Mitotic cell division is carried out as a stereotype. plant cell mitosis

mitotic cell division

The puppy grows and increases in size due to the division of somatic cells, called mitosis. Mitosis is not direct division somatic cell when complex changes in its nucleus and cytoplasm. After fertilization (ovogamy) of the egg by the spermatozoon (fusion, or copulation of gametes), a zygote(oocyst) - a new organism, consisting of only one cell. The process of growth and development of a new organism begins from the moment of the first mitotic division of this cell (maternal), when two daughter (more precisely, sister) cells, completely similar to it, arise from it, and continues until death.

Fig.1 Cell structure

During mitosis, the following occurs:

1- doubling of the substance of chromosomes;

2- change physical condition and chemical organization of chromosomes;

3- divergence of sister chromosomes to the poles of the cell;

4- subsequent division of the cytoplasm and full recovery two nuclei in new cells.

in mitosis life cycle nuclear genes: doubling, distribution and functioning. The period between cell divisions is called interphase, during which active life processes and preparation for the next division take place in it. The whole cycle of changes occurring in the cell

from one division to another is called mitotic cycle. The latter consists of two main periods - interphase and mitosis itself.

As a result of mitosis, two cells with identical chromosomes are formed from one cell. Thus, mitosis ensures the continuity and constancy of the number and set, that is, the qualitative specificity of chromosomes in successive generations of dividing cells (see Fig. 2).

In interphase, the period between two successive cell divisions, replication (autoduplication, or self-doubling) of DNA occurs in the nucleus, and hence the number of chromosomes in the cell (the formation of sister chromatids held together by the centromere, that is, the body that performs the function of the mechanical center of the chromosome), as well as despiralization of the latter.

In metaphase, or the central phase of nuclear division, a chromosome consisting of two chromatids turns into two daughter chromosomes.

Rice. 2. Mitosis

1 - three pairs of chromosomes; 2 - replication of chromosomes with the formation of sister chromatids connected in the region of the centromere; 3 - at the poles of the nuclear membrane, centrosomes are visible with asterial rays going to the centromeric region of the chromatids, lined up along the equator in order to separate the centromeres of sister chromatids and separate them to different poles; 4 - despiralization of chromosomes, restoration of the nuclear membrane and the formation of a cell septum with the formation of two daughter cells identical to the mother with exactly the same number of chromosomes as in it

In anaphase, the division and divergence of daughter chromosomes to the poles of the cell occurs, that is, the restoration of their proper number. In telophase, the final stage of cell division, the chromosomes take on the same form as before the division began, and the amount of DNA in each daughter nucleus is halved compared to the previous stages. Thus, both daughter cells contain equal quantities cytoplasm and identical sets of chromosomes and are ready to undergo mitosis.

Not all somatic cells of the body are constantly dividing. During embryonic development there is a differentiation of organs and tissues that develop along their specific, genetically incorporated path. Therefore, some cells turn into brain cells, others into blood cells, etc. Moreover, some of them divide constantly, while others only at a certain stage of development or, if necessary, are responsible, for example, for regenerative

(restorative) processes.

Cell division is the central moment of reproduction.

In the process of division, two cells arise from one cell. A cell, based on the assimilation of organic and inorganic substances, creates its own kind with a characteristic structure and functions.

In cell division, two main points can be observed: nuclear division - mitosis and division of the cytoplasm - cytokinesis, or cytotomy. The main attention of geneticists is still riveted to mitosis, since, from the point of view of chromosome theory, the nucleus is considered the "organ" of heredity.

During mitosis, the following occurs:

doubling of the substance of the chromosomes;
changes in the physical state and chemical organization of chromosomes;
divergence of daughter, or rather sister, chromosomes to the poles of the cell;
the subsequent division of the cytoplasm and the complete restoration of two new nuclei in sister cells.

Thus, the entire life cycle of nuclear genes is laid down in mitosis: duplication, distribution, and functioning; as a result of the completion of the mitotic cycle, sister cells end up with an equal “heritage”.

When dividing, the cell nucleus goes through five successive stages: interphase, prophase, metaphase, anaphase and telophase; some cytologists distinguish another sixth stage - prometaphase.

