home Audience 3 Amitosis, its stages and meaning. Direct cell division, or amitosis. What is amitosis

Amitosis, its stages and meaning. Direct cell division, or amitosis. What is amitosis

Familiarization with the information contained in this article will allow the reader to learn about one of the methods of cell division - amitosis. We will find out the features of this process, consider the differences from other types of division and much more.

What is amitosis

Amitosis is direct cell division. This process occurs thanks to the usual two parts. However, it may miss the spindle formation phase for division. And ligation occurs without chromatin condensation. Amitosis is a process characteristic of animal and plant cells, as well as simple organisms.

From history and research

Robert Remak described the process of amitosis for the first time in 1841, but the term itself arose much later. Already in 1882, histologist and biologist of German origin Walter Flemming proposed the modern name for the process itself. Amitosis of a cell in nature is a relatively rare phenomenon, but it can often occur because it is necessary.

Process Features

How does cell division occur? Amitosis most often occurs in cells that have reduced mitotic activity. Thus, many cells that should die as a result of old age or pathological changes can delay their death for some time.

Amitosis is a process in which the state of the nucleus during interphase retains its morphological characteristics: the nucleolus is clearly visible, as is its shell, DNA is not replicated, chromatin is proteinaceous, DNA and RNA are not spiralized, and there is no detection of chromosomes in the nucleus of a eukaryotic cell.

There is indirect cell division - mitosis. Amitosis, in contrast, allows the cell after division to maintain its activity as a functioning element. The spindle (a structure designed for chromosomal segregation) is not formed during amitosis, but the nucleus still divides, and the consequence of this process is the random distribution of hereditary information. The absence of the cytokinetic process results in the reproduction of cells with two nuclei, which in the future will not be able to enter into the typical mitotic cycle. Repeated repetition of amitosis can lead to the formation of cells with many nuclei.

Current situation

Amitosis as a concept began to appear in many textbooks back in the 80s of the twentieth century. Today, there are assumptions that all the processes that were previously subsumed under this concept are in fact incorrectly interpreted results of studies on poorly prepared microslides. Scientists believe that the phenomenon of cell division, accompanied by the destruction of the latter, could lead to the same misunderstood and interpreted data. However, some processes of eukaryotic cell division cannot be classified as either mitosis or meiosis. A striking example and confirmation of this is the process of division of the macronucleus (the nucleus of a ciliate cell, large in size), during which the segregation of some chromosome sections occurs, despite the fact that the spindle for division is not formed.

What causes the complication of studying the processes of amitosis? The fact is that this phenomenon is difficult to determine by its morphological characteristics. This definition is unreliable. The inability to clearly determine the process of amitosis by morphological signs is based on the fact that not every nuclear constriction is a sign of amitosis itself. And even its dumbbell-shaped form, which is clearly expressed in the core, can only belong to the transitional type. Also, nuclear constrictions can be a consequence of errors in the phenomenon of previous division by mitosis. Most often, amitosis occurs immediately after endomitosis (a method of doubling the chromosome number without dividing both the cell and its nucleus). Usually the process of amitosis leads to doubling. The repetition of this phenomenon creates a cell with many nuclei. Thus, amitosis creates cells with a polyploid chromosome set.

Conclusion

To summarize, we can say that amitosis is a process during which the cell divides in a direct manner, that is, the nucleus divides into two parts. The process itself is not capable of ensuring cell division into equal, identical halves. This also applies to information about the heredity of the cell.

This process has a number of sharp differences from the gradual division by mitosis. The main difference in the processes of amitosis and mitosis is the absence of destruction of the nuclear membrane and nucleolus during amitosis, as well as the occurrence of the process without the formation of a spindle, which ensures the division of information. Cytotomy in most cases does not divide.

Currently, there are no studies of the modern era that could clearly identify amitosis as a form of cell degeneration. The same applies to the perception of amitosis as a method of cell division due to the presence of a very small amount of division of the entire cell body. Therefore, amitosis may be better attributed to a regulatory process occurring inside cells.

Comment

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

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

Prerequisites for cell division

The basis of the development process of an organism is cell division. Note that the division of the cell nucleus always precedes the process of division of the cell itself. During 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 appears only during the division of another nucleus.

The development of a plant (growth and increase in volume and size) is a consequence of an increase in the number of living cells through their division. In single-celled organisms, division is the only way to reproduce.

