Mitosis phase during. Cell division

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

Mitosis

The process of indirect division, or mitosis, is most often found in nature. It is the basis for the division of all existing non-reproductive cells, namely muscle, nerve, epithelial and others.

Mitosis consists of four phases: prophase, metaphase, anaphase and telophase. The main role of this process is the uniform distribution of the genetic code from the parent cell to the two daughter cells. At the same time, the cells of the new generation are one to one similar to the maternal ones.

Rice. 1. Scheme of mitosis

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

• synthesis of protein and ATP molecules in the cell;
• chromosome duplication and formation of two sister chromatids;
• increase in the number of organelles in the cytoplasm.

Meiosis

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

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

Rice. 2. Meiosis scheme

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

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

Comparative characteristics

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

Phases	Mitosis	Meiosis
		First division	Second division
Interphase	The set of chromosomes of the mother cell is diploid. Protein, ATP and organic substances are synthesized. The chromosomes double and two chromatids are formed, connected by a centromere.	Diploid set of chromosomes. The same actions occur as during mitosis. The difference is the duration, especially during the formation of eggs.	Haploid set of chromosomes. There is no synthesis.
	Short phase. The nuclear membranes and nucleolus dissolve, and the spindle is formed.	Takes longer than mitosis. The nuclear envelope and nucleolus also disappear, and a fission spindle is formed. In addition, the process of conjugation (bringing together and merging homologous chromosomes) is observed. In this case, crossing over occurs - the exchange of genetic information in some areas. Afterwards the chromosomes separate.	The duration is a short phase. The processes are the same as in mitosis, only with haploid chromosomes.
Metaphase	Spiralization and arrangement of chromosomes in the equatorial part of the spindle are observed.	Similar to mitosis	The same as in mitosis, only with a haploid set.
	Centromeres are divided into two independent chromosomes, which diverge to different poles.	Centromere division does not occur. One chromosome, consisting of two chromatids, extends to the poles.	Similar to mitosis, only with a haploid set.
Telophase	The cytoplasm is divided into two identical daughter cells with a diploid set, and nuclear membranes with nucleoli are formed. The spindle disappears.	The duration of the phase is short. Homologous chromosomes are located in different cells with a haploid set. Cytoplasm does not divide in all cases.	The cytoplasm divides. Four haploid cells are formed.

Rice. 3. Comparative diagram of mitosis and meiosis

What have we learned?

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

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Mitosis (karyokinesis) is an indirect cell division in which the following phases are distinguished: prophase, metaphase, anaphase and telophase.

1. Prophase is characterized by:
1) chromonemata spiral, thicken and shorten.
2) the nucleoli disappear, i.e. The chromonema of the nucleolus is packed onto chromosomes that have a secondary constriction, which is called the nucleolar organizer.

3) two cell centers (centrioles) are formed in the cytoplasm and spindle filaments are formed.
4) at the end of prophase, the nuclear membrane disintegrates and the chromosomes end up in the cytoplasm. The set of prophase chromosomes is 2n4c.

2. Metaphase is characterized by:
1) the spindle threads are attached to the centromeres of the chromosomes and the chromosomes begin to move and line up at the equator of the cell.
2) metaphase is called the “passport of the cell”, because It is clearly visible that the chromosome consists of two chromatids. The chromosomes are maximally spiralized, the chromatids begin to repel each other, but are still connected at the centromere. At this stage, the karyotype of cells is studied, because the number and shape of chromosomes are clearly visible. The phase is very short.
The set of metaphase chromosomes is 2n4c.

3. Anaphase is characterized by:
1) the centromeres of chromosomes divide and sister chromatids move to the poles of the cell and become independent chromatids, which are called daughter chromosomes. At each pole in the cell there is a diploid set of chromosomes.
The set of anaphase chromosomes is 4n4c.

