The cell cycle and its periods. Cell cycle - mitosis: description of phases G0, G1, G2, S What is the cell cycle of a cell

The life cycle of a cell includes the beginning of its formation and the end of its existence as an independent unit. Let's start with the fact that a cell appears during the division of its mother cell, and ends its existence due to the next division or death.

The life cycle of a cell consists of interphase and mitosis. It is in this period that the period under consideration is equivalent to the cellular one.

Cell life cycle: interphase

This is the period between two mitotic cell divisions. The reproduction of chromosomes proceeds similarly to the reduplication (semi-conservative replication) of DNA molecules. In interphase, the cell nucleus is surrounded by a special two-membrane membrane, and the chromosomes are untwisted, and are invisible under ordinary light microscopy.

When staining and fixing cells, an accumulation of a strongly colored substance, chromatin, occurs. It is worth noting that the cytoplasm contains all the required organelles. This ensures the full existence of the cell.

In the life cycle of a cell, interphase is accompanied by three periods. Let's consider each of them in more detail.

Periods of the cell life cycle (interphases)

The first one is called resynthetic. The result of previous mitosis is an increase in the number of cells. Here, the transcription of newly made RNA (informational) molecules proceeds, and the molecules of the remaining RNA are systematized, proteins are synthesized in the nucleus and cytoplasm. Some substances of the cytoplasm are gradually broken down with the formation of ATP, its molecules are endowed with macroergic bonds, they transfer energy to where it is not enough. In this case, the cell increases, in size it reaches the mother. This period lasts a long time for specialized cells, during which they carry out their special functions.

The second period is known as synthetic(DNA synthesis). Its blockade can lead to a stop of the entire cycle. This is where the replication of DNA molecules takes place, as well as the synthesis of proteins that are involved in the formation of chromosomes.

DNA molecules begin to bind to protein molecules, as a result of which the chromosomes thicken. At the same time, reproduction of centrioles is observed, as a result, 2 pairs of them appear. The new centriole in all pairs is placed relative to the old one at an angle of 90°. Subsequently, each pair during the next mitosis moves away to the cell poles.

The synthetic period is characterized by both increased DNA synthesis and a sharp jump in the formation of RNA molecules, as well as proteins in cells.

Third period - postsynthetic. It is characterized by the presence of cell preparation for subsequent division (mitotic). This period lasts, as a rule, always less than others. Sometimes it falls out altogether.

Generation time duration

In other words, this is how long life cycle cells. The duration of the generation time, as well as individual periods, takes on different values ​​for different cells. This can be seen from the table below.

So, the shortest cell life cycle is in cambial. It happens that the third period completely falls out - the postsynthetic one. For example, in a 3-week-old rat in the cells of its liver, it decreases to half an hour, while the duration of the generation time is 21.5 hours. The duration of the synthetic period is the most stable.

In other situations, in the first period (presynthetic), the cell accumulates properties for the implementation of specific functions, this is due to the fact that its structure becomes more complex. If the specialization has not gone too far, it can go through the full life cycle of the cell with the formation of 2 new cells in mitosis. In this situation, the first period may increase significantly. For example, in the cells of the skin epithelium of a mouse, the generation time, namely 585.6 hours, falls on the first period - presynthetic, and in the cells of the periosteum of a rat cub - 102 hours out of 114.

The main part of this time is called the G0-period - this is the implementation of an intensive specific cell function. Many liver cells are in this period, as a result of which they have lost their ability to mitosis.

If a part of the liver is removed, most of its cells will go on to full life, first of the synthetic, then of the postsynthetic period, and at the end of the mitotic process. So, for various kinds of cell populations, the reversibility of such a G0-period has already been proven. In other situations, the degree of specialization increases so much that under typical conditions, cells can no longer divide mitotically. Occasionally, endoreproduction occurs in them. In some, it is repeated more than once, the chromosomes thicken so much that they can be seen with an ordinary light microscope.

Thus, we have learned that in the life cycle of a cell, interphase is accompanied by three periods: presynthetic, synthetic, and postsynthetic.

cell division

It underlies reproduction, regeneration, transmission of hereditary information, development. The cell itself exists only in the intermediate period between divisions.

