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 designated by the letter G1;
3rd - synthetic, it is designated by the letter S;
4th - postsynthetic, or premitotic, it is designated by the letter G 2,
and the mitotic period is represented by the letter M.
After mitosis, the next G1 period begins. During this period, the daughter cell's mass is 2 times less than the mother cell. This cell has 2 times less protein, DNA and chromosomes, i.e. normally there should be 2p chromosomes and 2c DNA.
What happens in the G1 period? At this time, transcription of RNA occurs on the surface of the DNA, which takes 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, DNA replication of chromosomes occurs. As a result, by the end of the S period the DNA content is 4c. But there will be 2n chromosomes, although in fact there will also be 4n, but the DNA of the chromosomes during this period is so intertwined that each sister chromosome in the mother 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. At this time, doubling of centrioles in cells can occur. Thus, a cell from the S period enters the G 2 period. At the beginning of the G2 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 duplication may occur. Mitochondria intensively synthesize ATP, 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 indicated by the letter G0. A cell entering this period loses its ability to undergo mitosis. Moreover, some cells lose their ability to mitosis temporarily, others permanently.
If a cell temporarily loses the ability to undergo 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 G2 period, undergo mitotic division.
Where in the body are cells located in the G0 period? Such 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, rapid restoration of liver parenchyma cells occurs.
Stem cells are also in the G0 period, but when a stem cell begins to divide, it goes through all the interphase periods: G1, S, G2.
Those cells that finally lose the ability to mitotic division undergo first initial differentiation and perform certain functions, and then final differentiation. At terminal differentiation, the cell is unable to return to the cell cycle and eventually dies. Where in the body are these cells located? Firstly, these are blood cells. Blood granulocytes that have undergone differentiation function for 8 days and then die. Red blood cells 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 peeled off from the surface of the epidermis. In the epidermis of the skin, cells can be in the G0 period, the G1 period, the G2 period and the S period.
Tissues with frequently dividing cells are more affected than tissues with rarely 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 an even distribution of chromosomal material between daughter cells. Mitosis is divided into 4 phases. The 1st phase is called prophase, 2nd - metaphase, 3rd - anaphase, 4th - telophase.
If a cell has a half (haploid) set of chromosomes, constituting 23 chromosomes (sex cells), then this set is designated by the symbol In chromosomes and 1c DNA, if diploid - 2p 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, spiralization of chromosomes occurs and they become visible in the form of thin threads and form a dense ball, i.e., a dense ball figure is formed. With the onset of late prophase, the chromosomes spiral 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 stop, and the nucleolus disappears. At the same time, fragmentation of the nuclear membrane occurs. Fragments of the nuclear membrane fold into small vacuoles. The amount of granular EPS in the cytoplasm decreases. The granular EPS tanks are fragmented into smaller structures. The number of ribosomes on the surface of the ER membranes decreases sharply. This leads to a decrease in protein synthesis by 75%. At this point, the cell center doubles. The resulting 2 cell centers begin to diverge towards the poles. Each of the newly formed cell centers consists of 2 centrioles: mother and daughter.
With the participation of cell centers, a fission spindle begins to form, which consists of microtubules. The chromosomes continue to spiral, resulting in the formation of a loose ball of chromosomes located in the cytoplasm. Thus, late prophase is characterized by a loose ball of chromosomes.
Metaphase. During metaphase, the chromatids of the maternal chromosomes become visible. Maternal chromosomes line up in the equatorial plane. If you look at these chromosomes from the equator of the cell, they are perceived as equatorial plate(lamina equatorialis). If you look at the same plate from the side of the pole, then it is perceived as mother star(monastr). During metaphase, spindle formation is completed. Two types of microtubules are visible in the spindle. 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 the kinetochores of the chromosomes. What are kinetochores? In the area of primary chromosome constrictions 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, the simultaneous separation of daughter chromosomes (chromatids) occurs, which begin to move, some to one, and others to the other pole. In this case, a double star appears, i.e. 2 daughter stars (diastr). The movement of stars is carried out thanks to the spindle and the fact that the poles of the cell themselves move somewhat away from each other.
