotic division occurs: the number of organelles increases, and centrioles also double.

Prophase

The main process with which prophase begins is the spiralization (or twisting) of chromosomes. They become more compact, denser, and eventually they can be seen with the most ordinary microscope.

Then a division spindle is formed, consisting of two centrioles with microtubules located at different poles of the cell. The genetic set, despite the change in the shape of the material, remains the same - 2n4c.

Prometaphase

Prometaphase is a continuation of prophase. Its main event is the destruction of the nuclear membrane, as a result of which chromosomes enter the cytoplasm and are located in the zone of the former nucleus. They are then placed in a line in the equatorial plane of the spindle, at which point prometaphase is completed. The set of chromosomes does not change.

Metaphase

During metaphase, chromosomes are completely spiralized, which is why they are usually studied and counted during this phase.

Then microtubules “stretch” from the poles of the chromosomes located on the equator of the cell and join them, ready to be pulled apart in different directions.

Anaphase

After the ends of microtubules are attached to the chromosome from different sides, their simultaneous divergence occurs. Each chromosome “breaks” into two chromatids, and from that moment on they are called daughter chromosomes.

The spindle threads shorten and pull the daughter chromosomes to the cell poles, with the chromosome set totaling 4n4c, and at each pole – 2n2c.

Telophase

Telophase completes mitotic cell division. Despiralization occurs - the unwinding of chromosomes, bringing them into a form in which it is possible to read information from them. Nuclear membranes are re-formed, and the fission spindle is destroyed as unnecessary.

Telophase ends with the separation of the cytoplasm and organelles, the separation of daughter cells from each other, and the formation of cell membranes in each of them. Now these cells are completely independent, and each of them enters anew into the first phase of life - interphase.

Conclusion

Much attention is paid to this topic in biology; in school lessons, students should understand that with the help of mitosis, all eukaryotic organisms reproduce, grow, recover from damage, and not a single cell renewal or regeneration can occur without it.

What is important is that mitosis ensures the constancy of genes over a number of generations, and therefore the constancy of properties that underlie heredity.

Mitosis (karyokinesis, indirect division) is the process of division of the nucleus of human, animal and plant cells, followed by division of the cell cytoplasm. In the process of division of the cell nucleus (see), several stages are distinguished. In the nucleus, which is in the period between cell division (interphase), (see) are usually represented by thin, long (Fig., a), intertwined threads; The nuclear membrane and nucleolus are clearly visible.

The nucleus at different phases of mitosis: a - interphase non-dividing nucleus; b - d - prophase stage; d - metaphase stage; e - anaphase stage; g and h - telophase stage; and - the formation of two daughter nuclei.

In the first stage of mitosis, the so-called prophase, chromosomes become clearly visible (Fig., b-d), they shorten and thicken, a gap appears along each chromosome, dividing it into two parts completely similar to each other, due to which each chromosome appears double . In the next stage of mitosis - metaphase, the nuclear membrane is destroyed, the nucleolus dissolves and the chromosomes find themselves lying in the cytoplasm of the cell (Fig., e). All chromosomes are arranged in one row along the equator, forming the so-called equatorial plate (star stage). The centrosome also undergoes changes. It is divided into two parts, diverging towards the poles of the cell; threads are formed between them, forming a biconical achromatin spindle (Fig. e. f).

Mitosis (from the Greek mitos - thread) is an indirect cell division, consisting in the uniform distribution of the double number of chromosomes between the two resulting daughter cells (Fig.). The process of mitosis involves two types of structures: chromosomes and the achromatin apparatus, which includes cell centers and a spindle (see Cell).

Schematic representation of the interphase nucleus and the various stages of mitosis: 1 - interphase; 2 - prophase; 3 - prometaphase; 4 and 5 - metaphase (4 - view from the equator, 5 - view from the cell pole); 6 - anaphase; 7 - telophase; 8 - late telophase, beginning of nuclear reconstruction; 9 - daughter cells at the beginning of interphase; NW - nuclear envelope; YAK - nucleolus; XP - chromosomes; C - centriole; B - spindle.

The first stage of mitosis - prophase - begins with the appearance in the cell nucleus of thin filaments - chromosomes (see). Each prophase chromosome consists of two chromatids, closely adjacent to each other in length; one of them is the chromosome of the mother cell, the other is newly formed due to the reduplication of its DNA onto the DNA of the mother chromosome in interphase (a pause between two mitoses). As prophase progresses, chromosomes spiral, causing them to shorten and thicken. By the end of prophase, the nucleolus disappears. In prophase, the development of the achromatin apparatus also occurs. In animal cells, cell centers (centrioles) bifurcate; around them, zones appear in the cytoplasm that strongly refract light (centrospheres). These formations begin to diverge in opposite directions, forming by the end of prophase two poles of the cell, which by this time often acquires a spherical shape. In the cells of higher plants there are no centrioles.