Life cycle (cell division) - the period of existence of the unit in question (begins from the moment of its appearance through the division of the mother cell), including the division itself. It ends with its own division or death.

Phases of the cell cycle

There are only six of them. The following phases of the cell life cycle are known:

The duration of the life cycle, as well as the number of phases in it, each cell has its own. So, in the nervous tissue, cells at the end of the initial embryonic period stop dividing, then only function throughout the life of the organism itself, and subsequently die. But the cells of the embryo in the stage of crushing first complete 1 division, and then immediately, bypassing the remaining phases, proceed to the next one.

Methods of cell division

Out of just two:

1. Mitosis is indirect cell division.
2. Meiosis- this is characteristic of such a phase as the maturation of germ cells, division.

Now we will learn more about what constitutes the life cycle of a cell - mitosis.

Indirect cell division

Mitosis is the indirect division of somatic cells. This is a continuous process, the result of which is first doubling, then the same distribution between the daughter cells of the hereditary material.

Biological significance of indirect cell division

It is as follows:

1. The result of mitosis is the formation of two cells, each containing the same number of chromosomes as the mother. Their chromosomes are formed by exact replication of the mother's DNA, as a result of which the genes of the daughter cells contain identical hereditary information. They are genetically identical to the parent cell. So, we can say that mitosis ensures the identity of the transmission of hereditary information to daughter cells from the mother.

2. The result of mitoses is a certain number of cells in the corresponding organism - this is one of the most important growth mechanisms.

3. A large number of animals and plants reproduce precisely asexually through mitotic cell division, therefore mitosis forms the basis of vegetative reproduction.

4. It is mitosis that provides complete regeneration of lost parts, as well as cell replacement, which occurs to a certain extent in any multicellular organisms.

Thus, it became known that the life cycle of a somatic cell consists of mitosis and interphase.

Mechanism of mitosis

The division of the cytoplasm and nucleus are 2 independent processes that proceed continuously, sequentially. But for the sake of convenience in studying the events occurring during the period of division, it is artificially distinguished into 4 stages: pro-, meta-, ana-, telophase. Their duration varies depending on the type of tissue, external factors, physiological state. The longest are the first and last.

Prophase

There is a noticeable increase in the core. As a result of spiralization, compaction and shortening of chromosomes occur. In the later prophase, the structure of chromosomes is already clearly visible: 2 chromatids, which are connected by a centromere. The movement of chromosomes to the equator of the cell begins.

From the cytoplasmic material in the prophase (late), a division spindle is formed, which is formed with the participation of centrioles (in animal cells, in a number of lower plants) or without them (cells of some protozoa, higher plants). Subsequently, 2-type spindle filaments begin to appear from the centrioles, more precisely:

• support, which connect the cell poles;
• chromosomal (pulling), which cross in metaphase to chromosomal centromeres.

At the end of this phase, the nuclear membrane disappears, and the chromosomes are located freely in the cytoplasm. Usually the core disappears a little earlier.

metaphase

Its beginning is the disappearance of the nuclear envelope. Chromosomes first line up in the equatorial plane, forming the metaphase plate. In this case, the chromosomal centromeres are strictly located in the equatorial plane. The spindle fibers attach to chromosomal centromeres, and some of them pass from one pole to the other without being attached.

Anaphase

Its beginning is the division of the centromeres of chromosomes. As a result, the chromatids are transformed into two separate daughter chromosomes. Further, the latter begin to diverge towards the cell poles. They usually take a special V-shape at this time. This divergence is carried out by accelerating the spindle threads. At the same time, the support threads are elongated, resulting in the distance of the poles from each other.

Telophase

Here the chromosomes gather at the cell poles, then disspiralize. Next, the division spindle is destroyed. The nuclear envelope of the daughter cells is formed around the chromosomes. This completes karyokinesis, followed by cytokinesis.

Mechanisms of entry of the virus into the cell

There are only two of them:

1. By fusion of the viral supercapsid and the cell membrane. As a result, the nucleocapsid is released into the cytoplasm. Subsequently, the realization of the properties of the virus genome is observed.

