What happens during mitosis. Mitosis, cell cycle

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

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

In the first stage of mitosis, the so-called prophase, the chromosomes become clearly visible (Fig., b-d), they shorten and thicken, a gap appears along each chromosome, dividing it into two parts that are completely similar to each other, due to which each chromosome is double . In the next stage of mitosis - metaphase, the nuclear envelope is destroyed, the nucleolus dissolves and the chromosomes are found to be 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, filaments are formed between them, forming a two-conical achromatin spindle (Fig., e. f).

Mitosis (from the Greek mitos - thread) is an indirect cell division, which consists in the uniform distribution of a doubled number of chromosomes between two resulting daughter cells (Fig.). Two types of structures are involved in the process of mitosis: chromosomes and the achromatic apparatus, which includes cell centers and a spindle (see Cell).

Schematic representation of the interphase nucleus and various stages of mitosis: 1 - interphase; 2 - prophase; 3 - prometaphase; 4 and 5 - metaphase (4 - view from the equator, 5 - view from the pole of the cell); 6 - anaphase; 7 - telophase; 8 - late telophase, the beginning of the reconstruction of the nuclei; 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 threads - 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 on the DNA of the maternal chromosome in interphase (a pause between two mitoses). As the prophase progresses, chromosomes spiralize, as a result of which they shorten and thicken. The nucleolus disappears towards the end of prophase. In prophase, the development of the achromatin apparatus also occurs. In animal cells, cell centers (centrioles) bifurcate; around them in the cytoplasm there are zones that strongly refract light (centrospheres). These formations begin to diverge in opposite directions, forming two poles of the cell by the end of prophase, which by this time often acquires a spherical shape. Centrioles are absent in the cells of higher plants.

Prometaphase is characterized by the disappearance of the nuclear envelope and the formation of a spindle-shaped filamentous structure (achromatin spindle) in the cell, some of the threads of which connect the poles of the achromatic 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 separate to opposite poles of the cell due to the contraction of the pulling threads of the spindle. From that time on, each chromatid is named as a daughter chromosome. The chromosomes that have diverged to the poles are assembled into compact groups, which is typical for the next stage of mitosis - telophase. In this case, the chromosomes begin to gradually despiralize, losing their dense structure; a nuclear shell appears around them - the process of reconstruction of the nuclei begins. There is an increase in the volume of new nuclei, nucleoli appear in them (the beginning of interphase, or the stage of the “resting nucleus”).

The process of separation of the nuclear substance of the cell - karyokinesis - is accompanied by the division of the cytoplasm (see) - cytokinesis. Animal cells in the telophase in the region of the equatorial zone develop a constriction, which, deepening, 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, which separates 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, the so-called amitosis, can occur.

See also Karyotype, Nucleus.

Mitosis- indirect cell division, which consists of nuclear division (karyotomy) and cytoplasm (cytotomy).

Mitosis is divided into prophase (early and late stages), prometaphase, metaphase, anaphase and telophase. The division itself takes a relatively short period of time - about 30 minutes.

Mitosis, or indirect cell division, is a method of dividing a eukaryotic cell in which each of the two newly formed cells receives genetic material identical to the original cell, that is, it leads to the formation of two full-fledged cells with a diploid set of chromosomes and evenly distributed cytoplasmic material.

Prophase. The first stage of mitosis is prophase. In the early prophase, condensation of chromosomes begins (the stage of a dense and loose tangle), the nucleolus undergoes disintegration, and centrioles are polarized.

At the beginning of prophase, pairs of centrioles move to different poles of the cell. At the same time, thin filaments are formed, radially diverging from each pair of centrioles - microtubules. Microtubules formed from one cell center stretch towards microtubules that polymerize in another cell center. As a result, they are intertwined. The nuclear membrane breaks up into vesicles (karyolysis), and the contents of the nucleus merge with the contents of the cytoplasmic matrix. On the membranes of the vesicles formed as a result of the breakdown of the karyolemma, receptor complexes and lamins are preserved.

In the late stage of prophase, condensation of chromosomes continues. They thicken and are clearly visible under light microscopy. Each chromosome consists of two sister chromatids connected by a centromere. At this stage, the mitotic spindle begins to form - a bipolar structure consisting of microtubules. It is organized by centrioles, which are part of the cell center, from which microtubules extend radially.

First, centrioles are located near the nuclear membrane, and then diverge, forming a bipolar mitotic spindle. This process involves polar microtubules interacting with each other as they elongate. The nucleus and nucleolus cease to exist as separate units. The cell becomes more elongated. During prophase, chromosomes are first seen as double threadlike structures. In the future, they acquire a rod-shaped form.