4. Telophase is characterized by:
Single-chromatid chromosomes despiral at the cell poles, nucleoli are formed, and the nuclear membrane is restored.
The set of telophase chromosomes is 2n2c.
Telophase ends with cytokinesis. Cytokinesis is the process of division of cytoplasm between two daughter cells. Cytokinesis occurs differently in plants and animals.
In an animal cell. A ring-shaped constriction appears at the equator of the cell, which deepens and completely laces the cell body. As a result, two new cells are formed that are half the size of the mother cell. There is a lot of actin in the constriction area, i.e. Microfilaments play a role in movement.
Cytokinesis proceeds by constriction.
In a plant cell. At the equator, in the center of the cell, as a result of the accumulation of vesicles of dictyosomes of the Golgi complex, a cell plate is formed, which grows from the center to the periphery and leads to the division of the mother cell into two cells. Subsequently, the septum thickens due to the deposition of cellulose, forming a cell wall. Cytokinesis proceeds through the septum.

Biological meaning of mitosis

As a result of mitosis, two daughter cells are formed with the same set of chromosomes as the mother cell.

Mitosis diagram

Accompanied by a halving of the number of chromosomes. It consists of two sequential divisions that have the same phases as mitosis. However, as shown in table “Comparison of mitosis and meiosis”, the duration of individual phases and the processes occurring in them differ significantly from the processes occurring during mitosis.

These differences are mainly as follows.

In meiosis prophase I longer lasting. What happens in it conjugation(connection of homologous chromosomes) and exchange of genetic information. In anaphase I centromeres, holding chromatids together, don't share, and one of the homologmeiosis of mitosis and egg chromosomes goes to the poles. Interphase before the second division very short, in it DNA is not synthesized. Cells ( halites), formed as a result of two meiotic divisions, contain a haploid (single) set of chromosomes. Diploidy is restored by the fusion of two cells - maternal and paternal. The fertilized egg is called zygote.

Mitosis and its phases

Mitosis, or indirect division, most widely distributed in nature. Mitosis underlies the division of all non-reproductive cells (epithelial, muscle, nerve, bone, etc.). Mitosis consists of four consecutive phases (see table below). Thanks to mitosis uniform distribution of the genetic information of the parent cell between the daughter cells is ensured. The period of cell life between two mitoses is called interphase. It is ten times longer than mitosis. A number of very important processes take place in it prior to cell division: ATP and protein molecules are synthesized, each chromosome doubles, forming two sister chromatids, held together by a common centromere, the number of main organelles of the cytoplasm increases.

In prophase spiral and as a result chromosomes thicken, consisting of two sister chromatids held together by a centromere. By the end of prophase the nuclear membrane and nucleoli disappear and the chromosomes are dispersed throughout the cell, the centrioles move to the poles and form spindle. In metaphase, further spiralization of chromosomes occurs. During this phase they are most clearly visible. Their centromeres are located along the equator. The spindle threads are attached to them.

In anaphase Centromeres divide, sister chromatids separate from each other and, due to the contraction of spindle filaments, move to opposite poles of the cell.

In telophase The cytoplasm divides, chromosomes unwind, and nucleoli and nuclear membranes are formed again. In animal cells the cytoplasm is laced, in plant- a septum is formed in the center of the mother cell. So from one original cell (mother) two new daughter cells are formed.

Table - Comparison of mitosis and meiosis

Phase	Mitosis	Meiosis
		1 division	2 division
Interphase	Chromosome set 2n. There is an intensive synthesis of proteins, ATP and other organic substances. The chromosomes double, each consisting of two sister chromatids held together by a common centromere.	Set of chromosomes 2n The same processes are observed as in mitosis, but longer, especially during the formation of eggs.	The set of chromosomes is haploid (n). There is no synthesis of organic substances.
Prophase	It does not last long, spiralization of chromosomes occurs, the nuclear membrane and nucleolus disappear, and a fission spindle is formed.	Longer lasting. At the beginning of the phase, the same processes occur as in mitosis. In addition, chromosome conjugation occurs, in which homologous chromosomes come together along their entire length and become twisted. In this case, an exchange of genetic information can occur (crossing of chromosomes) - crossing over. The chromosomes then separate.	Short; the same processes as in mitosis, but with n chromosomes.
Metaphase	Further spiralization of chromosomes occurs, their centromeres are located along the equator.	Processes similar to those in mitosis occur.
Anaphase	The centromeres holding sister chromatids together divide, each of them becomes a new chromosome and moves to opposite poles.	Centromeres do not divide. One of the homologous chromosomes, consisting of two chromatids held together by a common centromere, departs to opposite poles.	The same thing happens as in mitosis, but with n chromosomes.
Telophase	The cytoplasm divides, two daughter cells are formed, each with a diploid set of chromosomes. The spindle disappears and nucleoli form.	Does not last long. Homologous chromosomes end up in different cells with a haploid set of chromosomes. Cytoplasm does not always divide.	The cytoplasm divides. After two meiotic divisions, 4 cells with a haploid set of chromosomes are formed.