Between two successive cell divisions, the nucleus is in the interphase stage. During this period, the nucleus, during fixation and coloring, has a mesh structure formed by dyeing thin threads, which in the next phase form into chromosomes. Although the interphase is called differently resting nucleus phase, on the body itself, metabolic processes in the nucleus during this period are performed with the greatest activity.

Prophase is the first stage in the preparation of the nucleus for division. in prophase mesh structure nucleus gradually turns into chromosome threads. From the earliest prophase, even in light microscope the dual nature of chromosomes can be observed. This suggests that in the nucleus, it is in the early or late interphase that the most important process of mitosis takes place - doubling, or reduplication, of chromosomes, in which each of the maternal chromosomes builds a similar one - a daughter one. As a result, each chromosome looks longitudinally doubled. However, these halves of chromosomes, which are called sister chromatids, do not diverge in prophase, as they are held together by one common area - the centromere; the centromeric region is divided later. In prophase, the chromosomes undergo a process of twisting along their axis, which leads to their shortening and thickening. It should be emphasized that in prophase each chromosome in the karyolymph is located randomly.

In animal cells, even in late telophase or very early interphase, doubling of the centriole occurs, after which, in prophase, the daughter centrioles begin to converge to the poles and the formation of the astrosphere and spindle, called the new apparatus. At the same time, the nucleoli dissolve. An essential sign of the end of prophase is the dissolution of the nuclear membrane, as a result of which the chromosomes are in the total mass of the cytoplasm and karyoplasm, which now form the myxoplasm. This ends the prophase; the cell enters metaphase.

AT recent times between prophase and metaphase, researchers began to distinguish an intermediate stage called prometaphase. Prometaphase is characterized by the dissolution and disappearance of the nuclear membrane and the movement of chromosomes towards the equatorial plane of the cell. But by this time, the formation of the achromatin spindle has not yet been completed.

Metaphase called the end stage of the arrangement of chromosomes at the equator of the spindle. The characteristic arrangement of chromosomes in the equatorial plane is called the equatorial, or metaphase, plate. The arrangement of chromosomes in relation to each other is random. In metaphase, the number and shape of chromosomes are well revealed, especially when considering the equatorial plate from the poles of cell division. The achromatin spindle is fully formed: the spindle filaments acquire a denser consistency than the rest of the cytoplasm and are attached to the centromeric region of the chromosome. The cytoplasm of the cell during this period has the lowest viscosity.

Anaphase called the next phase of mitosis, in which chromatids divide, which can now be called sister or daughter chromosomes, diverge towards the poles. In this case, first of all, the centromeric regions repel each other, and then the chromosomes themselves diverge towards the poles. It must be said that the divergence of chromosomes in anaphase begins at the same time - "as if on command" - and ends very quickly.

In telophase, the daughter chromosomes despiralize and lose their visible individuality. The shell of the nucleus and the nucleus itself are formed. The nucleus is reconstructed reverse order compared to the changes it underwent in prophase. In the end, the nucleoli (or nucleolus) are also restored, and in the amount in which they were present in the parent nuclei. The number of nucleoli is characteristic of each cell type.

At the same time, the symmetrical division of the cell body begins. The nuclei of the daughter cells enter the state of interphase.

The figure above shows a diagram of the cytokinesis of animal and plant cells. AT animal cage division occurs by ligation of the cytoplasm of the mother cell. In a plant cell, the formation of a cell septum occurs with areas of spindle plaques that form a septum in the plane of the equator, called a phragmoplast. This ends the mitotic cycle. Its duration seems to depend on the type of tissue, physiological state body, external factors (temperature, light regime) and lasts from 30 minutes to 3 hours. According to various authors, the speed of passage of individual phases is variable.

Both internal and external factors environments that affect the growth of the organism and its functional state affect the duration of cell division and its individual phases. Since the nucleus plays a huge role in the metabolic processes of the cell, it is natural to believe that the duration of the phases of mitosis can change in accordance with the functional state of the organ tissue. For example, it has been established that the mitotic activity of various tissues during rest and sleep in animals is significantly higher than during wakefulness. In a number of animals, the frequency of cell divisions decreases in the light, and increases in the dark. It is also assumed that hormones influence the mitotic activity of the cell.