2. Through pinocytosis (receptor-mediated endocytosis). Here the virus binds at the site of the bordered fossa with receptors (specific). The latter bulges into the cell, and then transforms into the so-called bordered vesicle. This, in turn, contains the engulfed virion, fuses with a temporary intermediate vesicle called an endosome.

Intracellular replication of the virus

After entering the cell, the genome of the virus completely subordinates its life to its own interests. Through the protein-synthesizing system of the cell and its systems of energy generation, it embodies its own reproduction, sacrificing, as a rule, the life of the cell.

The figure below shows the life cycle of a virus in a host cell (Semliki forests - a representative of the genus Alphvirus). Its genome is represented by single-stranded positive non-fragmented RNA. There, the virion is equipped with a supercapsid, which consists of a lipid bilayer. About 240 copies of a number of glycoprotein complexes pass through it. The viral life cycle begins with its absorption on the host cell membrane, where it binds to a protein receptor. Penetration into the cell is carried out through pinocytosis.

Conclusion

The article considered the life cycle of a cell, its phases were described. It is described in detail about each period of the interphase.

cell cycle(cyclus cellularis) is the period from one cell division to another, or the period from cell division to its death. The cell cycle is divided into 4 periods.

The first period is mitotic;

2nd - postmitotic, or presynthetic, it is denoted by the letter G1;

3rd - synthetic, it is denoted by the letter S;

4th - postsynthetic, or premitotic, it is denoted by the letter G 2,

and the mitotic period - the letter M.

After mitosis, the next period G1 begins. During this period, the daughter cell is 2 times smaller in mass than the mother cell. In this cage 2 times less protein, DNA and chromosomes, i.e., normally, chromosomes should contain 2n and DNA - 2s.

What happens in period G1? At this time, transcription of RNA occurs on the surface of DNA, which take part in the synthesis of proteins. Due to proteins, the mass of the daughter cell increases. At this time, DNA precursors and enzymes involved in the synthesis of DNA and DNA precursors are synthesized. The main processes in the G1 period are the synthesis of proteins and cell receptors. Then comes the S period. During this period, chromosome DNA replication occurs. As a result, by the end of period S, the DNA content is 4c. But there will be 2p chromosomes, although in fact there will also be 4p, but the DNA of the chromosomes during this period is so mutually intertwined that each sister chromosome in the maternal chromosome is not yet visible. As their number increases as a result of DNA synthesis and the transcription of ribosomal, messenger and transport RNAs increases, protein synthesis naturally increases as well. At this time, doubling of centrioles in cells can occur. Thus, a cell from period S enters period G 2 . At the beginning of the G 2 period, the active process of transcription of various RNAs and the process of protein synthesis, mainly tubulin proteins, which are necessary for the division spindle, continue. Centriole doubling may occur. In mitochondria, ATP is intensively synthesized, which is a source of energy, and energy is necessary for mitotic cell division. After the G2 period, the cell enters the mitotic period.

Some cells may exit the cell cycle. The exit of a cell from the cell cycle is denoted by the letter G0. A cell entering this period loses the ability to mitosis. Moreover, some cells lose the ability to mitosis temporarily, others permanently.

In the event that a cell temporarily loses the ability to mitotic division, it undergoes initial differentiation. In this case, a differentiated cell specializes to perform a specific function. After initial differentiation, this cell is able to return to the cell cycle and enter the Gj period and, after passing through the S period and the G 2 period, undergo mitotic division.

Where in the body are cells in period G 0 ? These cells are found in the liver. But if the liver is damaged or part of it is surgically removed, then all the cells that have undergone initial differentiation return to the cell cycle, and due to their division, the liver parenchymal cells are quickly restored.

Stem cells are also in period G 0 , but when stem cell begins to divide, it goes through all periods of interphase: G1, S, G 2.

Those cells that finally lose the ability to mitotic division first undergo initial differentiation and perform certain functions, and then final differentiation. With final differentiation, the cell cannot return to the cell cycle and eventually dies. Where are these cells found in the body? First, they are blood cells. Blood granulocytes that have undergone differentiation function for 8 days, and then die. Blood erythrocytes function for 120 days, then they also die (in the spleen). Secondly, these are the cells of the epidermis of the skin. Epidermal cells undergo first initial, then final differentiation, as a result of which they turn into horny scales, which are then sloughed off from the surface of the epidermis. In the epidermis of the skin, cells can be in the G 0 period, the G1 period, the G 2 period, and the S period.