Comparison table between mitosis and meiosis.

Mitosis is conventionally divided into four phases: prophase, metaphase, anaphase and telophase.

Prophase. The two centrioles begin to diverge towards opposite poles of the nucleus. The nuclear membrane is destroyed; at the same time, special proteins combine to form microtubules in the form of threads. The centrioles, now located at opposite poles of the cell, have an organizing effect on the microtubules, which as a result line up radially, forming a structure reminiscent in appearance of an aster flower (“star”). Other filaments of microtubules extend from one centriole to another, forming a spindle. At this time, the chromosomes spiral and, as a result, thicken. They are clearly visible in a light microscope, especially after staining. Reading genetic information from DNA molecules becomes impossible: RNA synthesis stops and the nucleolus disappears. In prophase, the chromosomes split, but the chromatids still remain attached in pairs at the centromere. Centromeres also have an organizing effect on the spindle filaments, which now stretch from centriole to centromere and from it to another centriole.

Metaphase. In metaphase, chromosome spiralization reaches its maximum, and shortened chromosomes rush to the equator of the cell, located at an equal distance from the poles. Formed equatorial, or metaphase, plate. At this stage of mitosis, the structure of the chromosomes is clearly visible, they are easy to count and study their individual characteristics. Each chromosome has a region of primary constriction - the centromere, to which the spindle thread and arms are attached during mitosis. At the metaphase stage, the chromosome consists of two chromatids, connected to each other only at the centromere.

Rice. 1. Mitosis of a plant cell. A - interphase;
B, C, D, D- prophase; E, F-metaphase; 3, I - anaphase; K, L, M-telophase

IN anaphase the viscosity of the cytoplasm decreases, the centromeres are separated, and from this moment the chromatids become independent chromosomes. The spindle threads attached to the centromeres pull the chromosomes to the poles of the cell, while the chromosome arms passively follow the centromere. Thus, in anaphase, the chromatids of the chromosomes doubled in interphase precisely diverge to the poles of the cell. At this moment, the cell contains two diploid sets of chromosomes (4n4c).

Table 1. Mitotic cycle and mitosis

Phases	Process occurring in the cell
Interphase	Presynthetic period (G1)	Protein synthesis. RNA is synthesized on despiralized DNA molecules
Synthetic period (S)	DNA synthesis is the self-duplication of a DNA molecule. Construction of the second chromatid into which the newly formed DNA molecule passes: bichromatid chromosomes are obtained
Postsynthetic period (G2)	Protein synthesis, energy storage, preparation for division
Phases mitosis	Prophase	Bichromatid chromosomes spiral, nucleoli dissolve, centrioles separate, nuclear envelope dissolves, spindle filaments are formed
Metaphase	The spindle strands are attached to the centromeres of the chromosomes; bichromatid chromosomes are concentrated at the equator of the cell
Anaphase	Centromeres divide, single-chromatid chromosomes are stretched by spindle filaments to the cell poles
Telophase	Single-chromatid chromosomes despiral, a nucleolus is formed, the nuclear membrane is restored, a septum between cells begins to form at the equator, and spindle filaments dissolve