The reasons that determine the readiness of the cell for division are still unclear. There are reasons to assume several such reasons:

doubling of the mass of cellular protoplasm, chromosomes and other organelles, due to which nuclear-plasma relations are violated; for division, a cell must reach a certain weight and volume characteristic of the cells of a given tissue;
duplication of chromosomes;
secretion by chromosomes and other cell organelles of special substances that stimulate cell division.

The mechanism of divergence of chromosomes to the poles in the anaphase of mitosis also remains unclear. An active role in this process is apparently played by spindle filaments, which are protein filaments organized and oriented by centrioles and centromeres.

The nature of mitosis, as we have already said, varies depending on the type and functional state fabrics. Cells of different tissues are characterized different types Mitosis. In the described type of mitosis, cell division occurs in an equal and symmetrical manner. As a result of symmetrical mitosis, sister cells are hereditarily equivalent in respect of both nuclear genes and cytoplasm. However, in addition to symmetrical, there are other types of mitosis, namely: asymmetric mitosis, mitosis with delayed cytokinesis, division of multinucleated cells (syncytia division), amitosis, endomitosis, endoreproduction and polythenia.

In the case of asymmetric mitosis, sister cells are unequal in size, amount of cytoplasm, and also in relation to their future fate. An example of this is the unequal size sister (daughter) cells of the grasshopper neuroblast, animal eggs during maturation and during spiral fragmentation; during the division of nuclei in pollen grains, one of the daughter cells can further divide, the other cannot, etc.

Mitosis with a delay in cytokinesis is characterized by the fact that the cell nucleus divides many times, and only then does the division of the cell body occur. As a result of this division, multinucleated cells like syncytium are formed. An example of this is the formation of endosperm cells and the formation of spores.

Amitosis called direct fission of the nucleus without the formation of fission figures. In this case, the division of the nucleus occurs by "lacing" it into two parts; sometimes several nuclei are formed from one nucleus at once (fragmentation). Amitosis is constantly found in the cells of a number of specialized and pathological tissues, for example, in cancerous tumors. It can be observed under the influence of various damaging agents (ionizing radiation and high temperature).

Endomitosis called such a process when a doubling of nuclear fission occurs. In this case, the chromosomes, as usual, are reproduced in the interphase, but their subsequent divergence occurs inside the nucleus with the preservation of the nuclear envelope and without the formation of an achromatin spindle. In some cases, although the shell of the nucleus dissolves, however, the divergence of chromosomes to the poles does not occur, as a result of which the number of chromosomes in the cell multiplies even by several tens of times. Endomitosis occurs in cells of various tissues of both plants and animals. So, for example, A. A. Prokofieva-Belgovskaya showed that by endomitosis in the cells of specialized tissues: in the cyclops hypodermis, fat body, peritoneal epithelium and other tissues of the filly (Stenobothrus) - the set of chromosomes can increase 10 times. This increase in the number of chromosomes is associated with functional features differentiated tissue.

With polythenia, the number of chromosome threads multiplies: after reduplication along the entire length, they do not diverge and remain adjacent to each other. In this case, the number of chromosome threads within one chromosome is multiplied, as a result, the diameter of the chromosomes increases markedly. The number of such thin threads in a polytene chromosome can reach 1000-2000. In this case, the so-called giant chromosomes are formed. With polythenia, all phases of the mitotic cycle fall out, except for the main one - the reproduction of the primary strands of the chromosome. The phenomenon of polythenia is observed in the cells of a number of differentiated tissues, for example, in tissue salivary glands Diptera, in the cells of some plants and protozoa.

Sometimes there is a duplication of one or more chromosomes without any transformation of the nucleus - this phenomenon is called endoreproduction.

So, all phases of cell mitosis that make up are mandatory only for a typical process.

some cases, especially in differentiated tissues, the mitotic cycle undergoes changes. The cells of such tissues have lost the ability to reproduce the whole organism, and the metabolic activity of their nucleus is adapted to the function of the socialized tissue.