Tissues with rapidly dividing cells are more affected than tissues with infrequently dividing cells because a number of chemical and physical factors destroy spindle microtubules.

MITOSIS

Mitosis is fundamentally different from direct division or amitosis in that during mitosis there is a uniform distribution of chromosomal material between daughter cells. Mitosis is divided into 4 phases. 1st phase is called prophase 2nd - metaphase 3rd - anaphase, 4th - telophase.

If the cell has a half (haploid) set of chromosomes, comprising 23 chromosomes (sex cells), then such a set is indicated by the symbol In chromosomes and 1c DNA, if diploid - 2n chromosomes and 2c DNA (somatic cells immediately after mitotic division), an aneuploid set of chromosomes - in abnormal cells.

Prophase. Prophase is divided into early and late. During early prophase, chromosomes spiralize, and they become visible in the form of thin threads and form a dense ball, i.e., a dense ball is formed. With the onset of late prophase, the chromosomes coil even more, as a result of which the genes for the nucleolar chromosome organizers are closed. Therefore, rRNA transcription and the formation of chromosome subunits cease, and the nucleolus disappears. At the same time, fragmentation of the nuclear envelope occurs. Fragments of the nuclear envelope roll up into small vacuoles. In the cytoplasm, the amount of granular ER decreases. Cisterns of granular ER are fragmented into smaller structures. The number of ribosomes on the surface of ER membranes decreases sharply. This leads to a decrease in protein synthesis by 75%. By this time, doubling of the cell center occurs. The resulting 2 cell centers begin to diverge towards the poles. Each of the newly formed cell centers consists of 2 centrioles: maternal and daughter.

With the participation of cell centers, the division spindle begins to form, which consists of microtubules. Chromosomes continue to spiral, and as a result, a loose tangle of chromosomes is formed, located in the cytoplasm. Thus, late prophase is characterized by a loose tangle of chromosomes.

Metaphase. During metaphase, the chromatids of the maternal chromosomes become visible. Maternal chromosomes line up in the plane of the equator. If you look at these chromosomes from the side of the cell equator, then they are perceived as equatorial plate(lamina equatorialis). In the event that you look at the same plate from the side of the pole, then it is perceived as mother star(monaster). During metaphase, the formation of the fission spindle is completed. 2 types of microtubules are visible in the spindle of division. Some microtubules are formed from the cell center, i.e. from the centriole, and are called centriolar microtubules(microtubuli cenriolaris). Other microtubules begin to form from kinetochore chromosomes. What are kinetochores? In the area of ​​primary constrictions of chromosomes there are so-called kinetochores. These kinetochores have the ability to induce self-assembly of microtubules. This is where the microtubules begin, which grow towards the cell centers. Thus, the ends of the kinetochore microtubules extend between the ends of the centriolar microtubules.

Anaphase. During anaphase, there is a simultaneous separation of daughter chromosomes (chromatids), which begin to move one to one, others to the other pole. In this case, a double star appears, i.e. 2 child stars (diastr). The movement of stars is carried out due to the spindle of division and the fact that the poles of the cell themselves are somewhat removed from each other.

Mechanism, movement of child stars. This movement is ensured by the fact that the ends of the kinetochore microtubules slide along the ends of the centriolar microtubules and pull the chromatids of the daughter stars towards the poles.

