Amitosis. Its types and biological significance

Remark 1

An integral property of all living organisms is reproduction or reproduction of their own kind.

At any level of organization, living matter is represented by elementary units, that is, it is discrete; and discreteness is one of the properties of the living. Organelles are structural units for a cell and its integrity is due to the constant reproduction of new organelles instead of worn out ones. Every organism is made up of cells. And the development and existence of the organism is ensured by the reproduction of cells.

Prerequisites preceding nuclear and cell division

The basis of reproduction is cell division. Nuclear division always precedes cell division. In the process of historical development, the nucleus, like other cell organelles, arose perhaps as a result of specialization and differentiation of individual sections of the cytoplasm. However, in the process of individual development of cells, the nucleus arises only from the nucleus as a result of division.

The growth of a plant organism (an increase in its size) occurs due to an increase in the number of cells by division. In unicellular organisms, cell division is both a way of their reproduction - an increase in their weight, and reproduction - an increase in the number of individuals of a given species.

Each cell grows over a given time, and in the process of its growth, the ratio between the growing volume of cells and its growing surface changes all the time.

The growth of the surface, of course, lags in its absolute terms behind the growth of the volume, since the surfaces increase quadratically, while the volume increases cubically.

Remark 2

As you know, the cell is fed through the surface. Therefore, at a certain time, the surface cannot "provide" the volume of the cell and it begins to divide intensively.

There are four ways of cell division:

amitosis,
mitosis,
endomitosis
meiosis.

Amitosis

Definition 1

Amitosis (from Greek a - negative particle and mitos - thread) - direct division of the nucleus, which occurs by restructuring the nuclear substance, without the formation of chromosomes.

The phenomenon of amitosis was first described by the German biologist R. Remarque (1841). The term "amitosis" was introduced by the German histologist W. Fleming (1882). Amitosis is much less common than mitosis. It occurs by constriction of the nucleolus, nucleus, and then the cytoplasm. In contrast to mitosis, during amitosis in the nucleus, there is no condensation of chromosomes, but only their doubling, and the physicochemical properties of the cytoplasm do not change. According to the physiological value, three types of amitotic distribution are distinguished:

generative amitosis is a full-fledged cell division, the daughter cells of which are capable of mitotic distribution and normal functioning.
reactive amitosis - caused by inadequate actions on the body.
degenerative amitosis - distribution associated with the processes of self-destruction and cell death.

Amitosis process

With amitotic type of cell division, the splitting of the nucleus is accompanied by cytoplasmic narrowing. During amitosis, the nucleus first lengthens and then acquires dumbbells. The depression or narrowing increases in size and eventually divides the nucleus into two nuclei; nuclear division is followed by a narrowing of the cytoplasm, which divides the cell into two identical or approximately identical halves. Without the occurrence of any nuclear event, two daughter cells are formed. Due to auxetic growth, the cell enlarges. The nucleus expands and eventually forms a dumbbell-shaped structure with the appearance of a median constriction.

Two narrowings appear on the middle part of the cell membrane. The constriction of the nucleus gradually deepens and divides the nucleus into two daughter nuclei without the formation of any spindle fiber. The invaginated cells also move inward, and the parent cell divides into two equal-sized daughter cells.

Amitosis is observed in young, completely normally developed cells (in the daughter of the bulb, root tissues). But more often it is inherent in highly differentiated and older cells. Amitosis is also inherent in low-level organisms - yeast, bacteria, etc. The disadvantage of amitosis is that in this process of cell division there is no possibility of genetic recombination and there is the possibility of expression of unwanted recessive genes.

The meaning of amitosis

Remark 3

The essence of amitosis is that the nucleus, followed by the contents of the cell, is divided into two parts - daughter cells without any preliminary changes in the structure of the organelles, including the nucleus.

Moreover, the nucleus is divided into two parts, even without prior dissolution of the nuclear envelope. There is no fission spindle formation, which is characteristic of other types of fission.

After the division of the nucleus, the protoplast and the entire cell begin to divide into two parts, but in cases where the nucleus is fragmented into several parts, multinucleated cells are formed. During amitosis, there is no uniform distribution of the substance of the nucleus between the daughter nuclei, that is, their biological uniformity is not ensured. However, the formed cells do not lose their structural organization and vital activity.