In the prophase of mitosis, the EPS and the Golgi complex break up into vesicles. Such temporary destruction of organelles plays an essential role in the uniform distribution of cytoplasmic material.

prometaphase. This is a continuation of the late prophase. During prometaphase, kinetochores (centromeres) are formed that function as centers of organization of kinetochore microtubules. The departure of kinetochores from each chromosome in both directions and their interaction with the polar microtubules of the mitotic spindle is the reason for the movement of chromosomes.

metaphase. In this phase, the chromosomes are distributed around the equator and form a metaphase plate. If the metaphase plate falls in a tangent cut, then it is visible as a parent star. The degree of chromosome condensation reaches its maximum level. Each chromosome is held by a pair of kinetochores and associated kinetochore microtubules directed to opposite poles of the mitotic spindle.

The chromosome contains a DNA molecule and DNA-binding proteins. Chromatin in the chromosome forms numerous loops, contains many densely packed nucleosomes. In prophase and metaphase, mammalian chromosomes are either x- or y-shaped. The x chromosomes have a so-called primary constriction (centromere) that connects the arms of the chromosomes. The sections of the metaphyseal chromosome from the centromere to both ends are called the arms of the chromosome. The arms are double structures consisting of adjacent s-chromosomes. The primary constriction contains kinetochores.

If the arms of the chromosomes are equal, then such chromosomes are called metacentric. Chromosomes that have short and long arms are called acrocentric. Arms that are almost equal or not very different in size have submetacentric chromosomes.

In one of the poles of the chromosome arm, you can sometimes find a narrowed area - a secondary constriction. The distal area of the shoulder behind the secondary constriction is called the satellite. The secondary constriction contains the nucleolar organizer zone.

The centromeres of all d-chromosomes (with a double set of DNA) are located in the same plane - this is the equatorial plane of the cell. It crosses the cell at right angles to the longitudinal axis of the spindle. The centromere has a kinetochore, a small disc-shaped structure that lies on both sides of the centromeric region of the d chromosome. Kinetochores are so small that they can only be seen with an electron microscope. In the active state, kinetochores behave like centrioles, that is, they serve as centers for the organization of microtubules (kinetochore microtubules). Kinetochores show their activity only from the moment of destruction of the nuclear envelope and when interacting with tubulins.

Among the microtubules of the fission spindle, several types are distinguished: kinetochore, polar and astral.

Kinetochore microtubules attach one pole to the kinetochore of the chromosome, and the other to one of the diplosomes and pull apart the chromosomes. Polar microtubules are directed from centrioles (diplosomes) to the center of the spindle, where they mutually overlap with similar microtubules of the opposite diplosome.

Astral microtubules are directed from the diplosome to the cell surface. The last two types of microtubules serve for uniform distribution of cytoplasmic material and cytokinesis.

Anaphase. It begins with the divergence of the daughter chromosomes to the poles of the emerging cells. This occurs with the direct participation of microtubules and proceeds at a rate of about 1 µm/min.

Due to the divergence from each d-chromosome, two s-chromosomes are formed. As a result, each cell receives an identical diploid set of s chromosomes. As the chromosomes diverge towards the poles, the kinetochore microtubules shorten and the division spindle elongates. In addition to the disassembly of kinetochore microtubules, the process of divergence of the genetic material is provided by the elongation of polar microtubules and the functional activity of translocator proteins.

Conventionally, early and late anaphase are distinguished, depending on the degree of separation of the genetic material to opposite poles. In general, this is the shortest stage of mitosis in time.

Telophase. This is the final stage of mitosis. In telophase, the chromatids approach the poles, the uniform distribution of the cytoplasmic material of the cell continues, including extranuclear heredity; the nuclear membrane is formed, the nucleoli are again formed. Telophase is completed by cell cytokinesis with the division of one mother cell into two daughter cells.

In the early telophase, condensed s-chromosomes are located in opposite poles of the cell near the cell centers and do not yet change their orientation.

The processes of elongation of the dividing cell continue. The plasmalemma retracts between the two daughter nuclei in a plane perpendicular to the long axis of the fission spindle, and two new cells begin to contour.

In the late telophase, decondensation of chromosomes begins and nuclear envelopes are formed by fusion of vesicles from the previously disintegrated karyolemma, and nucleoli are formed. The fission furrow deepens, and a cytoplasmic bridge remains between the daughter cells, which is further separated by the cell membrane, which leads to the autonomy of the daughter cells.