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

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

Comment

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

The following types of cell division are distinguished:

Amitosis.
Mitosis.
Endomitosis.
Meiosis.

What is amitosis in biology definition

What is amitosis

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

This phenomenon was described by the biologist of German origin R. Remarque. The term was proposed by histologist V. 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 their biological significance, they are distinguished:

Generative – characterized by full cell division.
Reactive – occurs as a result of an inadequate effect 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 occurs and the cytoplasm narrows. The size of the constriction constantly increases, ultimately leading to the division of the nucleus into two independent ones. The process of nuclear division ends with a narrowing of the cytoplasm, dividing the cell into two identical parts, without straightening the chromosomes inside the newly created cells. This is what distinguishes mitosis from amitosis.

Amitosis briefly

During the process of division, the cell nucleus is split. During the process of amitosis, the cell nucleus gradually lengthens, after which it acquires ganglia. The size of the constriction constantly increases, ultimately leading to the division of the nucleus into two independent ones; the process ends with a narrowing of the cytoplasm, dividing the cell into two approximately equal parts. Two daughter cells are formed without any cellular events occurring, causing the cell to increase in volume. The core expands to form an hourglass-shaped structure.

Constrictions form in the middle part of the membrane. Which gradually deepen, dividing the core into two daughter ones. The invagination moves into the cell. After which, 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. And also, amitosis can occur in low-level organisms. The disadvantage of this process is the lack of possibility of genetic recombination, which provokes the possibility of the appearance of damaged genes.

Amitosis biological significance

The meaning of amitosis

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

Note that the cell nucleus is divided into two equal parts, without previous dissolution of the nuclear membrane. Also, the cell lacks a spindle.

Upon completion of the process, the protoplast and the entire cell mass are divided into two equal parts, but in the case of division of the nucleus into equal parts, new multinuclear cellular structures are formed. During the process of division, cellular substance is not distributed between the nuclei.

For a long time it was believed that amitosis is a pathological process inherent only to affected cells. However, the latest scientific research has not confirmed this point of view. Scientists have proven that the process of amitosis occurs more often in young cells that do not have developmental defects. This type of division is inherent in algae, onions, and tradescantia. In addition, it is found in cells characterized by high metabolic activity.

However, this type of division is not typical for cells, the biological function of which 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.

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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 “inquiries into the structure of cork by means of magnifying lenses.” In 1674, Anthony van Leeuwenhoek discovered 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 the advent of 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 coined the term “cell theory” and introduced this theory to the scientific community. The emergence of cytology is closely connected with the creation of cell 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 cell theory was the statement 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 called mitosis and meiosis. Already 10 years after this, it was possible to establish the main genetic features of these types of division. It was found that before mitosis, chromosomes are doubled and distributed evenly between daughter cells, so that the daughter cells retain the same number of chromosomes. Before meiosis, chromosomes also double. but in the first (reduction) division, bichromatid chromosomes diverge to the poles of the cell, so that cells with a haploid set are formed, the number of chromosomes in them is half that in the mother cell. It was found that the number, shape and size of chromosomes - karyotype - are 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 chromosomal theory of heredity.

1. Amitosis: concept and essence

Amitosis (or direct cell division) occurs less frequently in somatic eukaryotic cells 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 (mammalian embryonic membrane cells, tumor cells, etc.). With amitosis, the interphase state of the nucleus is morphologically preserved, the nucleolus and nuclear envelope are clearly visible. There is no DNA replication.

Rice. 1 Amitosis

Chromatin spiralization does not occur, chromosomes are not detected. The cell retains its characteristic functional activity, which almost completely disappears during mitosis. During amitosis, only the nucleus divides, without the formation of a fission spindle, so the hereditary material is distributed randomly. The absence of cytokinesis leads to the formation of binucleate cells, which are subsequently unable to enter the normal mitotic cycle. With repeated amitoses, multinucleated cells can form.