Prometaphase is characterized by the disappearance of the nuclear membrane and the formation in the cell of a fusiform filamentous structure (achromatin spindle), some of the threads of which connect the poles of the achromatin apparatus (interzonal threads), and others - each of the two chromatids with opposite poles of the cell (pulling threads). Chromosomes, lying randomly in the prophase nucleus, begin to move to the central zone of the cell, where they are located in the equatorial plane of the spindle (metakinesis). This stage is called metaphase.

During anaphase, the partners of each pair of chromatids diverge to opposite poles of the cell due to the contraction of the pulling spindle threads. From this time on, each chromatid receives the name of a daughter chromosome. Chromosomes that have diverged to the poles gather into compact groups, which is characteristic of the next stage of mitosis - telophase. In this case, the chromosomes begin to gradually despiral, losing their dense structure; a nuclear envelope appears around them - the process of nuclear reconstruction begins. The volume of new nuclei increases, and nucleoli appear in them (the beginning of interphase, or the stage of the “resting nucleus”).

The process of separation of the nuclear substance of a cell - karyokinesis - is accompanied by the separation of the cytoplasm (see) - cytokinesis. In animal cells in telophase, a constriction appears in the equatorial zone, which, as it deepens, leads to the division of the cytoplasm of the original cell into two parts. In plant cells, in the equatorial plane, a cell septum is formed from small vacuoles of the endoplasmic reticulum, separating two new cell bodies from each other.

In principle, close to mitosis is endomitosis, i.e., the process of doubling the number of chromosomes in cells, but without separating the nuclei. Following endomitosis, direct division of nuclei and cells, so-called amitosis, can occur.

See also Karyotype, Cell nucleus.

What are mitosis and meiosis and what phases do they have? cells with some differences. During meiosis, four daughter nuclei are formed from the mother nucleus, in which the number of chromosomes is reduced by half. Mitosis also occurs, but in this type only two daughter cells are formed with the same chromosomes as the parents.

So is meiosis? These are biological division procedures that produce cells with specific chromosomes. Reproduction by mitosis occurs in multicellular, complex living organisms.

Stages

Mitosis occurs in two stages:

1. Doubling information at the gene level. Here, mother cells distribute genetic information among themselves. At this stage, chromosomes change.
2. Mitotic stage. It consists of time periods.

Cell formation occurs in several stages.

Phases

Mitosis is divided into several phases:

• telophase;
• anaphase;
• metaphase;
• prophase.

These phases occur in a certain sequence and have their own characteristics.

In any complex multicellular organism, mitosis most often involves cell division according to an undifferentiated type. During mitosis, the mother cell divides into daughter cells, usually two. One of them becomes a stem and continues division, and the second stops dividing.

Interphase

Interphase is the cell's preparation for division. Typically this stage lasts up to twenty hours. At this time, many different processes take place, during which cells prepare for mitosis.

During this period, protein division occurs and the number of organelles in the DNA structure increases. By the end of division, the genetic molecules double, but the number of chromosomes does not change. Identical DNAs are spliced ​​and are two chromatids in one molecule. The resulting chromatids are identical and sister.

After the completion of interphase, mitosis proper begins. It consists of prophase, metaphase, anaphase and telophase.

Prophase

The first phase of mitosis is prophase. It lasts about an hour. It is conventionally divided into several stages. At the initial stage in the prophase of mitosis, the nucleolus enlarges, as a result of which molecules are formed. By the end of the phase, each chromosome already consists of two chromatids. The nucleoli and nuclear membranes dissolve, all the elements in the cell are in disarray. Further, in the prophase of mitosis, achromatin division is formed, some of the threads pass through the entire cell, and some are connected to the central elements. During this process, the content of the genetic code remains unchanged.

The number of chromosomes does not change in prophase of mitosis. What else happens? In prophase of mitosis, the nuclear membrane disintegrates, resulting in spiral chromosomes ending up in the cytoplasm. Particles of the disintegrated nuclear membrane form small membrane vesicles.

In the prophase of mitosis, the following happens: the animal cell becomes round, but in plants it does not change shape.

Metaphase

After prophase comes metaphase. In this phase, chromosome spiralization reaches its peak. The shortened chromosomes begin to move towards the center of the cell. During movement, they are located equally in both parts. Here the metaphase plate is formed. When examining a cell, chromosomes are clearly visible. It is during metaphase that they are easy to count.