The formation of a cell membrane that separates two new cells from each other occurs with the contraction of microfilaments in the region of the cytoplasmic bridge and due to the transport of vesicles that merge with each other.

After cytotomy (cell division), the vesicles merge in the cells, forming the EPS and the Golgi complex.

Mitosis and the mitotic cycle are not automatic phenomena - they are regulated by various factors. The most studied are cyclin-dependent kinases (protein kinases). These proteins are abbreviated as Cdk. These proteins are similar in all cells of animal organisms. These protein kinases phosphorylate proteins that control individual stages of the mitotic cycle, bind special proteins - cyclins. Only the complex of Cdk with cyclins controls the mitotic cycle.

Each stage of the mitotic cycle has its own cyclin, which triggers a complex of biological reactions of the cell. In the initial stage of the presynthetic period of interphase, the cell does not enter the Go period due to the complexes of Cdk4 and Cdk6 with cyclin D.

In the second half of the G 1 period, Cdk2 with cyclin E becomes the leading controlling complex. In the synthetic period, cyclin changes, but protein kinase remains. So, at the beginning of the S-period, the leading complex is diclin A-Cdk2, and then - cyclin B-Cdk2. In the C 2 period, it is not cyclin that changes, but protein kinase. As a result, the control complex is referred to as cyclin B-Cdk1. This last complex actually introduces the cell into mitosis and is called the mitosis-stimulating factor.

Cyclin B-Cdk1 is able to phosphorylate histone H1. This phosphorylated histone is involved in the folding (condensation) of the DNA strand. But this is not enough. In the prometaphase of mitosis, the mitosis-stimulating factor also phosphorylates a group of proteins, the complex of which is called condensin, and its formation is just triggered by phosphorylation. Under the action of histone H1 and condensin, the chromosomes fit into metaphase structures. This process requires the use of ATP.

In addition, under the influence of the mitosis-stimulating factor, phosphorylation of the lamins of the inner surface of the nuclear membrane occurs in prophase. As a result, A - and C-lamins go into a dissolved state. The structural integrity of the shell is broken, and it breaks up into a system of bubbles. This may also occur in the EPS with the Golgi complex.

Under the influence of a mitosis-stimulating factor, microtubule polymerization and blockade of myosin light chains occur in prophase, which prevents premature cell cytotomy.

Cell division is regulated by two groups of factors: mitogenic and antimitogenic, or kalons. Mitogenic factors are produced in tissues (tissue hormones) and activate cell division, while the cell population increases. Mitogenic include growth factors of fibroblasts, epidermis, platelets, transforming growth factors, etc.

Mitogenic factors induce cell division through the activation of tyrosine kinase. This stimulates the formation of a number of transcription factors, the so-called early and delayed response genes. A change in their activity stimulates the formation of cyclin-dependent kinases and cyclins. This, in turn, induces cells to divide.

The concentration of growth factors is relatively low, and as soon as the number of cells increases significantly, growth factors become insufficient, and cells stop dividing and begin to differentiate. Some authors believe that the mechanism of termination of division and the beginning of differentiation is controlled by special biologically active substances - kalons or other regulators. An example of such a regulator is iodinated thyroid hormones - triiodothyronine and tetraiodothyronine. These hormones activate the processes of cell differentiation and block division. Important in this regard is the effect of tetraiodothyronine on the differentiation of neurons, and therefore, with its deficiency, cretinism develops, accompanied by mental retardation (oligophrenia).

An example of an anti-mitogenic factor is tumor necrosis factor. It blocks the formation of a complex of mitogen-activating protein kinases through a number of intracellular mediators (sphingosine). Ultimately, the content of cyclin D complexes with Cdk6 and Cdk4 decreases, and cell division stops.

A variant of mitosis is fragmentation - this is cell division, when an increase in the mother cell does not occur during a short interphase. As a result, after each division, the cell size decreases. Cleavage is characteristic of the formation of a multicellular organism (blastula) from a unicellular embryo (zygote) in the early stages of embryonic development.

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The growth and development of living organisms is impossible without the processes of cell division. One of them is mitosis - the process of division of eukaryotic cells, in which genetic information is transmitted and stored. In this article, you will learn more about the features of the mitotic cycle, get acquainted with the characteristics of all phases of mitosis, which will be included in the table.