For a long time, there was an opinion in science that amitosis is a pathological phenomenon inherent only in pathologically altered cells. However, recent research does not support this view. Many studies (Karolinskaya, 1951 and others) have shown that amitosis is also observed in young, normally developed cells. This type of cell and nucleus division was observed in the cells of the internodes of Chara algae, in the cells of onion, tradescantia. In addition, amitosis also occurs in specialized tissues with a high activity of metabolic processes, namely: in the cells of the tapetum of microsporangia, in the endosperm of the seeds of some plants, and the like.

However, this type of separation does not occur in cells where complete genetic information must be preserved, such as eggs and embryonic cells. Therefore, according to a number of scientists, amitosis cannot be considered a full-fledged way of cell reproduction.

Send your good work in the knowledge base is simple. Use the form below

Students, graduate students, young scientists who use the knowledge base in their studies and work will be very grateful to you.

Posted on http:// www. all best. en/

Amitosis: its types and meaning

Plan

Introduction

1. Amitosis: concept and essence

2. Types of amitosis

Conclusion

Bibliography

Introduction

Term "cell" first used by Robert Hooke in 1665 when describing his "research on the structure of cork with the help of magnifying lenses." In 1674, Anthony van Leeuwenhoek established that the substance inside the cell is organized in a certain way. He was the first to discover cell nuclei. At this level, the idea of a cell lasted for more than 100 years.

The study of the cell accelerated in the 1830s with improved microscopes. In 1838-1839, the botanist Matthias Schleiden and the anatomist Theodor Schwann almost simultaneously put forward the idea of the cellular structure of the body. T. Schwann proposed the term "cell theory" and presented this theory to the scientific community. The emergence of cytology is closely connected with the creation of the cellular theory, the broadest and most fundamental of all biological generalizations. According to the cellular theory, all plants and animals consist of similar units - cells, each of which has all the properties of a living thing.

The most important addition to the cellular theory was the assertion of the famous German naturalist Rudolf Virchow that each cell is formed as a result of the division of another cell.

In the 1870s, two methods of eukaryotic cell division were discovered, later named mitosis and meiosis. Already 10 years later, it was possible to establish the main genetic features of these types of division. It was found that before mitosis, chromosomes are duplicated and distributed evenly between daughter cells, so that the daughter cells retain the same number of chromosomes. Prior to meiosis, chromosomes also double. but in the first (reduction) division, two-chromatid chromosomes diverge to the poles of the cell, so that cells with a haploid set are formed, the number of chromosomes in them is two times less than in the mother cell. It was found that the number, shape and size of chromosomes - the karyotype - is the same in all somatic cells of animals of a given species, and the number of chromosomes in gametes is half as much. Subsequently, these cytological discoveries formed the basis of the chromosome theory of heredity.

1. Amitosis: concept and essence

Amitosis (or direct cell division) occurs in somatic eukaryotic cells less frequently than mitosis. It was first described by the German biologist R. Remak in 1841, the term was proposed by the histologist W. Flemming later - in 1882. 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. DNA replication is absent.

Rice. 1 Amitosis

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. 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.

This concept still appeared in some textbooks until the 1980s. At present, it is believed that all the phenomena attributed to amitosis are the result of an incorrect interpretation of insufficiently prepared microscopic preparations, or the interpretation of phenomena accompanying cell destruction or other pathological processes as cell division. At the same time, some variants of eukaryotic nuclear fission cannot be called mitosis or meiosis. Such, for example, is the division of the macronuclei of many ciliates, where, without the formation of a spindle, segregation of short fragments of chromosomes occurs.

Amitosis - (from Greek a - negative part, and mitos - thread; synonym: direct division, fragmentation). This is the name of a special form of cell division, which differs from ordinary mitosis (fission with fibrous metamorphosis of the nucleus) in its simplicity. According to the definition of Flemming "a, who established this form (1879), "amitosis is such a form of cell and nuclear division in which there is no formation of a spindle and correctly formed chromosomes and the movement of the latter in a certain order."

The nucleus, without changing its character, directly or after preliminary division of the nucleolus, splits into two parts by ligation or the formation of a one-sided fold. After the division of the nucleus, in some cases, the cell body also divides, also by ligation and splitting. Sometimes the nucleus breaks up into several parts of equal or unequal size. A. has been described in all organs and tissues of both vertebrates and invertebrates; at one time it was thought that the protozoa divide exclusively in a direct way, but the fallacy of this view was soon proved. The main sign for ascertaining A. was the presence of binuclear cells, and along with them, cells with large nuclei that show folds and intercepts; amitotic division of the cell body was observed extremely rarely, it had to be concluded on the basis of indirect considerations.--

On the question of the essence and meaning of A., various views were expressed:

1. A. is the primary and simplest method of division (Strassburger, Waldeyer, Car-po); it occurs, for example, during wound healing, when cells "do not have time" to share mitosis (Balbiani, Henneguy), it is sometimes observed in embryos (Maximov). fragmentation interphase cell amitosis

2. A. is an abnormal way of division, occurs under pathological conditions, in obsolete tissues, sometimes in cells with increased secretion and assimilation, and marks the end of division; cells after A. can no longer divide mitotically, so A. has no regenerative value (Flemming, Ziegler, Rath).