Embryonic and meristem cells that have not lost the function of reproducing the whole organism and related to undifferentiated tissues retain full cycle mitosis, on which asexual and vegetative reproduction is based.

The cells of a multicellular organism are extremely diverse in their functions. According to their specialization, cells have different duration life. For example, nervous and muscle cells after finishing embryonic period development cease to divide and function throughout the life of the organism. Cells of other tissues bone marrow, epidermis, epithelium small intestine- in the process of performing their function, they quickly die and are replaced by new ones as a result of continuous cell reproduction.

Thus, the life cycle of cells in renewing tissues includes functionally vigorous activity and division period. Cell division underlies the development and growth of organisms, their reproduction, and also ensures the self-renewal of tissues throughout the life of the organism and the restoration of their integrity after damage.

The most widespread form of cell reproduction in living organisms is indirect division, or mitosis. Mitosis is characterized by complex transformations of the cell nucleus, accompanied by the formation of specific structures-chromosomes. Chromosomes are constantly present in the cell, but in the period between two divisions - interphase - they are in a despiralized state and therefore are not visible in a light microscope. In interphase, preparation for mitosis is carried out, consisting mainly in doubling (reduplication) of DNA. The totality of processes that occur during the preparation of a cell for division, as well as during mitosis itself, is called mitotic cycle. The figure shows that after the completion of division, the cell can enter the period of preparation for DNA synthesis, denoted by the symbol G1 . At this time, RNA and proteins are intensively synthesized in the cell, and the activity of enzymes involved in DNA synthesis increases. The cell then proceeds to synthesize DNA. The two helices of the old DNA molecule separate, and each becomes a template for the synthesis of new DNA strands. As a result, each of the two daughter molecules necessarily includes one old helix and one new one. The new molecule is absolutely identical to the old one. This is a deep biological meaning: in this way, in countless cell generations, the continuity of genetic information is preserved.

The duration of DNA synthesis in different cells is not the same and ranges from several minutes in bacteria to 6-12 hours in mammalian cells. After completion of DNA synthesis - phase S mitotic cycle - the cell does not immediately begin to divide. The period from the end of DNA synthesis to the beginning of mitosis is called the phase G2. During this period, the cell completes its preparation for mitosis: ATP accumulates, achromatin spindle proteins are synthesized, and centrioles double.

The process of proper mitotic cell division consists of four phases: prophase, metaphase, anaphase and telophase.

AT prophase the volume of the nucleus and the cell as a whole increases, the cell rounds, decreases or stops its functional activity(for example, amoeboid movement in protozoa and in leukocytes of higher animals). Specific cell structures (cilia, etc.) often disappear. The centrioles diverge in pairs towards the poles, the chromosomes spiralize and, as a result, thicken and become visible. Reading genetic information from DNA molecules becomes impossible: RNA synthesis stops, the nucleolus disappears. Between the poles of the cell, the threads of the spindle of division are stretched - an apparatus is formed that ensures the divergence of chromosomes to the poles of the cell. Throughout the prophase, the spiralization of chromosomes continues, which become thick and short. At the end of prophase, the nuclear membrane disintegrates and the chromosomes are scattered randomly in the cytoplasm.

AT metaphase spiralization of chromosomes reaches a maximum, and shortened chromosomes rush to the equator of the cell, located at an equal distance from the poles. An equatorial, or metaphase, plate is formed. At this stage of mitosis, the structure of chromosomes is clearly visible, it is easy to count them 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 in the centromere region.

All somatic cells of any organism contain a strictly defined number of chromosomes. In all organisms belonging to the same species, the number of chromosomes in the cells is the same: in house flies - 12, in Drosophila - 8, in corn - 20, in garden strawberries - 56, in river cancer - 116, in humans - 46, in chimpanzees , cockroach and pepper - 48. As can be seen, the number of chromosomes does not depend on the height of the organization and does not always indicate phylogenetic relationship. The number of chromosomes, therefore, does not serve as a species-specific feature. The set of features of the chromosome set (karyotype) - the shape, size and number of chromosomes - is characteristic of only one kind of plant or animal.

