Mitosis. Its essence, phases, biological significance. Amitosis. Amitosis, its mechanisms and biological significance What is cell amitosis

AMITOSIS (amitosis; Greek negative prefix a-, mitos - thread + -ōsis) direct nuclear fission - division of the cell nucleus into two or more parts without the formation of chromosomes and achromatic spindle; during amitosis, the nuclear membrane and nucleolus are preserved and the nucleus continues to function actively.

Direct nuclear fission was first described by Remak (R. Bemak, 1841); the term "amitosis" was proposed by Flemming (W. Flemming, 1882).

Usually amitosis begins with the division of the nucleolus, then the nucleus divides. Its division can proceed in different ways: either a partition appears in the nucleus - the so-called nuclear plate, or it is gradually laced, forming two or more daughter nuclei. With the help of cytophotometric research methods, it was found that in about 50% of cases of amitosis, DNA is evenly distributed between the daughter nuclei. In other cases, division ends with the appearance of two unequal nuclei (meroamitosis) or many small unequal nuclei (fragmentation and budding). Following the division of the nucleus, the division of the cytoplasm (cytotomy) occurs with the formation of daughter cells (Fig. 1); if the cytoplasm does not divide, one two- or multinuclear cell appears (Fig. 2).

Amitosis is characteristic of a number of highly differentiated and specialized tissues (vegetative ganglion neurons, cartilaginous, glandular cells, blood leukocytes, blood vessel endothelial cells, etc.), as well as for malignant tumor cells.

Benshshghoff (A. Benninghoff, 1922), based on the functional purpose, proposed to distinguish three types of amitosis: generative, reactive and degenerative.

Generative amitosis- this is a complete division of nuclei, after which mitosis becomes possible (see). Generative amitosis is observed in some protozoa, in polyploid nuclei (see Chromosomal set); in this case, a more or less ordered redistribution of the entire hereditary apparatus occurs (for example, the division of the macronucleus in ciliates).

A similar picture is observed in the division of some specialized cells (liver, epidermis, trophoblast, etc.), where amitosis is preceded by endomitosis - intranuclear doubling of the set of chromosomes (see Meiosis); the resulting endomitosis and the polyploid nuclei then undergo amitosis.

Reactive amitosis due to the influence on the cell of various damaging factors - radiation, chemicals, temperature, and more. It can be caused by metabolic disorders in the cell (during starvation, tissue denervation, etc.). This type of amitotic nuclear division, as a rule, does not end with cytotomy and leads to the appearance of multinucleated cells. Many researchers tend to consider reactive amitosis as an intracellular compensatory reaction that ensures the intensification of cell metabolism.

Degenerative amitosis- nuclear division associated with the processes of degradation or irreversible cell differentiation. With this form of amitosis, fragmentation, or budding, of nuclei occurs, which is not associated with DNA synthesis, which in some cases is a sign of incipient tissue necrobiosis.

The question of the biological significance of amitosis has not been finally resolved. However, there is no doubt that amitosis is a secondary phenomenon compared to mitosis.

Bibliography: Klishov A. A. Histogenesis, regeneration and tumor growth of skeletal muscle tissue, p. 19, L., 1971; Knorre A. G. Embryonic histogenesis, p. 22, L., 1971; Mikhailov V.P. Introduction to Cytology, p. 163, L., 1968; Guide to Cytology, ed. A. S. Troshina, vol. 2, p. 269, M. - L., 1966; Bucher O. Die Amitose der tierischen und menschlichen Zelle, Protoplasmalogia, Handb. Protoplasmaforsch., hrsg. v. L. V. Heilbrunn u. F. Weber, Bd 6, Wien, 1959, Bibliogr.

Yu. E. Ershikova.

Plan 2

1. Amitosis 3

1.1. The concept of amitosis 3

1.2. Features of amitotic division of the cell nucleus 4

1.3. Amitosis value 6

2. Endomitosis 7

2.1. The concept of endomitosis 7

2.2. Examples of Endomitosis 8

2.3. Significance of endomitosis 8

3. References 10

1.1. The concept of amitosis

Amitosis (from Greek a - negative particle and mitosis)- direct division of the interphase nucleus by ligation without transformation of chromosomes.

During amitosis, there is no uniform divergence of chromatids to the poles. 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 be carried out in several ways.

The most common type of amitosis is the ligation of the nucleus in two. This process begins with the division of the nucleolus. The constriction deepens, and the nucleus is divided in two.

After this, the division of the cytoplasm begins, but this does not always happen. If amitosis is limited only by nuclear division, then this leads to the formation of bi- and multinuclear cells. During amitosis, budding and fragmentation of nuclei can also occur.