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

Amitosis - (from Greek a - negative part, and mitos - thread; synonym: direct division, fragmentation). This is the name for a special form of cell division, which differs from ordinary mitosis (division with fibrous metamorphosis of the nucleus) in its simplicity. According to the definition of Flemming, who established this form (1879), “amitosis is 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 lacing or the formation of a one-sided fold. Following 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 protozoa divided exclusively in a direct way, but this view was soon proven wrong. The main sign for ascertaining A. was the presence of binucleate cells, and along with them, cells with large nuclei that exhibit folds and interceptions; amitotic division of the cell body was observed extremely rarely; it had to be concluded on the basis of indirect considerations.--

Various views have been expressed on the issue of the essence and meaning of A.:

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

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

3. A. does not represent a method of cell reproduction; in one part of the cases of A., a simple disintegration of the nucleus occurs under the influence of physical and mechanical moments (pressure, squeezing of the cell with something, the formation and deepening of folds due to changes in the osmotic pressure of the nucleus), in other cases described as A., abortive occurs (not reached the end) mitosis; depending on the stage at which mitosis breaks off, the resulting cells are with a large laced nucleus or binuclear (Karpov)." - Over the past two decades, the question of A. has been debated less frequently, and all three views have been expressed: i.e., unity in views on A. has not been achieved.

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

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

With amitosis, only nuclear division is often observed: in this case, bi- and multinucleated cells can appear. If nuclear division is followed by cytoplasmic division, then the distribution of cellular components, like DNA, is arbitrary.

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

With 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, which is apparently unchanged; division of the cell body - cytotomy, as a rule, does not occur (Fig.); Usually, amitosis does not ensure uniform division of the nucleus and its individual components.

Fig. 2 Amitotic division of rabbit connective tissue cell nuclei in tissue culture.

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

In mammals, tissues with both 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) are known. Bi- and multinucleate cells differ from mononuclear diploid cells (see Diploid) in their larger sizes, more intense synthetic activity, and an increased number of various structural formations, including chromosomes. Binuclear polyploid cells differ from mononuclear polyploid cells mainly in the larger nuclear surface. 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, fasting, 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 formed through Amitosis as the body ages. The idea of Amitosis as a form of cell degeneration is not supported by modern research. The view of Amitosis as a form of cell division is also untenable; There are only isolated 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, ligation or fragmentation of the nucleus occurs without identifying chromosomes and forming a spindle. One of the forms of amitosis may be genome segregation - multiple ligation of a polyploid nucleus with the formation of small daughter nuclei.

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

The complexity of the genome organization: “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, aging or pathologically altered cells and leads to the formation of multinucleated cells. In recent years, the existence of amitosis as a method of normal cell reproduction has been denied.

In tissues that are completing their life activities, or under 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 with subsequent ligation of the nucleus. The resulting binucleate cells can undergo cytotomy.

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

Amitosis is generative;

Degenerative;

Reactive.

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

Reactive amitosis caused by any inappropriate effects on the body.

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

Conclusion

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

Cell division leads to the formation of two or many daughter cells from one mother 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: the nucleus divides many times, and then around each of them a part of the cytoplasm of the mother cell is separated. In this case, several daughter cells are immediately formed from one initial cell.

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

Bibliography

1. Biology / Ed. Chebysheva. N.V. - M.: GOU VUNMC, 2005.

2. Congenital malformations // Series of educational literature “Education of nurses”, module 10. - M.: Geotar-med, 2002.

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

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

5. A manual on biology for pre-university education of foreign students / Ed. Chernyshova V.N., Elizarova L.Yu., Shvedova L.P. - M.: GOU VUNMC Ministry of Health of the Russian Federation, 2004.

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

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Peculiarities

Amitosis is a simpler process than mitosis or meiosis. Amitosis in eukaryotes is quite rare and is more common in prokaryotes. It is a faster and more economical process than mitosis. Observed during rapid tissue restoration. Amitosis divides aging cells and tissue cells that will not further divide mitotically. Most often, this is a group of cells that perform strictly defined functions.

Amitosis is observed:

The main features of amitosis, compared to mitosis:

not accompanied by restructuring of the entire cell;
the spindle is missing;
chromatin spiralization does not occur;
chromosomes are not detected;
lack of DNA replication (doubling);
genetic material is distributed unevenly;
the resulting cell is not capable of mitosis.

Rice. 1. Mitosis and amitosis.

Amitosis can occur in tumor tissues. With an uneven distribution of genetic material, defective eukaryotic cells with disrupted intracellular processes are formed.

Mechanism

Amitosis is a simple and rare method of cell division that has been little studied. It is known that amitosis occurs due to simple constriction (invagination) of the karyolemma - the nuclear membrane, which leads to the division of the parent cell into two parts. During division, the cell is in interphase, i.e. in a state of growth and development, in no way preparing for division. The process of amitosis is described in the table.