The number of chromosomes in somatic cells is always paired. This is due to the fact that in these cells there are two chromosomes of the same shape and size: one comes from the paternal, the other from the maternal organism. Chromosomes that are the same shape and size and carry the same genes are called homologous. The chromosome set of a somatic cell, in which each chromosome has a pair, is called double, or diploid set, and is denoted by 2n. The amount of DNA corresponding to the diploid set of chromosomes is designated as 2c. Only one of each pair of homologous chromosomes enters the germ cells, so the chromosome set of gametes is called single or haploid.

The study of the details of the structure of the chromosomes of the metaphase plate is very great importance for the diagnosis of human diseases caused by violations of the structure of chromosomes.

AT anaphase the viscosity of the cytoplasm decreases, the centromeres separate, and from that moment on, the chromatids become independent chromosomes. The spindle fibers attached to the centromeres pull the chromosomes to the poles of the cell, while the arms of the chromosomes passively follow the centromere. Thus, in the anaphase, the chromatids of the doubled chromosomes still in the interphase exactly diverge towards the poles of the cell. At this moment, there are two diploid sets of chromosomes (4n4c) in the cell.

In the final stage - telophase - chromosomes unwind, despiralize. The nuclear envelope is formed from the membrane structures of the cytoplasm. In animals, the cell is divided into two smaller ones by the formation of a constriction. In plants, the cytoplasmic membrane arises in the middle of the cell and extends to the periphery, dividing the cell in half. After the formation of the transverse cytoplasmic membrane plant cells have a cellulose wall. Thus, two daughter cells are formed from one cell, in which 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 formed as a result of mitosis contain the same set of chromosomes and the same genes. Therefore, mitosis is a method of cell division, which consists in the exact distribution of genetic material between daughter cells.

As a result of mitosis, both daughter cells receive a diploid set of chromosomes.

Mitosis is inhibited by high temperature, high doses ionizing radiation, the action of plant poisons. One of these poisons, colchicine, is used in cytogenetics: it can be used to stop mitosis at the stage of the metaphase plate, which makes it possible to count the number of chromosomes and give each of them an individual characteristic, i.e., to carry out karyotyping.

Table Mitotic cycle and mitosis ( T.L. Bogdanov. Biology. Tasks and exercises. Allowance for entering universities. M., 1991 )

		The process that takes place in the cell
Interphase (phase between cell divisions)	Presynthetic period	Protein synthesis. RNA is synthesized on uncoiled DNA molecules
	Synthetic period	DNA synthesis is the self-doubling of the DNA molecule. Construction of the second chromatid, into which the newly formed DNA molecule passes: two-chromatid chromosomes are obtained
	Postsynthetic period	Protein synthesis, energy storage, preparation for division
	Prophase (first phase of division)	Two-chromatid chromosomes spiralize, nucleoli dissolve, centrioles diverge, nuclear membrane dissolves, spindle fibers form
Phases of mitosis	Metaphase (phase of accumulation of chromosomes)	Spindle threads attach to the centromeres of chromosomes, two-chromatid chromosomes are concentrated at the equator of the cell
	Anaphase (phase of divergence of chromosomes)	Centromeres divide, single chromatid chromosomes are stretched by spindle threads to the poles of the cell
	Telophase (end phase of division)	Single-chromatid chromosomes are despiralized, the nucleolus is formed, the nuclear envelope is restored, a partition between cells begins to form at the equator, the fission spindle threads dissolve

Features of mitosis in plants and animals

Time from one to the next. It takes place in two successive stages - interphase and division itself. The duration of this process is different and depends on the type of cells.

Interphase is the period between two cell divisions, the time from the last division to cell death or loss of the ability to divide.

During this period, the cell grows and doubles its DNA, as well as mitochondria and plastids. In the interphase, other organic compounds. The synthesis process is most intense in the synthetic period of the interphase. At this time, nuclear chromatids double, energy is accumulated, which will be used during division. The number of cell organelles and centrioles also increases.

Interphase occupies almost 90% cell cycle. After it, mitosis takes place, which is the main method of cell division in eukaryotes (organisms whose cells contain a formed nucleus).