3. A. is not a method of cell reproduction; in one part of A.'s cases, a simple disintegration of the nucleus occurs under the influence of physical and mechanical moments (pressure, pinching the cell with something, the formation and deepening of folds due to a change in the osmotic pressure of the nucleus), in other cases, described as A., there is an abortive (not completed) mitosis; depending on the stage, mitosis breaks off at a cut, cells with a large ligated nucleus or binuclear (Karpov) are obtained. "-- Over the past two decades, the question of A. has been debated less frequently, and all three views are expressed: in views on A. not achieved.

During amitosis, the spindle of division is not formed and the chromosomes are indistinguishable in a light microscope. Such a division occurs in unicellular organisms (for example, this is how large polyploid nuclei of ciliates divide), as well as in some highly specialized cells of plants and animals with weakened physiological activity, degenerating, doomed to death, or during various pathological processes, such as malignant growth, inflammation, etc. . P.

Amitosis can be observed in the tissues of a growing potato tuber, seed endosperm, pistil ovary walls, and parenchyma of leaf petioles. In animals and humans, this type of division is characteristic of the cells of the liver, cartilage, and cornea of the eye.

With amitosis, only nuclear division is often observed: in this case, two- and multi-nuclear cells can arise. If the division of the nucleus is followed by the division of the cytoplasm, then the distribution of cellular components, like DNA, is carried out arbitrarily.

Amitosis, unlike mitosis, is the most economical way of division, since the energy costs are very small.

In Amitosis, in contrast to mitosis, or indirect nuclear division, the nuclear membrane and nucleoli are not destroyed, the fission spindle is not formed in the nucleus, the chromosomes remain in a working (despiralized) state, the nucleus is either laced or a septum appears in it, externally unchanged; division of the cell body - cytotomy, as a rule, does not occur (Fig.); Amitosis usually does not provide uniform division of the nucleus and its individual components.

Figure 2 Amitotic nuclear division of rabbit connective tissue cells in tissue culture.

The study of Amitosis is complicated by the unreliability of its definition by morphological features, since not every constriction of the nucleus means Amitosis; even pronounced "dumbbell" constrictions of the nucleus can be transient; nuclear constrictions can also be the result of an incorrect previous mitosis (pseudoamitosis). Amitosis usually follows endomitosis. In most cases, in Amitosis, only the nucleus divides and a binuclear cell appears; with repeated amitosis, multinucleated cells can form. Very many binuclear and multinuclear cells are the result of amitosis (a certain number of binuclear cells are formed during mitotic division of the nucleus without dividing the cell body); they contain (in total) polyploid chromosome sets (see Polyploidy).

In mammals, tissues are known both with mononuclear and binuclear polyploid cells (cells of the liver, pancreas and salivary glands, nervous system, bladder epithelium, epidermis), and only with binuclear polyploid cells (mesothelial cells, connective tissues). Two multinucleated cells differ from single-nuclear diploid cells (see Diploid) in larger sizes, more intense synthetic activity, and an increased number of various structural formations, including chromosomes. Binuclear and multinuclear cells differ from mononuclear polyploid cells mainly in the larger surface area of the nucleus. This is the basis for the idea of amitosis as a way to normalize nuclear-plasma relations in polyploid cells by increasing the ratio of the surface of the nucleus to its volume. During amitosis, the cell retains its characteristic functional activity, which almost completely disappears during mitosis. In many cases, amitosis and binuclearity accompany compensatory processes occurring in tissues (for example, during functional overload, starvation, after poisoning or denervation). Amitosis is usually observed in tissues with reduced mitotic activity. This, apparently, explains the increase in the number of binuclear cells with aging of the body, which are formed by Amitosis. Ideas about Amitosis as a form of cell degeneration are not supported by modern research. The view of Amitosis as a form of cell division is also untenable; there are only single observations of amitotic division of the cell body, and not just its nucleus. It is more correct to consider Amitosis as an intracellular regulatory reaction.