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Cytokinesis does not always occur during amitosis, i.e. division of the cell body - the cytoplasm with all its contents. In this case, two or more nuclei are formed under one shell (multinuclear cell), which can lead to the formation of colonies (yeast).

Rice. 2. Yeast budding.

Meaning

Amitosis has biological significance for the rapid restoration of tissues and the reproduction of unicellular eukaryotic and prokaryotic organisms. Amitosis is characteristic of yeasts that reproduce asexually (by budding, division), bacteria, and leukocytes.

Bacteria and other prokaryotes do not have a nucleus. Therefore, amitosis occurs somewhat differently. First, circular DNA attached to a fold of the cytoplasmic membrane (mesosome) doubles. Then, a constriction forms between the two DNAs attached to the mesosomes, dividing the cell in half.

Rice. 3. Division of prokaryotes.

What have we learned?

We found out how mitosis differs from amitosis, how direct cell division occurs, and what role it plays in nature. Amitosis is the fastest method of division, which helps restore damaged tissue in a short period of time. Characteristic of eukaryotes (rare) and prokaryotes. Direct cell division does not require preparation: chromosome spiralization, DNA doubling, or creation of a division spindle. With this method, the cell divides unevenly: daughter cells may differ in size and amount of genetic information.

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Mitosis(from the Greek mitos - thread), or karyokinesis (Greek karyon - core, kinesis - movement), or indirect division. This is a process during which chromosome condensation occurs and daughter chromosomes are evenly distributed between daughter cells. Mitosis includes five phases: prophase, prometaphase, metaphase, anaphase and telophase. IN prophase chromosomes condense (twist), become visible and are arranged in the form of a ball. The centrioles divide into two and begin to move towards the cell poles. Between the centrioles, filaments consisting of the protein tubulin appear. The formation of a mitotic spindle occurs. IN prometaphase the nuclear membrane disintegrates into small fragments, and the chromosomes immersed in the cytoplasm begin to move towards the equator of the cell. In metaphase chromosomes are installed at the equator of the spindle and become maximally compacted. Each chromosome consists of two chromatids connected to each other by centromeres, and the ends of the chromatids diverge, and the chromosomes take an X-shape. In anaphase daughter chromosomes (former sister chromatids) move to opposite poles. The assumption that this is achieved by contraction of the spindle filaments has not been confirmed.

Many researchers support the sliding filament hypothesis, according to which neighboring spindle microtubules, interacting with each other and contractile proteins, pull chromosomes towards the poles. In telophase daughter chromosomes reach the poles, despiral, a nuclear envelope is formed, and the interphase structure of the nuclei is restored. Then comes the division of the cytoplasm - cytokinesis. In animal cells, this process manifests itself in the constriction of the cytoplasm due to the retraction of the plasmalemma between two daughter nuclei, and in plant cells, small EPS vesicles merge to form a cell membrane from within the cytoplasm. The cellulose cell wall is formed due to the secretion that accumulates in dictyosomes.

The duration of each phase of mitosis is different - from several minutes to hundreds of hours, which depends on both external and internal factors and the type of tissue.

Violation of cytotomy leads to the formation of multinucleated cells. If the reproduction of centrioles is disrupted, multipolar mitoses can occur.

Amitosis

This is a direct division of the cell nucleus, which maintains the interphase structure. In this case, chromosomes are not detected, spindle formation and their uniform distribution do not occur. The core is divided by constriction into relatively equal parts. The cytoplasm can divide by a constriction, and then two daughter cells are formed, but it may not divide, and then binucleate or multinucleate cells are formed.

Amitosis as a method of cell division can occur in differentiated tissues, such as skeletal muscle, skin cells, and also in pathological tissue changes. However, it is never found in cells that need to preserve complete genetic information.

11. Meiosis. Stages, biological significance.

Meiosis(Greek meiosis - reduction) - a method of dividing diploid cells with the formation of four daughter haploid cells from one maternal diploid cell. Meiosis consists of two successive nuclear divisions and a short interphase between them. The first division consists of prophase I, metaphase I, anaphase I and telophase I.