The concept of "mitotic cycle"

All processes that occur in a cell, from one division to another, and ending with the production of two daughter cells, is called the mitotic cycle. The life cycle of a cell is also a state of rest and a period of performance of its direct functions.

The main stages of mitosis are:

Self-duplication or reduplication of the genetic code, which is transmitted from the mother cell to two daughter cells. The process affects the structure and formation of chromosomes.
cell cycle- consists of four periods: presynthetic, synthetic, postsynthetic and, in fact, mitosis.

The first three periods (presynthetic, synthetic and postsynthetic) refer to the interphase of mitosis.

Some scientists call the synthetic and postsynthetic period the preprophase of mitosis. Since all stages occur continuously, smoothly passing from one to another, there is no clear separation between them.

The process of direct cell division, mitosis, occurs in four phases, corresponding to the following sequence:

Table "Phases of mitosis"

*No. p / p*	*Phase*	*Characteristic*
		In the prophase of mitosis, the nuclear membrane and nucleolus dissolve, the centrioles diverge to different poles, the formation of microtubules, the so-called spindle threads, begins, and the chromatids condense in the chromosomes.
	metaphase	At this stage, the chromatids in the chromosomes condense to the maximum and line up in the equatorial part of the spindle, forming a metaphase plate. Centriole filaments attach to chromatid centromeres or stretch between poles.
		It is the shortest phase during which the separation of chromatids occurs after the collapse of the centromeres of chromosomes. The couple diverges to different poles and begins an independent lifestyle.
	Telophase	It is the final stage of mitosis, in which the newly formed chromosomes acquire their normal size. A new nuclear envelope with a nucleolus inside is formed around them. The spindle threads disintegrate and disappear, the process of division of the cytoplasm and its organelles (cytotomy) begins.

The process of cytotomy in an animal cell occurs with the help of a fission furrow, and in a plant cell - with the help of a cell plate.

Atypical forms of mitosis

In nature, atypical forms of mitosis are sometimes found:

Amitosis - a method of direct nuclear division, in which the structure of the nucleus is preserved, the nucleolus does not disintegrate, and the chromosomes are not visible. The result is a binuclear cell.

Rice. 2. Amitosis

Politenia - DNA cells multiply, but without an increase in the content of chromosomes.
Endomitosis - during the process after DNA replication, there is no division of chromosomes into daughter chromatids. In this case, the number of chromosomes increases tenfold, polyploid cells appear, which can lead to mutations.

Rice. 3. Endomitosis

What have we learned?

The process of indirect division of eukaryotic cells takes place in several stages, each of which has its own characteristics. The mitotic cycle consists of the stages of interphase and direct cell division, which consists of four phases: prophase, metaphase, anaphase and telophase. Sometimes in nature there are atypical methods of division, these include amitosis, polythenia and endomitosis.

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Among all the interesting and rather complex topics in biology, it is worth highlighting two processes of cell division in the body - meiosis and mitosis. At first it may seem that these processes are the same, since in both cases cell division occurs, but in fact there is a big difference between them. First of all, you need to deal with mitosis. What is this process, what is the interphase of mitosis and what role do they play in the human body? More about this and will be discussed in this article.

The complex biological process that is accompanied by cell division and the distribution of chromosomes between these cells - all this can be said about mitosis. Thanks to him, chromosomes containing DNA are evenly distributed between the daughter cells of the body.

There are 4 main phases of the mitosis process. All of them are interconnected, since the phases smoothly pass from one to another. The prevalence of mitosis in nature is due to the fact that it is he who participates in the process of division of all cells, including muscle, nerve, and so on.

Briefly about interphase

Before entering the state of mitosis, the cell that divides goes into the period of interphase, that is, it grows. The duration of interphase can take more than 90% of the total time of cell activity in the normal mode..

Interphase is divided into 3 main periods:

phase G1;
S-phase;
phase G2.

All of them pass in a certain sequence. Let's consider each of these phases separately.

Interphase - main components (formula)

Phase G1

This period is characterized by the preparation of the cell for division. It increases in volume for the next phase of DNA synthesis.

S-phase

This is the next stage in the process of interphase, in which the cells of the body divide. As a rule, the synthesis of most cells occurs for a short period of time. After cell division, the cells do not increase in size, but the last phase begins.

Phase G2

The final stage of interphase, during which cells continue to synthesize proteins, while increasing in size. During this period, the cell still has nucleoli. Also in the last part of the interphase, duplication of chromosomes occurs, and the surface of the nucleus at this time is covered with a special shell that has a protective function.