Telophase. During telophase, the movement of daughter stars stops and nuclei begin to form. Chromosomes undergo despiralization, a nuclear envelope (nucleolemma) begins to form around the chromosomes. Since the DNA fibrils of chromosomes undergo despiralization, transcription begins

RNA on discovered genes. Since the DNA fibrils of the chromosomes are despiralized, rRNA begins to be transcribed in the form of thin threads in the region of the nucleolar organizers, i.e., the fibrillar apparatus of the nucleolus is formed. Then, ribosomal proteins are transported to rRNA fibrils, which are complexed with rRNA, resulting in the formation of ribosome subunits, i.e., the granular component of the nucleolus is formed. This occurs already in the late telophase. cytotomy, i.e., constriction formation. With the formation of a constriction along the equator, the cytolemma is invaginated. The invagination mechanism is as follows. Along the equator are tonofilaments, consisting of contractile proteins. It is these tonofilaments that draw in the cytolemma. Then there is a separation of the cytolemma of one daughter cell from another such daughter cell. So, as a result of mitosis, new daughter cells are formed. Daughter cells are 2 times smaller in mass compared to the parent. They also have less DNA - corresponds to 2c, and half the number of chromosomes - corresponds to 2p. So, mitotic division ends the cell cycle.

The biological significance of mitosis is that due to division, the body grows, physiological and reparative regeneration of cells, tissues and organs.

cell division- a set of processes due to which two or more daughter cells are formed from one mother cell. Cell division is the biological basis of life. In the case of unicellular organisms, new organisms are formed due to cell division. In multicellular organisms, cell division is associated with asexual and sexual reproduction, growth and restoration of many of their structures. The primary task of cell division is the transfer of hereditary information to the next generation. Prokaryotic cells do not have a formed nucleus, so their cell division into two smaller daughter cells, known as binary Separation, made easier and faster. There are several types of cell division in eukaryotes:

mitotic division- division, in which two daughter cells with the same set of chromosomes are formed from one mother cell (for somatic cells)

meiotic division - division, in which four daughter cells with a half (haploid) set of chromosomes are formed from one mother cell (in organisms with sexual reproduction)

budding - division in which two daughter cells are formed from one mother cell, one of which is larger than the other (for example, in yeast)

multiple division(schizogony) - division in which many daughter cells are formed from one mother cell (for example, in malarial plasmodium).

Cell division is part of the cell cycle. cell cycle- this is the period of existence of a cell from one division to another. The duration of this period is different in different organisms (for example, in bacteria - 20-30 minutes, for human leukocytes - 4-5 days) and depends on age, temperature, amount of DNA, cell type, and the like. In unicellular organisms, the cell cycle coincides with the life of an individual, and in multicellular organisms, in body cells that are continuously dividing, it coincides with the mitotic cycle. The molecular processes that occur during the cell cycle are sequential. It is impossible to carry out the cell cycle in the opposite direction. An important feature of all eukaryotes is that the transverse phases of the cell cycle are subject to precise coordination. One phase of the cell cycle is replaced by another in a strictly established order, and before the start of the next phase, all biochemical processes characteristic of the previous phase must be completed properly. Cell cycle disruptions can lead to chromosomal abnormalities. For example, part of the chromosomes may be lost, inadequately distributed between two daughter cells, and the like. Similar chromosomal abnormalities are characteristic of cancer cells. There are two main classes of regulatory molecules that direct the cell cycle. These are cyclins and cyclin-dependent enzyme kinases. L. Gartwell, R. Hunt and P. Nurse received Nobel Prize in Medicine and Physiology 2001 for the discovery of these central molecules in the regulation of the cell cycle.

The main periods of the cell cycle are interphase, mitosis and cytokinesis.

cell cycle= Interphase + Mitosis + Cytokinesis

Interphase (lat. Inter - between, phasis - appearance) - the period between cell divisions or from cell division to its death.

The duration of interphase, as a rule, is up to 90% of the time of the entire cell cycle. The main sign of interphase cells is the despiralized state of chromatin. In cells that have lost the ability to divide (for example, neurons), interphase will be the period from the last mitosis to cell death.

Interphase ensures cell growth, doubling of DNA molecules, synthesis of organic compounds, reproduction of mitochondria, it accumulates energy in ATP, which is necessary to ensure cell division.

Interphase includes presynthetic, synthetic and postsynthetic periods. Presynthetic period(G1-phase) - characterized by cell growth. During this period, which is the longest, cells grow, differentiate and perform their functions. In differentiated cells that no longer divide, there is no G1 phase in the cell cycle. Such cells are in a dormant period (G0-phase). Synthetic period(S-phase) is the period in which the main event is DNA duplication. Each chromosome in this period becomes two-chromatid. Postsynthetic period(G2-phase) - the period of immediate preparation for mitosis.