IN telophase chromosomes unwind and despiral. The nuclear envelope is formed from the membrane structures of the cytoplasm. At this time, the nucleolus is restored. This completes nuclear division (karyokinesis), then division of the cell body (or cytokinesis) occurs. When animal cells divide, a groove appears on their surface in the equatorial plane, gradually deepening and dividing the cell into two halves - daughter cells, each of which has a nucleus. In plants, division occurs through the formation of a so-called cell plate that separates the cytoplasm: it arises in the equatorial region of the spindle, and then grows in all directions, reaching the cell wall (i.e., grows from the inside out). The cell plate is formed from material supplied by the endoplasmic reticulum. Each of the daughter cells then forms a cell membrane on its side and finally cellulose cell walls are formed on both sides of the plate. Features of the course of mitosis in animals and plants are given in Table 2.

Table 2. Features of mitosis in plants and animals

This is how two daughter cells are formed from one cell, in which the hereditary information exactly copies the information contained in the mother cell. Starting from the first mitotic division of a fertilized egg (zygote), all daughter cells resulting from mitosis contain the same set of chromosomes and the same genes. Therefore, mitosis is a method of cell division that involves the precise distribution of genetic material between daughter cells. As a result of mitosis, both daughter cells receive a diploid set of chromosomes.

The entire process of mitosis takes in most cases from 1 to 2 hours. The frequency of mitosis varies between tissues and species. For example, in human red bone marrow, where 10 million red blood cells are formed every second, 10 million mitoses should occur every second. And in nervous tissue, mitoses are extremely rare: for example, in the central nervous system, cells generally stop dividing in the first months after birth; and in the red bone marrow, in the epithelial lining of the digestive tract and in the epithelium of the renal tubules, they divide until the end of life.

Regulation of mitosis, the question of the trigger mechanism of mitosis.

The factors that induce a cell to undergo mitosis are not precisely known. But it is believed that the factor of the ratio of the volumes of the nucleus and cytoplasm (nuclear-plasma ratio) plays a major role. According to some data, dying cells produce substances that can stimulate cell division. The protein factors responsible for the transition to the M phase were initially identified based on cell fusion experiments. The fusion of a cell located at any stage of the cell cycle with a cell located in the M phase leads to the entry of the nucleus of the first cell into the M phase. This means that in a cell in the M phase there is a cytoplasmic factor capable of activating the M phase. Later, this factor was secondarily discovered in experiments on the transfer of cytoplasm between frog oocytes at different stages of development, and was called the “maturation promoting factor” MPF (maturation promoting factor). Further study of MPF showed that this protein complex determines all M-phase events. The figure shows that nuclear membrane breakdown, chromosome condensation, spindle assembly, and cytokinesis are regulated by MPF.

Mitosis is inhibited by high temperature, high doses of ionizing radiation, and the action of plant poisons. One such poison is called colchicine. With its help, you can stop mitosis at the stage of the metaphase plate, which allows you to count the number of chromosomes and give each of them an individual characteristic, i.e., carry out karyotyping.

Amitosis (from the Greek a - negative particle and mitosis)-direct division of the interphase nucleus by ligation without transformation of chromosomes. During amitosis, uniform divergence of chromatids to the poles does not occur. And this division does not ensure the formation of genetically equivalent nuclei and cells. Compared to mitosis, amitosis is a shorter and more economical process. Amitotic division can occur in several ways. The most common type of amitosis is the lacing of the nucleus into two parts. This process begins with the division of the nucleolus. The constriction deepens and the core splits in two. After this, the separation of the cytoplasm begins, but this does not always happen. If amitosis is limited only to nuclear division, then this leads to the formation of bi- and multinucleated cells. During amitosis, budding and fragmentation of nuclei can also occur.

A cell that has undergone amitosis is subsequently unable to enter the normal mitotic cycle.

Amitosis occurs in cells of various tissues of plants and animals. In plants, amitotic division occurs quite often in the endosperm, in specialized root cells and in storage tissue cells. Amitosis is also observed in highly specialized cells with weakened viability or degenerating, during various pathological processes such as malignant growth, inflammation, etc.

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 filaments 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.

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.

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