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

Amitosis is found in the cells of various plant and animal tissues. In plants, amitotic division is quite common in the endosperm, in specialized root cells, and in cells of storage tissues.

Amitosis is also observed in highly specialized cells with impaired viability or degenerating, in various pathological processes such as malignant growth, inflammation, etc.

1.2. Features of amitotic division of the cell nucleus

It is known that the formation of polynuclear cells occurs due to four mechanisms: as a result of the fusion of mononuclear cells, in the case of blockade of cytokinesis, due to multipolar mitoses, and during amitotic nuclear division.

Unlike the first three, well-studied mechanisms, amitosis is rarely the object of study, and the amount of information on this issue is extremely limited.

Amitosis is important in the formation of multinucleated cells and is a step-by-step process during which nuclear stretching, invagination of the karyolemma, and nucleus constriction into parts occur sequentially.

Although the volume of reliable information on the molecular and subcellular mechanisms of amitosis is insufficient, there is information about the participation of the cell center in the implementation of this process. It is also known that if nuclei are segmented due to the action of microfilaments and microtubules, then the role of cytoskeletal elements in amitotic division is not excluded.

Direct fission, accompanied by the formation of nuclei that differ in volume, may indicate an unbalanced distribution of chromosomal material, which is refuted by data obtained in the course of studies conducted using light and electron microscopy methods. These contradictions may indicate the use of various methods of morphometric analysis and evaluation of the results obtained, which underlie certain conclusions.

Regeneration in pathological and physiological conditions is carried out by amitosis, which also occurs with an increase in the functional activity of the tissue, for example, amitosis is due to an increase in the number of binuclear cells that make up the glandular epithelium of the mammary glands during lactation. Therefore, to consider amitotic nuclear fission only as a sign of a pathological nature, it should be recognized as a one-sided approach to the study of this issue, and to reject the facts confirming the compensatory significance of this phenomenon.

Amitosis has been noted in cells of various origins, including cells of some tumors; therefore, its involvement in oncogenesis cannot be denied. An opinion is expressed about the presence of amitosis in intact cells cultivated in vitro, although it is possible to classify them as such only conditionally, since incubation itself is an influencing factor that changes the morphological and functional characteristics of cells extracted from the body.

The fundamental importance of amitosis in the implementation of intracellular processes is evidenced by the fact of its existence in many types of cells and under different conditions.

Since the role of amitotic division of polyploid nuclei in the formation of polynuclear cells is considered proven, in this case, the main meaning of amitosis is to establish optimal nuclear-cytoplasmic relationships that allow cells to adequately perform various functions.

The existence of amitosis in multinuclear cells of various origins and their formation due to several mechanisms, including due to amitotic nuclear division, has been shown.

Summarizing the presented information, we can conclude that amitosis, which results in the formation of polynuclear cells, has a staged nature and takes part in ensuring the adequate functioning of cells and tissues of the body under physiological and pathological conditions.

However, the amount of information about the features of the formation of multinuclear fibroblasts as a result of the amitotic division of their nuclei, depending on the influence of various factors, probably cannot be considered sufficient. At the same time, obtaining such data is necessary for understanding many aspects of the functioning and morphogenesis of these cells.

Amitosis , or direct cell division (from the Greek α - a particle of negation and the Greek μίτος - “thread”) - cell division by simply dividing the nucleus in two.

It was first described by the German biologist Robert Remak in 1841, and the term was proposed by the histologist Walter Flemming in 1882. Amitosis is a rare but sometimes necessary occurrence. In most cases, amitosis is observed in cells with reduced mitotic activity: these are aging or pathologically altered cells, often doomed to death (cells of the embryonic membranes of mammals, tumor cells, etc.).

During amitosis, the interphase state of the nucleus is morphologically preserved, the nucleolus and the nuclear membrane are clearly visible. No DNA replication . Spiralization of chromatin does not occur, chromosomes are not detected. The cell retains its inherent functional activity, which almost completely disappears during mitosis. During amitosis, only the nucleus divides, and without the formation of a fission spindle, therefore, the hereditary material is distributed randomly.

If the amount of the original genetic material is taken as 100%, and the amount of genetic material in the divided cells is denoted x and y , then

x = 100% -y, a y = 100% -x .

The absence of cytokinesis leads to the formation of binuclear cells, which are subsequently unable to enter into a normal mitotic cycle. With repeated amitoses, multinucleated cells can form.

Amitosis is a direct cell division. It occurs in some specialized cells or in cells where it is not necessary to preserve genetic information from generation to generation.