Major events during interphase

Mitosis is the main type of eukaryotic cell division. This section consists of 4 phases ( prophase, metaphase, anaphase, telophase) and lasts from several minutes to 2-3 hours.

Tsntokinez(or cytotomy) - division of the cytoplasm of a eukaryotic cell, which occurs after the division of the nucleus has occurred in the cell (mitosis). In most cases, the cytoplasm and organelles of the cell are distributed approximately equally among the daughter cells. An exception is oogenesis, during which the future egg cell receives almost all of the cytoplasm and organelles, while the polar bodies contain almost none of them and soon die off. In cases where nuclear division is not accompanied by cytokinesis, multinucleated cells are formed (for example, cross-blinking muscle fibers). Cytokinesis occurs immediately after telophase. In animal cells, during telophase, the plasma membrane begins to fold inwards at the equator level (under the action of microfilaments) and divides the cell in half. In plant cells at the equator, a body is formed from microfilaments - phragmoblast. Mitochondria, ER, Golgi apparatus, ribosomes move to it. Bubbles from the Golgi apparatus combine and form a cell plate, which grows and merges with the cell wall of the mother cell.

BIOLOGY +apoptosis is a phenomenon of programmed cell death. Unlike another type of cell death - necrosis- during apoptosis, there is no destruction of the cytoplasmic membrane and, accordingly, the content of the cell does not enter the extracellular environment. characteristic feature is the fragmentation of DNA by a specific enzyme endonuclease into fragments. The process of apoptosis is necessary for the physiological regulation of the number of cells in the body, for the destruction of old cells, for autumn leaf fall, for the cytotoxic effect of killer lymphocytes, for the embryogenesis of the body, etc. Violation of normal cell apoptosis leads to uncontrolled cell reproduction and the appearance of a tumor.

The reproduction and development of organisms, the transmission of hereditary information, and regeneration are based on cell division. The cell as such exists only in the time interval between divisions.

The period of existence of a cell from the moment it begins to form by dividing the mother cell (i.e., the division itself is also included in this period) until the moment of its own division or death is called vital or cell cycle.

The cell life cycle is divided into several phases:

• fission phase (this phase when mitotic division occurs);
• growth phase (immediately after division, cell growth begins, it increases in volume and reaches some specific size);
• resting phase (in this phase, the fate of the cell in the future has not yet been determined: the cell can start preparing for division, or follow the path of specialization);
• phase of differentiation (specialization) (comes at the end of the growth phase - at this time the cell receives certain structural and functional features);
• maturity phase (the period of functioning of the cell, the performance of certain functions, depending on specialization);
• aging phase (a period of weakening of the vital functions of a cell, which ends with its division or death).

The duration of the cell cycle and the number of phases included in it are different in cells. For example, cells of the nervous tissue after the end of the embryonic period stop dividing and function throughout the life of the organism, and then die. Another example, embryonic cells. At the stage of crushing, having completed one division, they immediately move on to the next, bypassing, at the same time, all other phases.

There are the following methods of cell division:

1. mitosis or karyokinesis - indirect division;
2. meiosis or reduction division - division, which is characteristic of the phase of maturation of germ cells or the formation of spores in higher spore plants.

Mitosis is a continuous process, as a result of which, first, doubling occurs, and then a uniform distribution of hereditary material between daughter cells. As a result of mitosis, two cells appear, each of them contains the same number of chromosomes as contained in the mother cell. Because the chromosomes of the daughter cells are derived from the maternal chromosomes with the help of precise DNA replication, their genes have exactly the same hereditary information. Daughter cells are genetically identical to the parent cell.
Thus, during mitosis, the exact transmission of hereditary information from parent to daughter cells occurs. The number of cells in the body increases as a result of mitosis, which is one of the main growth mechanisms. It should be remembered that cells with different chromosome sets can divide by mitosis - not only diploid (somatic cells of most animals), but also haploid (many algae, gametophytes of higher plants), triploid (endosperm of angiosperms) or polyploid.