Mechanism, movements of daughter 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 motion of daughter stars stops and cores begin to form. Chromosomes undergo despiralization, and 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 despiralization of chromosome DNA fibrils occurs, rRNA in the form of thin threads begins to be transcribed in the region of nucleolar organizers, i.e., the fibrillar apparatus of the nucleolus is formed. Then ribosomal proteins are transported to the rRNA fibrils, which are complexed with rRNA, resulting in the formation of ribosomal subunits, i.e., a granular component of the nucleolus is formed. This occurs already in late telophase. Cytotomy, i.e., the formation of a constriction. When a constriction forms along the equator, the cytolemma invaginates. The mechanism of invagination is as follows. Tonofilaments, consisting of contractile proteins, are located along the equator. These tonofilaments retract the cytolemma. Then the cytolemma of one daughter cell separates from another similar daughter cell. Thus, as a result of mitosis, new daughter cells are formed. Daughter cells are 2 times less in mass compared to the mother. They also have less DNA - corresponds to 2c, and half the number of chromosomes - corresponds to 2p. Thus, mitotic division ends the cell cycle.
Biological significance of mitosis is that due to division, the growth of the body, physiological and reparative regeneration of cells, tissues and organs occurs.
The G1, S and G2 phases of the cell cycle are collectively called interphase. A dividing cell spends most of its time in interphase as it grows in preparation for division. The mitosis phase involves nuclear separation followed by cytokinesis (division of the cytoplasm into two separate cells). At the end of the mitotic cycle, two different ones are formed. Each cell contains identical genetic material.
The time required to complete cell division depends on its type. For example, cells in the bone marrow, skin cells, stomach and intestinal cells divide quickly and constantly. Other cells divide as needed, replacing damaged or dead cells. These types of cells include cells from the kidneys, liver, and lungs. Others, including nerve cells, stop dividing after maturation.
Scheme of the main phases of the cell cycle
The two main periods of the eukaryotic cell cycle include interphase and mitosis:
During this period, the cell doubles and synthesizes DNA. It is estimated that a dividing cell spends about 90-95% of its time in interphase, which consists of the following 3 phases:
During mitosis and cytokinesis, the contents of the mother cell are evenly distributed between the two daughter cells. Mitosis has five phases: prophase, prometaphase, metaphase, anaphase and telophase.
Cytokinesis
Cytokinesis is the process of separation of the cytoplasm in eukaryotic cells that produces various daughter cells. Cytokinesis occurs at the end of the cell cycle after mitosis or.
During animal cell division, cytokinesis occurs when the contractile ring forms a split furrow that pinches the cell membrane in half. The cell plate is built, which divides the cell into two parts.
Once the cell has completed all phases of the cell cycle, it returns to the G1 phase and the entire cycle repeats again. The body's cells are also capable of entering a state of rest, called the Gap 0 (G0) phase, at any point in their life cycle. They can remain in this stage for a very long period of time until signals are given to move through the cell cycle.
Cells that contain genetic mutations are permanently placed in the G0 phase to prevent them from replicating. When the cell cycle goes wrong, normal cell growth is disrupted. Can develop that gain control of their own growth signals and continue to reproduce unchecked.
Not all cells divide through the process of mitosis. Organisms that reproduce sexually also undergo a type of cell division called meiosis. Meiosis occurs in and is similar to the process of mitosis. However, after a complete cell cycle, meiosis produces four daughter cells. Each cell contains half the number of chromosomes of the original (parent) cell. This means that the sex cells are . When haploid male and female sex cells come together in a process called , they form one called a zygote.
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 cell existence from the moment of its formation 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 life cycle of a cell is divided into several phases:
The duration of the cell cycle and the number of phases included in it are different for cells. For example, after the end of the embryonic period, cells of nervous tissue stop dividing and function throughout the life of the organism, and then die. Another example is embryonic cells. At the crushing stage, having completed one division, they immediately move on to the next, bypassing all other phases.