In prophase I paired chromosomes, each of which consists of two chromatids, approach each other (this process is called conjugation of homologous chromosomes), cross over (crossing over), forming bridges (chiasmata), and then exchange sections. Crossing over involves recombination of genes. After crossing over, the chromosomes are separated.

In metaphase I paired chromosomes are located along the equator of the cell; spindle strands are attached to each chromosome.

In anaphase I bichromatid chromosomes diverge to the cell poles; in this case, the number of chromosomes at each pole becomes half that in the mother cell.

Then comes telophase I– two cells with a haploid number of bichromatid chromosomes are formed; Therefore, the first division of meiosis is called reduction.

Telophase I is followed by a short interphase(in some cases, telophase I and interphase are absent). In the interphase between two divisions of meiosis, chromosome duplication does not occur, because each chromosome already consists of two chromatids.

The second division of meiosis differs from mitosis only in that it is carried out by cells with a haploid set of chromosomes; in the second division, prophase II is sometimes absent.

In metaphase II bichromatid chromosomes are located along the equator; the process occurs in two daughter cells at once.

In anaphase II Single-chromatid chromosomes move to the poles.

In telophase II in four daughter cells, nuclei and partitions (in plant cells) or constrictions (in animal cells) are formed. As a result of the second division of meiosis, four cells with a haploid set of chromosomes (1n1c) are formed; the second division is called equational (equalization) (Fig. 18). These are gametes in animals and humans or spores in plants.

The significance of meiosis is that it creates a haploid set of chromosomes and conditions for hereditary variability due to crossing over and probabilistic divergence of chromosomes

12.Gametogenesis: ovo - and spermatogenesis.

Gametogenesis- process of formation of eggs and sperm.

Spermatogenesis- from Greek sperma, gen. n. spermatos - seed and...genesis), the formation of differentiated male germ cells - sperm; in humans and animals - in the testes, in lower plants - in antheridia.

In most higher plants, spermatozoa are formed in the pollen tube, more often called sperm. Spermatogenesis begins simultaneously with the activity of the testicle under the influence of sex hormones during puberty in adolescence and then proceeds continuously (in most men almost until the end of life), has a clear rhythm and uniform intensity. Spermatogonia, containing a double set of chromosomes, divide by mitosis, leading to the emergence of subsequent cells - first-order spermatocytes. Further, as a result of two successive divisions (meiotic divisions), 2nd order spermatocytes are formed, and then spermatids (spermatogenesis cells immediately preceding the sperm). During these divisions, the number of chromosomes is halved. Spermatids do not divide, enter the final period of spermatogenesis (the period of sperm formation) and, after a long differentiation phase, turn into sperm. This occurs through gradual elongation of the cell, changes, and elongation of its shape, as a result of which the cell nucleus of the spermatid forms the head of the sperm, and the membrane and cytoplasm form the neck and tail. In the last phase of development, the sperm heads are closely adjacent to the Sertoli cells, receiving nutrition from them until full maturation. After this, the sperm, already mature, enter the lumen of the testicular tubule and then into the epididymis, where they accumulate and are excreted from the body during ejaculation.

Oogenesis- the process of development of female gametes, ending with the formation of eggs. During a woman's menstrual cycle, only one egg matures. The process of oogenesis is fundamentally similar to spermatogenesis and also goes through a number of stages: reproduction, growth and maturation. Eggs are formed in the ovary, developing from immature germ cells - oogonia, containing a diploid number of chromosomes. Oogonia, like spermatogonia, undergo successive mitotic

divisions that are completed by the time the fetus is born. Then comes the period of growth of oogonia, when they are called first-order oocytes. They are surrounded by a single layer of cells - the granulosa membrane - and form the so-called primordial follicles. A female fetus on the eve of birth contains about 2 million of these follicles, but only about 450 of them reach the stage of second-order oocytes and leave the ovary during ovulation. The maturation of the oocyte is accompanied by two successive divisions leading to

reducing the number of chromosomes in a cell by half. As a result of the first division of meiosis, a large oocyte of the second order and the first polar body are formed, and after the second division - a mature one, capable of fertilization and further

development of an egg with a haploid set of chromosomes and a second polar body. Polar bodies are small cells that play no role in oogenesis and are eventually destroyed.

13.Chromosomes. Their chemical composition, supramolecular organization (DNA packaging levels).

Amitosis, its stages and meaning. Direct cell division, or amitosis. What is amitosis

What is amitosis

From history and research

Process Features

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