There are many species of plants and animals that reproduce asexually with only one mitotic cell division, i.e. Mitosis is the basis of asexual reproduction. Thanks to mitosis, the replacement of cells and the regeneration of lost body parts occur, which is always present to one degree or another in all multicellular organisms. Mitotic cell division proceeds under complete genetic control. Mitosis is the central event of the mitotic cell cycle.

Mitotic cycle - a complex of interconnected and chronologically determined events that occur during the preparation of the cell for division and during the cell division itself. In different organisms, the duration of the mitotic cycle can vary greatly. The shortest mitotic cycles are found in the crushing eggs of some animals (for example, in a goldfish, the first divisions of crushing occur every 20 minutes). The most common duration of mitotic cycles is 18-20 hours. There are also cycles lasting several days. Even in different organs and tissues of the same organism, the duration of the mitotic cycle can be different. For example, in mice, epithelial tissue cells duodenum divide every 11 hours, the jejunum - every 19 hours, and in the cornea of ​​the eye - every 3 days.

What exactly factors induce a cell to mitosis is not known to scientists. There is an assumption that the nuclear-cytoplasmic ratio (the ratio of the volumes of the nucleus and cytoplasm) plays the main role here. There is also evidence that dying cells produce substances that can stimulate cell division.

There are two main events in the mitotic cycle: interphase and actually division .

New cells are formed in two sequential processes:

1. mitosis leading to doubling of the nucleus;
2. cytokinesis - division of the cytoplasm, in which two daughter cells appear, which each contain one daughter nucleus.

The cell division itself usually takes 1-3 hours, therefore, the main part of the cell's life takes place in the interphase. Interphase The time interval between two cell divisions is called. The duration of interphase is usually up to 90% of the entire cell cycle. Interphase consists of three periods: presynthetic or G 1 , synthetic or S, and postsynthetic or G2.

Presynthetic period is the longest period of interphase, its duration is from 10 hours to several days. Immediately after division, the features of the organization of the interphase cell are restored: the formation of the nucleolus is completed, an intensive synthesis of proteins in the cytoplasm occurs, leading to an increase in the mass of cells, a supply of DNA precursors, enzymes that catalyze the reaction of DNA replication, etc. are formed. Those. in the presynthetic period, processes of preparation for the next period of the interphase, the synthetic one, take place.

Duration synthetic the period may vary: in bacteria it is several minutes, in mammalian cells it can reach up to 6-12 hours. During the synthetic period, doubling of DNA molecules occurs - the main event of interphase. In this case, each chromosome becomes two-chromatid, and their number does not change. Simultaneously with DNA replication in the cytoplasm, an intensive process of synthesis of proteins that make up chromosomes occurs.

Despite the fact that the period G 2 is called postsynthetic , the processes of synthesis at this stage of the interphase continue. It is called postsynthetic only because it begins after the end of the process of DNA synthesis (replication). If in the presynthetic period growth and preparation for DNA synthesis is carried out, then in the postsynthetic period, the cell is prepared for division, which is also characterized by intensive synthesis processes. During this period, the process of synthesis of proteins that make up the chromosomes continues; energy substances and enzymes are synthesized, which are necessary to ensure the process of cell division; the spiralization of chromosomes begins, the proteins necessary for building the mitotic apparatus of the cell (the division spindle) are synthesized; there is an increase in the mass of the cytoplasm and greatly increases the volume of the nucleus. At the end of the postsynthetic period, the cell begins to divide.

From Wikipedia, the free encyclopedia

Cell cycle- this is the period of existence of a cell from the moment of its formation by dividing the mother cell to its own division or death.

Length of the eukaryotic cell cycle

The length of the cell cycle varies from cell to cell. Rapidly multiplying cells of adult organisms, such as hematopoietic or basal cells of the epidermis and small intestine, can enter the cell cycle every 12-36 hours. Short cell cycles (about 30 minutes) are observed during the rapid crushing of eggs of echinoderms, amphibians and other animals. Under experimental conditions, many cell culture lines have a short cell cycle (about 20 h). In most actively dividing cells, the period between mitoses is approximately 10-24 hours.

Phases of the eukaryotic cell cycle

The eukaryotic cell cycle consists of two periods:

• The period of cell growth, called "interphase", during which DNA and proteins are synthesized and preparations are made for cell division.
• The period of cell division, called "phase M" (from the word mitosis - mitosis).