After the formation of the metaphase plate, the set of chromosomes inherent in this cell type is analyzed. This occurs by blocking chromosome segregation using alkaloids.

Each organism has its own set of chromosomes. For example, corn has 20, and garden strawberries have 56. The human body has fewer chromosomes than berries, only 46.

Anaphase

All processes occurring in prophase of mitosis end and anaphase begins. During this process, all chromosomal connections are broken and begin to move in opposite directions from each other. In anaphase, related chromosomes become independent. They end up in different cells.

The phase ends with the divergence of chromatids to the poles of the cell. Also here the distribution of hereditary information between daughter and mother cells occurs.

Telophase

Chromosomes are located at the poles. Under a microscope, they become difficult to see, as a nuclear shell forms around them. The fission spindle is completely destroyed.

In plants, the membrane forms in the center of the cell, gradually spreading to the poles. It divides the mother cell into two parts. Once the membrane has fully grown, a cellulose wall appears.

Features of mitosis

Cell division can be inhibited due to high temperatures, exposure to poisons, and radiation. When studying cell mitosis in various multicellular organisms, poisons can be used that inhibit mitosis at the metaphase stage. This allows you to study chromosomes in detail and carry out karyotoping.

Mitosis in the table

Consider the phases of cell division in the table below.

The process of the stages of mitosis can also be traced in the table.

Mitosis in animals and plants

The features of this process can be described in a comparative table.

So, we examined the process of cell division in animal organisms and plants, as well as their features and differences.

Mitosis- the main method of division of eukaryotic cells, in which the doubling first occurs, and then the hereditary material is evenly distributed between 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 a brief description of the 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 generations of cells. It underlies processes such as growth, regeneration, asexual reproduction, etc.

is a special method of dividing 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 of the 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. An essential component of 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 chooses its future fate: death or return to the mitotic cycle.

Go to lectures No. 12"Photosynthesis. Chemosynthesis"

Go to lectures No. 14"Reproduction of Organisms"

The G1 phase is characterized by the resumption of intensive biosynthesis processes, which during mitosis sharply slows down, and for a short time of cytokinesis, it stops altogether. The total protein content increases continuously during this phase. For most cells, there is a critical point in the G1 phase, the so-called restriction point. During its passage, internal changes occur in the cell, after which the cell must go through all subsequent phases of the cell cycle. The boundary between the S and G2 phases is determined by the appearance of a substance - an S-phase activator.

The G2 phase is considered as the period of preparation of the cell for the onset of mitosis. Its duration is shorter than other periods. In it, the synthesis of fission proteins (tubulin) occurs and phosphorylation of proteins involved in chromatin condensation is observed.

During prophase, two parallel processes occur. This is the gradual condensation of chromatin, the appearance of clearly visible chromosomes and disintegration of the nucleolus, as well as the formation of a spindle, which ensures the correct distribution of chromosomes between daughter cells. These two processes are spatially separated by the nuclear envelope, which persists throughout prophase and is destroyed only at its end. The center of microtubule organization in most animal and some plant cells is the cell center or centrosome. In an interphase cell it is located on the side of the nucleus. In the central part of the centrosome there are two centrioles, immersed in its material at right angles to each other. Numerous tubes formed by the protein tubulin extend from the peripheral part of the centrosome. They also exist in the interphase cell, forming a cytoskeleton in it. Microtubules are in a state of very rapid assembly and disassembly. They are unstable and their array is constantly updated. For example, in fibroblast cells in in vitro culture, the average lifetime of microtubules is less than 10 minutes. At the beginning of mitosis, microtubules of the cytoplasm disintegrate, and then their restoration begins. First, they appear in the near-nuclear zone, forming a radiant structure - a star. The center of its formation is the centrosome. Microtubules are polar structures because the tubulin molecules from which they are formed are oriented in a certain way. One end of it lengthens three times faster than the others. The fast growing ends are called plus ends, the slow growing ends are called minus ends. Plus the ends are oriented forward in the direction of growth. The centriole is a small cylindrical organelle about 0.2 µm thick and 0.4 µm long. Its wall is formed by nine groups of triplets of tubules. In a triplet, one tube is complete and the two adjacent to it are incomplete. Each triplet is inclined towards the central axis. Adjacent triplets are interconnected by cross-links. New centrioles arise only by duplicating existing ones. This process coincides with the time of DNA synthesis in the S phase. In the G1 period, the centrioles forming a pair move apart by several microns. Then, on each of the centrioles, in its middle part, a daughter centriole is built at a right angle. The growth of daughter centrioles is completed in the G2 phase, but they are still immersed in a single mass of centrosomal material. At the beginning of prophase, each pair of centrioles becomes part of a separate centrosome, from which a radial bundle of microtubules, a star, extends. The formed stars move away from each other on two sides of the nucleus, subsequently becoming the poles of the fission spindle.