The significance of Amitosis for the organism is not unambiguous, since it can be regenerative and generative.

Regenerative , has a positive meaning, as it occurs when you need to quickly restore the integrity of the body. After surgery, injuries, burns. Cells rapidly divide and form a scar.

Generative , occurs normally during the division of ovarian follicular cells. Usually, once a month, 1 egg matures and the follicular cells surrounding it begin to rapidly divide, forming a mature follicle. After the egg leaves it, it fills with the corpus luteum and then dissolves, and a scar forms in its place. That is, in this case, exact mechanisms for the distribution of genetic information are not needed, since the follicle dies anyway.

But this mechanism also has its disadvantages: since the genetic information in the daughter cells changes randomly, these cells, if they do not die physiologically, are the sources of ovarian cancer. As you know, cystic and tumor processes in the ovaries occur quite often.

Degenerative Mitosis occurs in senescent, pathologically altered cells. For example, in inflammation or in the cells of malignant tumors.

Reactive Mitosis is observed when a cell is exposed to chemical or physical factors.

Thus, amitosis leads to the formation of cells with unequal genetic information. After cell division by amitosis, the cell loses the ability to divide by mitosis.

Comment

The process of procreation is a basic characteristic that characterizes all living beings.

At all organizational levels, living matter is represented by the simplest structural units, from which it can be concluded that all matter is discrete, and discreteness itself is the main property of a living organism. The structural units of the cell are organelles, and its integrity is determined by their constant reproduction, instead of damaged or worn ones. All living organisms are made up of cells, the process of reproduction of which determines their very existence.

Background of cell division

The basis of the development process of the body is cell division. Note that the division of the cell nucleus always outstrips the process of division of the cell itself. In the process of development, the cell nucleus, like other components of the cell itself, arose in the process of specialization of the cytoplasm. The nucleus of a new cell arises only in the process of division of another nucleus.

The development of a plant (growth and its increase in volume and size) is a consequence of an increase in the number of living cells by dividing them. In unicellular organisms, division is the only way to reproduce.

Living cells grow and develop throughout the entire period of their existence, and in the process of growth there is a constant change in the relationship between its growing volume and surface.

The surface of the cell lags behind in absolute terms from the rate of growth of its volume, this is explained by the fact that the area of ​​the cell increases in arithmetic progression, and the growth of its volume in geometric progression.

It is well known that the nutrition of the cell is carried out through its own surface. In a certain period of time, the surface area cannot provide the required volume, as a result, it begins to divide at an increased rate.

There are the following types of cell division:

• Amitosis.
• Mitosis.
• Endomitosis.
• Meiosis.

What is amitosis in biology definition

Amitosis, briefly and clearly, is the process of division of the cell nucleus, which occurs by restructuring the intranuclear substance, without creating new chromosomes.

This phenomenon was described by the German-born biologist R. Remarque. The term was proposed by the histologist W. Fleming. Amitosis is more common than mitosis. The process of amitosis is carried out by constriction of the nucleus, nucleolus and cytoplasm. Unlike other methods of cell division, chromosome compensation does not occur, but their doubling occurs. According to the biological significance, they distinguish:

• Generative - characterized by full cell division.
• Reactive - occur as a result of inadequate impact on the cell.
• Degenerative - distribution is the result of the process of cell death.

With this type of division, the splitting of the cell nucleus leads to narrowing of the cytoplasm. The size of the constriction is constantly increasing, eventually leading to the division of the nucleus into two independent ones. The process of nuclear division ends with the narrowing of the cytoplasm, dividing the cell into two identical parts, without straightening the chromosomes inside the newly created cells. What distinguishes mitosis from amitosis.

Amitosis briefly

In the process of division, the cell nucleus is split. In the process of amitosis, the cell nucleus gradually lengthens, after which it acquires ganglia. The size of the constriction is constantly increasing, eventually leading to the division of the nucleus into two independent ones, the process ends with the narrowing of the cytoplasm, dividing the cell into two approximately identical parts. Two daughter cells are formed without the occurrence of cellular events, due to which the cell expands in volume. The nucleus expands to form an hourglass-shaped structure.

Constrictions form on the middle part of the membrane. Which gradually deepen, dividing the core into two children. The invagination moves into the cell. After that, the parent cell is divided into two (equal in size).

Amitosis is characteristic of healthy cells that do not have pathologies. But more often it occurs in highly differentiated, old cells. Also, amitosis can occur in low-level organisms. The disadvantage of this process is the lack of the possibility of genetic recombination, which provokes the possibility of the appearance of damaged genes.