On a note! At the end of the third phase, mitosis occurs. It also includes several stages, after which cell division occurs (this process in medicine is called cytokinesis).

Stages of mitosis

As noted earlier, mitosis is divided into 4 stages, but sometimes there may be more. Below are the main ones.

Table. Description of the main phases of mitosis.

Phase name, photo	Description
	During prophase, chromosomes spiralize, as a result of which they take a twisted shape (it is more compact). All synthetic processes in the cell of the body are stopped, so ribosomes are no longer produced.
	Many experts do not distinguish prometaphase as a separate phase of mitosis. Often, all the processes that occur in it are referred to as prophase. During this period, the cytoplasm envelops the chromosomes, which freely move around the cell up to a certain point.
	The next phase of mitosis, which is accompanied by the distribution of condensed chromosomes on the equatorial plane. During this period, microtubules are renewed on an ongoing basis. In metaphase, the chromosomes are arranged so that their kinetochores are in a different direction, that is, they are directed towards opposite poles.
	This phase of mitosis is accompanied by the separation of the chromatids of each of the chromosomes from each other. The growth of microtubules stops, they are now starting to disassemble. Anaphase does not last long, but during this period of time the cells have time to disperse closer to different poles in approximately equal numbers.
	This is the last stage during which chromosome decondensation begins. Eukaryotic cells complete their division, and a special shell is formed around each set of human chromosomes. When the contractile ring contracts, the cytoplasm separates (in medicine, this process is called cytotomy).

Important! The duration of the complete process of mitosis, as a rule, is no more than 1.5-2 hours. The duration may vary depending on the type of cell being divided. Also, the duration of the process is influenced by external factors, such as light conditions, temperature, and so on.

What biological role does mitosis play?

Now let's try to understand the features of mitosis and its importance in the biological cycle. First of all, it provides many vital processes of the organism, among which - embryonic development.

Mitosis is also responsible for the restoration of tissues and internal organs of the body after various types of damage, resulting in regeneration. In the process of functioning, cells gradually die off, but with the help of mitosis, the structural integrity of tissues is constantly maintained.

Mitosis ensures the preservation of a certain number of chromosomes (it corresponds to the number of chromosomes in the mother cell).

Video - Features and types of mitosis

Mitosis- the main method of division of eukaryotic cells, in which doubling first occurs, and then a uniform distribution of hereditary material between daughter cells.

Mitosis is a continuous process in which there are 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 make up 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- the number of chromosomes, With- the number of DNA molecules) - cell growth, activation of biological synthesis processes, preparation for the next period.

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

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

Prophase (2n 4c) - the dismantling of nuclear membranes, the divergence of centrioles to different poles of the cell, the formation of fission spindle threads, the "disappearance" of the nucleoli, the condensation of two-chromatid chromosomes.

metaphase (2n 4c) - alignment of the most condensed two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes.

Anaphase (4n 4c) - the 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, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy). Cytotomy in animal cells occurs due to the fission furrow, in plant cells - due to the cell plate.

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

The 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 in a number of cell generations. Underlies such processes as growth, regeneration, asexual reproduction, etc.

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

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

Interphase 1(at the beginning - 2 n 2c, at the end - 2 n 4c) - the synthesis and accumulation of substances and energy necessary for the implementation of both divisions, an increase in cell size and the 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 fission spindle filaments, "disappearance" of nucleoli, condensation of two-chromatid chromosomes, conjugation of homologous chromosomes and crossing over. Conjugation- the process of convergence and interlacing of homologous chromosomes. A pair of conjugating homologous chromosomes is called bivalent. Crossing over is the process of exchanging 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 human oogenesis), diakinesis(terminalization of chiasma).

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 the fission spindle threads 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 moves 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 two-chromatid chromosomes, the division of the cytoplasm. In many plants, a cell from anaphase 1 immediately transitions 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 fibers.

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

Anaphase 2 (2n 2With) - the 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, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy) with the formation of four haploid cells as a result.

The biological significance of meiosis. Meiosis is the central event of gametogenesis in animals and sporogenesis in plants. Being the basis of combinative variability, meiosis ensures the 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 to death cells. After amitosis, the cell is unable to return to the normal mitotic cycle.

cell cycle

cell cycle- the life of a cell from the moment of its appearance to division or death. An obligatory component of the cell cycle is the mitotic cycle, which includes a period of preparation for division and mitosis proper. In addition, there are periods of rest in the life cycle, during which the cell performs its own functions and chooses its further fate: death or return to the mitotic cycle.

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