During mitosis, the chromosomes are compacted, and a special apparatus is also formed, which is responsible for uniform distribution hereditary information between cells that are formed as a result of this process.

It goes through several stages. The stages of mitosis are characterized individual characteristics and a certain duration.

Phases of mitosis

During mitotic cell division, the corresponding phases of mitosis pass: prophase, after it comes metaphase, anaphase, the final one is telophase.

The phases of mitosis are characterized by the following features:

Which biological significance mitosis process?

The phases of mitosis contribute to the accurate transmission of hereditary information to daughter cells, regardless of the number of divisions. At the same time, each of them receives 1 chromatid, which helps to maintain the constancy of the number of chromosomes in all cells that are formed as a result of division. It is mitosis that ensures the transfer of a stable set of genetic material.

Remember!

How, according to cell theory, there is an increase in the number of cells?

Do you think that the lifespan of different types of cells in a multicellular organism is the same? Justify your opinion.

At the time of birth, a baby weighs an average of 3–3.5 kg and is about 50 cm tall, a brown bear cub whose parents reach a weight of 200 kg or more weighs no more than 500 g, and a tiny kangaroo weighs less than 1 gram. A beautiful swan grows from a gray nondescript chick, a nimble tadpole turns into a sedate toad, and a huge oak tree grows from an acorn planted near the house, which a hundred years later pleases new generations of people with its beauty. All these changes are possible due to the ability of organisms to grow and develop. The tree will not turn into a seed, the fish will not return to the eggs - the processes of growth and development are irreversible. These two properties of living matter are inextricably linked with each other, and they are based on the ability of the cell to divide and specialize.

The growth of ciliates or amoeba is an increase in size and complication of the structure of an individual cell due to biosynthesis processes. But the growth of a multicellular organism is not only an increase in the size of cells, but also their active division - an increase in the number. The growth rate, developmental features, the size to which a certain individual can grow - all this depends on many factors, including the influence of the environment. But the main, determining factor in all these processes is hereditary information, which is stored in the form of chromosomes in the nucleus of each cell. All cells of a multicellular organism originate from a single fertilized egg. In the process of growth, each newly formed cell must receive exact copy genetic material, so that, having a common hereditary program of the organism, to specialize and, performing its specific function, to be an integral part of the whole.

In connection with differentiation, i.e., division into different types, the cells of a multicellular organism have an unequal life span. For example, nerve cells stop dividing at the same time prenatal development, and during the life of the organism, their number can only decrease. Once having arisen, they no longer divide and live as long as the tissue or organ of which they are part, cells that form striated muscle tissues in animals and storage tissues in plants. Red bone marrow cells are constantly dividing to form blood cells, which have a limited lifespan. In the process of performing their functions, cells of the skin epithelium quickly die, therefore, in germinal zone epidermal cells divide very intensively. Cambial cells and growth cone cells in plants are actively dividing. The higher the specialization of cells, the lower their ability to reproduce.

There are about 10 14 cells in the human body. About 70 billion cells of the intestinal epithelium and 2 billion erythrocytes die every day. The shortest-lived cells are the intestinal epithelium, whose lifespan is only 1-2 days.

The life cycle of a cell. The period of life of a cell from the moment of its appearance in the process of division to death or the end of subsequent division called life cycle. The cell arises in the process of division of the mother cell and disappears during its own division or death. The life cycle length of different cells varies greatly and depends on cell type and conditions external environment(temperature, presence of oxygen and nutrients). For example, the life cycle of an amoeba is 36 hours, and bacteria can divide every 20 minutes.

The life cycle of any cell is a set of events occurring in the cell from the moment it arises as a result of division and until death or subsequent mitosis. The life cycle may include a mitotic cycle consisting of preparation for mitosis − interphase and the division itself, as well as the stage of specialization - differentiation, during which the cell performs its specific functions. The duration of interphase is always longer than the division itself. In the cells of the intestinal epithelium of rodents, the interphase lasts an average of 15 hours, and division takes place in 0.5–1 hour. During interphase, biosynthesis processes are actively going on in the cell, the cell grows, forms organelles and prepares for the next division. But certainly the most important process that occurs during interphase in preparation for division is DNA duplication (§).