Interphase consists of several periods:

• G 1 -phase (from the English. gap- gap), or the phase of initial growth, during which mRNA, proteins, and other cellular components are synthesized;
• S-phases (from English. synthesis- synthesis), during which the DNA of the cell nucleus is replicated, centrioles are also doubled (if they exist, of course).
• G 2 -phase, during which there is preparation for mitosis.

Differentiated cells that no longer divide may lack the G 1 phase in the cell cycle. Such cells are in the resting phase G 0 .

The period of cell division (phase M) includes two stages:

• karyokinesis (nucleus division);
• cytokinesis (division of the cytoplasm).

In turn, mitosis is divided into five stages.

The description of cell division is based on the data of light microscopy in combination with microfilming and on the results of light and electron microscopy of fixed and stained cells.

Cell cycle regulation

The regular sequence of changing periods of the cell cycle is carried out during the interaction of proteins such as cyclin-dependent kinases and cyclins. Cells in the G0 phase can enter the cell cycle when exposed to growth factors. Various growth factors, such as platelet, epidermal, and nerve growth factors, by binding to their receptors, trigger an intracellular signaling cascade, ultimately leading to the transcription of genes for cyclins and cyclin-dependent kinases. Cyclin-dependent kinases become active only when interacting with the corresponding cyclins. The content of various cyclins in the cell changes throughout the entire cell cycle. Cyclin is a regulatory component of the cyclin-cyclin-dependent kinase complex. Kinase is the catalytic component of this complex. Kinases are not active without cyclins. On the different stages cell cycle, various cyclins are synthesized. Thus, the content of cyclin B in frog oocytes reaches a maximum by the time of mitosis, when the entire cascade of phosphorylation reactions catalyzed by the cyclin B/cyclin-dependent kinase complex is triggered. By the end of mitosis, cyclin is rapidly degraded by proteinases.

Cell cycle checkpoints

To determine the completion of each phase of the cell cycle, it is necessary to have checkpoints in it. If the cell "passes" the checkpoint, then it continues to "move" through the cell cycle. If any circumstances, such as DNA damage, prevent the cell from passing through a checkpoint, which can be compared to a kind of checkpoint, then the cell stops and another phase of the cell cycle does not occur, at least until obstacles that prevented the cage from passing through the checkpoint were removed. There are at least four cell cycle checkpoints: a checkpoint in G1 where DNA integrity is checked before entering S-phase, a checkpoint in S-phase where DNA replication is checked for correctness, a checkpoint in G2 where damages missed are checked for when passing previous checkpoints, or obtained at subsequent stages of the cell cycle. In the G2 phase, the completeness of DNA replication is detected, and cells in which DNA is underreplicated do not enter mitosis. At the spindle assembly checkpoint, it is checked whether all kinetochores are attached to microtubules.

Cell cycle disorders and tumor formation

Violation of the normal regulation of the cell cycle is the cause of most solid tumors. In the cell cycle, as already mentioned, the passage of checkpoints is possible only if the previous stages are completed normally and there are no breakdowns. Tumor cells are characterized by changes in the components of the checkpoints of the cell cycle. When cell cycle checkpoints are inactivated, dysfunction of several tumor suppressors and proto-oncogenes is observed, in particular p53, pRb, Myc and Ras. The p53 protein is one of the transcription factors that initiates the synthesis of the p21 protein, which is an inhibitor of the CDK-cyclin complex, which leads to cell cycle arrest in the G1 and G2 periods. Thus, a cell whose DNA is damaged does not enter the S phase. When mutations lead to the loss of p53 protein genes, or when they change, cell cycle blockade does not occur, cells enter mitosis, which leads to the appearance of mutant cells, most of which are not viable, while others give rise to malignant cells.

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Literature

1. Kolman J., Rem K., Wirth Y., (2000). ‘Visual biochemistry’,
2. Chentsov Yu.S., (2004). ‘Introduction to Cell Biology’. M.: ICC "Akademkniga"
3. Kopnin B. P., ‘Mechanisms of action of oncogenes and tumor suppressors’

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