Prometaphase begins with the rapid disintegration of the nuclear envelope into membrane fragments indistinguishable from EPS fragments. They are moved to the cell periphery by chromosomes and spindles. A protein complex is formed at the centromeres of chromosomes, which in electronic photographs looks like a lamellar three-layer structure - a kinetochore. Both chromatids carry one kinetochore; it is to this kinetochore that the protein microtubules of the spindle are attached. Using the methods of molecular genetics, it was found that the information determining the specific design of kinetochores is contained in the DNA nucleotide sequence in the centromere region. Spindle microtubules attached to chromosome kinetochores play a very important role; firstly, they orient each chromosome relative to the spindle so that its two kinetochores face opposite poles of the cell. Secondly, microtubules move chromosomes so that their centromeres are in the plane of the cell's equator. This process in mammalian cells takes from 10 to 20 minutes and is completed by the end of prometaphase. The number of microtubules associated with each kinetochore varies among species. In humans there are from 20 to 40, in yeast - 1. The plus ends of microtubules are associated with chromosomes. In addition to kinetochore microtubules, the spindle also contains polar microtubules, which extend from opposite poles and are stitched together at the equator by special proteins. Microtubules that extend from the centrosome and are not included in the spindle are called astral; they form a star.

Metaphase. Occupies a significant part of mitosis. It is easily recognized by two features: the bipolar structure of the spindle and the metaphase chromosome plate. This is a relatively stable state of the cell; many cells can be left in metaphase for several hours or days if they are treated with substances that depolymerize the spindle tubes. Once the agent is removed, the mitotic spindle is able to recover and the cell is able to complete mitosis.

Anaphase begins with the rapid synchronous splitting of all chromosomes into sister chromatids, each of which has its own kinetochore. The splitting of chromosomes into chromatids is associated with DNA replication in the centromere region. Replication of such a small area occurs in a few seconds. The signal for the onset of anaphase comes from the cytosol and is associated with a short-term rapid increase in the concentration of calcium ions by 10 times. Electron microscopy showed that membrane vesicles rich in calcium accumulate at the spindle poles. In response to the anaphase signal, sister chromatids begin to move towards the poles. This is associated first with the shortening of kinetochore tubes (anaphase A), and then with the moving apart of the poles themselves, associated with the elongation of polar microtubules (anaphase B). The processes are relatively independent, as indicated by their different sensitivity to poisons. In different organisms, the contribution of anaphase A and anaphase B to the final chromosome segregation is different. For example, in mammalian cells, anaphase B begins after anaphase A and ends when the spindle reaches a length 1.5-2 times greater than in metaphase. In protozoa, anaphase B predominates, causing the spindle to lengthen 15 times. Shortening of kinetochore tubes occurs through their depolymerization. Subunits are lost from the plus end, i.e. from the kinetochore side, as a result the kinetochore moves along with the chromosome to the pole. As for pole microtubules. Then in anaphase they assemble and elongate as the poles diverge. By the end of anaphase, the chromosomes are completely separated into two identical groups at the poles of the cell.

Nuclear and cytoplasmic divisions are related. The mitotic spindle plays an important role. In animal cells, already in anaphase, a cleavage furrow appears in the plane of the spindle equator. It is laid at right angles to the long axis of the mitotic spindle. The formation of the furrow is caused by the activity of the contractile ring, which is located under the cell membrane. It consists of the thinnest threads - actin filaments. The contractile ring has sufficient force to bend a thin glass needle inserted into the cell. As the groove deepens, the thickness of the contractile ring does not increase, since some of the filaments are lost as its radius decreases. After cytokinesis is completed, the contractile ring completely disintegrates, and the plasma membrane in the area of ​​the cleavage furrow contracts. For some time, a body of remnants of closely packed microtubules remains in the contact zone of newly formed cells. In plant cells that have a rigid cell wall, the cytoplasm is divided by the formation of a new wall at the border between daughter cells. Plant cells do not have a contractile ring. A phragmoplast is formed in the equatorial plane of the cell, gradually expanding from the center of the cell to its periphery until the growing cell plate reaches the plasma membrane of the mother cell. The membranes fuse, completely separating the resulting cells.