Cell division. Mitosis" class="img-responsive img-thumbnail">

Rice. 52. Phases of mitosis

The two helices of the DNA molecule diverge and a new polynucleotide chain is synthesized on each of them. DNA replication occurs with highest precision, which is provided by the principle of complementarity. New DNA molecules are absolutely identical copies of the original, and after the duplication process is completed, they remain connected in the centromere region. DNA molecules that make up a chromosome after reduplication are called chromatids.

There is a deep biological meaning in the exactness of the reduplication process: a violation of copying would lead to a distortion of hereditary information and, as a result, to a disruption in the functioning of daughter cells and the whole organism as a whole.

If DNA duplication did not occur, then with each cell division, the number of chromosomes would be halved and pretty soon there would be no chromosomes left in each cell at all. However, we know that in all cells of the body of a multicellular organism, the number of chromosomes is the same and does not change from generation to generation. This constancy is achieved through mitotic cell division.

Mitosis. Mitosis- this is a division, during which there is a strictly identical distribution of exactly copied chromosomes between daughter cells, which ensures the formation of genetically identical - identical - cells.

The whole process of mitotic division is conditionally divided into four phases: prophase, metaphase, anaphase and telophase (Fig. 52).

AT prophase chromosomes begin to actively spiralize - twist and acquire a compact shape. As a result of such packaging, reading information from DNA becomes impossible and RNA synthesis stops. Chromosome spiralization is prerequisite successful separation of genetic material between daughter cells. Imagine some small room, the entire volume of which is filled with 46 threads, total length which are hundreds of thousands of times larger than the size of this room. This is the nucleus of the human cell. In the process of reduplication, each chromosome doubles, and we already have 92 entangled strands in the same volume. It is almost impossible to divide them equally without getting confused and without tearing. But wind these threads into balls, and you can easily distribute them into two equal groups - 46 balls in each. Something similar happens during mitotic division.

By the end of prophase, the nuclear membrane breaks up, and the spindle fibers are stretched between the poles of the cell - an apparatus that ensures an even distribution of chromosomes.

AT metaphase spiralization of chromosomes becomes maximum, and compact chromosomes are located in the equatorial plane of the cell. At this stage, it is clearly seen that each chromosome consists of two sister chromatids connected at the centromere. The spindle fibers are attached to the centromere.

Anaphase flows very quickly. The centromeres split in two, and from that moment the sister chromatids become independent chromosomes. The spindle fibers attached to the centromeres pull the chromosomes to the poles of the cell.

On the stage telophase daughter chromosomes, gathered at the poles of the cell, unwind and stretch. They again turn into chromatin and become poorly distinguishable in a light microscope. New nuclear membranes form around the chromosomes at both poles of the cell. Two nuclei are formed containing the same diploid sets of chromosomes.

Rice. 53. Significance of mitosis: A - growth (root tip); B - vegetative propagation (budding of yeast); B - regeneration (lizard tail)

Mitosis ends with the division of the cytoplasm. Simultaneously with the divergence of chromosomes, the organelles of the cell are approximately evenly distributed along the two poles. in animal cells cell membrane begins to bulge inward, and the cell divides by constriction. In plant cells, the membrane is formed inside the cell in the equatorial plane and, spreading to the periphery, divides the cell into two equal parts.

Meaning of mitosis. As a result of mitosis, two daughter cells arise, containing the same number of chromosomes as there were in the nucleus of the mother cell, i.e., cells identical to the parent cell are formed. AT normal conditions no changes in genetic information occur during mitosis, therefore mitotic division supports genetic stability cells. Mitosis underlies growth, development and vegetative reproduction multicellular organisms. Thanks to mitosis, the processes of regeneration and replacement of dying cells are carried out (Fig. 53). In unicellular eukaryotes, mitosis ensures asexual reproduction.

Review questions and assignments

1. What is the life cycle of a cell?

2. How does DNA duplication occur in the mitotic cycle? What is the meaning of this process?

3. What is the preparation of a cell for mitosis?

4. Describe sequentially the phases of mitosis.

5. What is the biological significance of mitosis?

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