Amitosis biological significance

Amitosis is characterized by the division of the cell nucleus and the contents of the cell into two equal parts - without structural changes.

Note that the cell nucleus is divided into two equal parts, without prior dissolution of the nuclear envelope. Also, there is no spindle in the cell.

Upon completion of the process, the division of the protoplast and the entire mass of the cell into two equal parts occurs, but in the case of division of the nucleus into equal parts, new multinuclear cellular structures are formed. In the process of division, there is no distribution of cellular substance between the nuclei.

For a long time it was believed that amitosis is a pathological process inherent only in affected cells. However, recent scientific studies have not confirmed this point of view. Scientists have proven that the process of amitosis is more common in young cells that do not have developmental defects. This type of division is inherent in algae, onions, tradescantia. In addition, it is found in cells with high metabolic activity.

However, this type of division is not characteristic of cells whose biological function is reduced to the safest storage and transmission of genetic information. For example, in germ cells or embryonic cells. Due to this, amitosis is not considered a full-fledged method of cell reproduction.

Atypical forms of mitosis include amitosis, endomitosis, and polythenia.

Amitosis sometimes also called simple division. Amitosis is direct cell division by constriction or invagination. During amitosis, there is no condensation of chromosomes and no division apparatus is formed. Amitosis does not provide an even distribution of chromosomes between daughter cells. Usually amitosis is characteristic of senescent cells. During amitosis, the cell nucleus retains the structure of the interphase nucleus, and complex restructuring of the entire cell, chromosome spiralization, as during mitosis, does not occur. There is no evidence for an even distribution of DNA between two cells during amitotic division, so it is believed that DNA during this division can be distributed unevenly between two cells. Amitosis is quite rare in nature, mainly in unicellular organisms and in some cells of multicellular animals and plants. There are several forms of amitosis:

• uniform, when two equal nuclei are formed;
• uneven - unequal nuclei are formed;
• fragmentation - the nucleus breaks up into many small nuclei, of the same size or not.

The first two types of division cause the formation of two cells from one. In cartilage cells, loose connective and some other tissues, nucleolus division occurs, followed by nuclear division by constriction. In a binuclear cell, a circular constriction of the cytoplasm appears, which, when deepened, causes a complete division of the cell into two. Example. Isogenic groups appear in the cartilage, i.e., groups originating from one cell. Such cells are specialized to perform certain functions in the body, but are unable to divide mitotically. In the process of amitosis in the nucleus, the division of the nucleoli occurs, followed by division of the nucleus by a constriction, the cytoplasm is also divided by a constriction.

Amitosis fragmentation causes the formation of multinucleated cells. In some cells of the epithelium, the liver, the process of division of the nucleoli in the nucleus is observed, after which the entire nucleus is laced with an annular constriction. This process ends with the formation of two nuclei. Such a binuclear or multinuclear cell no longer divides mitotically, after a while it ages or dies. Thus, amitosis is a division that occurs without spiralization of chromosomes and without spindle formation. It is also unknown whether DNA synthesis is synthesized before the onset of amitosis and how DNA is distributed between the daughter nuclei. Whether the previous DNA synthesis occurs before the onset of amitosis and how it is distributed between the daughter nuclei is unknown. When certain cells divide, sometimes mitosis alternates with amitosis.

The biological significance of amitosis Some scientists consider this method of cell division to be primitive, while others consider it to be a secondary phenomenon. Amitosis, compared with mitosis, is much less common in multicellular organisms and can be attributed to an inferior method of cell division that has lost the ability to divide. The biological significance of the processes of amitotic division:

• there are no processes that ensure the uniform distribution of the material of each chromosome between two cells;
• the formation of multinucleated cells or an increase in the number of cells.

Endomitosis. In this type of division, after DNA replication, chromosomes do not separate into two daughter chromatids. This leads to an increase in the number of chromosomes in a cell, sometimes by tens of times in comparison with the diploid set. This is how polyploid cells are formed. Normally, this process takes place in intensively functioning tissues, for example, in the liver, where polyploid cells are very common. However, from a genetic point of view, endomitosis is a genomic somatic mutation.

Polythenia. There is a multiple increase in the content of DNA (chromonemes) in the chromosomes without an increase in the content of the chromosomes themselves. At the same time, the number of chromonemes can reach 1000 or more, while the chromosomes become gigantic. During polythenia, all phases of the mitotic cycle fall out, except for the reproduction of primary DNA strands. This type of division is observed in some highly specialized tissues (liver cells, cells of the salivary glands of Diptera). Drosophila polytene chromosomes are used to build cytological maps of genes in chromosomes.