2. Types of amitosis

Amitosis - direct division of the cell (nucleus). In this case, the ligation or fragmentation of the nucleus occurs without the detection of chromosomes and the formation of a fission spindle. One of the forms of amitosis can be genome segregation - multiple ligation of the polyploid nucleus with the formation of small daughter nuclei.

Segregation - the process of segregation of chromosomes in mitosis or meiosis. Segregation ensures the constancy of the number of chromosomes in cell divisions.

The complexity of the organization of the genome: “silent” DNA - A significant part of the nucleotide sequences in eukaryotes is replicated, but not transcribed at all, the mosaic structure of genes (introns are a section of DNA that is part of a gene, but does not contain information about the amino acid sequence of a protein, exons are a DNA sequence , which is presented in mature RNA), mobile genetic elements are DNA sequences that can move within the genome.

As a rule, amitosis occurs in polyploid, obsolete or pathologically altered cells and leads to the formation of multinucleated cells. In recent years, the existence of amitosis as a means of normal cell reproduction has been denied.

In tissues that complete their life activity, or in pathological conditions, direct cell division can be observed without detecting chromosomes in the nucleus - amitosis. It is characterized by a change in the shape and number of nucleoli, followed by ligation of the nucleus. The resulting binuclear cells can undergo cytotomy.

According to the physiological significance, three types of amitotic division are distinguished:

Amitosis generative;

Degenerative;

Reactive.

Generative amitosis - full-fledged cell division, the daughter cells of which are subsequently capable of mitotic division and normal functioning characteristic of them.

Reactive amitosis caused by any inappropriate effects on the body.

Degenerative amitosis - division associated with the processes of degeneration and cell death.

Conclusion

The ability to divide the most important property of cells. Without division, it is impossible to imagine an increase in the number of unicellular beings, the development of a complex multicellular organism from a single fertilized egg, the renewal of cells, tissues, and even organs lost during the life of the organism. Cell division is carried out in stages. At each stage of division, certain processes occur. They lead to the doubling of the genetic material (DNA synthesis) and its distribution between daughter cells. The period of a cell's life from one division to the next is called the cell cycle.

Cell division leads to the formation of two or more daughter cells from one parent cell. If the division of the nucleus of the mother cell is immediately accompanied by the division of its cytoplasm, two daughter cells appear. But it also happens this way: the nucleus divides many times, and only then a part of the cytoplasm of the mother cell separates around each of them. In this case, several daughter cells are formed from one initial cell at once.

Amitosis , or direct division, is the division of the interphase nucleus by constriction without the formation of a fission spindle (chromosomes are generally indistinguishable in a light microscope). Such division occurs in unicellular organisms (for example, polyploid large ciliate nuclei divide by amitosis), as well as in some highly specialized cells of plants and animals with weakened physiological activity, degenerating, doomed to death, or during various pathological processes, such as malignant growth, inflammation and etc.

Bibliography

1. Biology / Ed. Chebyshev. N.V. - M.: GOU VUNMTS, 2005.

2. Congenital malformations // A series of educational literature "Education of nurses", module 10. - M .: Geotar-med, 2002.

3. Medical genetics / Ed. Bochkova N.P. - M.: Mastery, 2001.

4. Orekhova. V.A., Lazhkovskaya T.A., Sheybak M.P. Medical genetics. - Minsk: Higher School, 1999.

5.Biology manual for pre-university education of foreign students / Ed. Chernyshova V.N., Elizarova L.Yu., Shvedova L.P. - M.: GOU VUNMTs MZ RF, 2004.

6.Yarygin V.N., Volkov I.N. etc. Biology. - M.: Vlados, 2001.

Hosted on Allbest.ru

...

Amitosis (biology): it all starts with the cell

It's hard to imagine, but the cells present in the tiny fetus eventually give rise to all the cells that make up the body of an adult. Bone and flesh, organs and tissues are the products of thousands of generations of cell division. Most plant and animal cells replicate by separating into two identical daughter cells. Simple division, which is the means of asexual reproduction of single-celled organisms such as bacteria and protozoa, is called amitosis. It is also a way of reproduction or growth in the fetal membranes of some vertebrates.

The splitting of the nucleus is accompanied by cytoplasmic narrowing. In the process of division, the nucleus elongates and then takes on an elongated shape, then it increases in size and, ultimately, is divided into two halves. This process is accompanied by a narrowing of the cytoplasm, which divides the cell into two equal or approximately identical parts. Thus, two daughter cells are formed.

Discovery of cell division

In the 19th century, Flemming, a professor at the Institute of Anatomy in Kiel (Germany), first documented the details of cell division. He was highly regarded as an innovator in this field, largely due to technology such as the use of microscopes to study biological tissues. Flemming experimented with the technique of using dyes to stain specimens that he wanted to examine under a microscope. He discovered some positive properties of aniline dyes and came to the conclusion that different types of fabrics absorb with different intensity depending on their chemical composition. This made it possible to reveal structures and processes that were invisible before.

Fleming was interested in the process of cell division. He began a series of live observations under a microscope using stained animal tissue samples and found that a certain mass of material inside the nucleus absorbs the dye quite well. After a while, it began to be called "chromatin" (from the Greek saturated). Today, the process of splitting one nucleus into two is called mitosis, and the division itself is called cytokinesis. But what is amitosis? Scientists began to think about this issue only in the 20th century.

Key Difference Between Mitosis and Amitosis

Mitosis is the process in which cells arrange their chromosomes into two identical sets. Amitosis is a process that occurs in the absence of mitosis in cells. Life is beautiful and complex. It's amazing how everything around grows, changes and develops. Mitosis is an integral part of the cell cycle, which basically involves a series of events leading a cell to divide and create two daughter cells. So there are exact copies of the parent cell. This is followed by cytokinesis, which separates the cytoplasm, organelles, and membrane.

Another way of division is amitosis. This concept can be classified as a form of closed mitosis. During this process, the mother cell also produces two daughter cells, but they are not identical to each other or to the parent cell. Amitosis is also sometimes referred to as direct cell division, during which the cell and its nucleus split into two halves. However, unlike mitosis, no complex changes occur in the nucleus.

Amitosis to the rescue

In 1882, the scientific term amitosis appeared in medicine. Where it has already been observed, a normal mitotic cycle is no longer possible. Previously called a primitive form, amitosis in the modern sense is a qualitatively peculiar process of nuclear fission, which appeared on the basis of mitotic transformations. Sometimes amitosis is observed in various pathological phenomena, for example, inflammatory processes or malignant tumors.

Amitosis is also discussed when the cell has lost the ability to mitosis. Most often this happens already in adulthood. An example is the human body. The cells of the circulatory system lose their ability to mitosis, therefore, when they are damaged (for example, by a heart attack), they cannot recreate or replace themselves. Remarkably, skin cells continue to replicate and replace themselves throughout their lives and ours. Amitosis may be accompanied by cell division, or it may be limited to nuclear division without division of the cytoplasm, which leads to the formation of multinucleated cells. Basically, this process occurs in degenerating cells that are doomed to death, especially in the embryonic membranes of mammals.

Main features of amitosis

The activity of the cell is preserved, but the hereditary material is distributed in a chaotic manner.
Lack of cytokinesis, this can lead to the formation of cells with multiple nuclei.
The resulting cells are no longer capable of mitosis.
Difficulties in identification, sometimes amitosis can be the result of incorrectly proceeding mitosis.
Most often found in unicellular organisms, as well as in plant and animal cells with weakened physiological activity and other deviations from the norm.

The question of what exactly is amitosis is still controversial. A large number of scientists and biologists dispute the fact that this is simply a form of cell division, calling it an internal regulatory response of the cell.

Accent placement: AMITO`Z

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 achromatin spindle; with A., 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 A. 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 gradually ligates, 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).

A. is characteristic of a number of highly differentiated and specialized tissues (neurons of autonomic ganglia, cartilaginous, glandular cells, blood leukocytes, endothelial cells of blood vessels, etc.), as well as for cells of malignant tumors.

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

Generative A. is a full-fledged nuclear fission, after which it becomes possible mitosis(cm.). Generative A. is observed in some protozoa, in polyploid nuclei (see. Chromosomal set); at the same time, 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 certain specialized cells (liver, epidermis, trophoblast, etc.), where A. is preceded by endomitosis - intranuclear doubling of the set of chromosomes (see. Meiosis); the resulting endomitosis and polyploid nuclei are then subjected to A.

Reactive A. due to the influence on the cell of various damaging factors - radiation, chemical. drugs, temperature, etc. 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 A. as an intracellular compensatory reaction that ensures the intensification of cell metabolism.

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

Question about biol. the value of A. has not been finally resolved. However, there is no doubt that A. is a secondary phenomenon in comparison with mitosis.