The following methods of cell division exist:
Mitosis is a continuous process, as a result of which the doubling first occurs, and then the hereditary material is evenly distributed between daughter cells. As a result of mitosis, two cells appear, each of them containing the same number of chromosomes as were contained in the mother cell. Because The chromosomes of daughter cells are derived from the mother's chromosomes through precise DNA replication, and their genes have exactly the same hereditary information. Daughter cells are genetically identical to the parent cell.
Thus, during mitosis, the exact transfer 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 mechanisms of growth. 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 using only one mitotic cell division, i.e. Mitosis underlies asexual reproduction. Thanks to mitosis, cell replacement and regeneration of lost body parts occurs, which is always present to one degree or another in all multicellular organisms. Mitotic cell division occurs under complete genetic control. Mitosis is the central event of the cell's mitotic cycle.
Mitotic cycle - a complex of interconnected and chronologically determined events that occur during the preparation of a cell for division and during cell division itself. The duration of the mitotic cycle can vary greatly among different organisms. The shortest mitotic cycles are found in the cleavage eggs of some animals (for example, in a goldfish, the first cleavage divisions 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, cells of the epithelial tissue of the duodenum divide every 11 hours, the jejunum - every 19 hours, and in the cornea of the eye - every 3 days.
Scientists do not know exactly what factors induce a cell to undergo mitosis. There is an assumption that the main role here is played by the nuclear-cytoplasmic ratio (the ratio of the volumes of the nucleus and cytoplasm). 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 itself division .
New cells are formed through two sequential processes:
The cell division itself usually takes 1-3 hours, therefore the main part of the cell's life is spent in interphase. Interphase is the period of time between two cell divisions. The duration of interphase usually accounts for up to 90% of the entire cell cycle. Interphase consists of three periods: presynthetic or G 1, synthetic or S, and postsynthetic or G 2.
Presynthetic period is the longest period of interphase, its duration ranges from 10 hours to several days. Immediately after division, the organizational features of the interphase cell are restored: the formation of the nucleolus is completed, intensive protein synthesis occurs in the cytoplasm, leading to an increase in cell mass, a supply of DNA precursors, enzymes that catalyze the DNA replication reaction, etc. are formed. Those. During the presynthetic period, preparation processes take place for the next period of interphase - the synthetic period.
Duration synthetic The period may vary: in bacteria it is a few minutes, in mammalian cells it can be up to 6-12 hours. During the synthetic period, the doubling of DNA molecules occurs - the main event of interphase. In this case, each chromosome becomes bichromatid, and their number does not change. Simultaneously with DNA replication in the cytoplasm, an intensive process of synthesis of proteins that make up the chromosomes occurs.
Despite the fact that the period G 2 is called postsynthetic , synthesis processes continue at this stage of interphase. It is called post-synthetic 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 take place, 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 that are necessary to ensure the process of cell division are synthesized; spiralization of chromosomes begins, proteins necessary for the construction of the mitotic apparatus of the cell (division spindle) are synthesized; there is an increase in the mass of the cytoplasm and the volume of the nucleus greatly increases. At the end of the postsynthetic period, the cell begins to divide.
Cell cycle
The cell cycle is the period of existence of a cell from the moment of its formation by division of the mother cell until its own division or death. Contents [show]
The length of the cell cycle varies among different cells. Rapidly reproducing 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 rapid fragmentation of eggs of echinoderms, amphibians and other animals. Under experimental conditions, many cell culture lines have a short cell cycle (about 20 hours). For most actively dividing cells, the period between mitoses is approximately 10-24 hours.
The eukaryotic cell cycle consists of two periods:
A period of cell growth called “interphase,” during which DNA and proteins are synthesized and preparation for cell division occurs.
The period of cell division, called “phase M” (from the word mitosis - mitosis).
Interphase consists of several periods:
G1 phase (from the English gap - gap), or the initial growth phase, during which the synthesis of mRNA, proteins, and other cellular components occurs;
S-phase (from the English synthesis - synthetic), during which DNA replication of the cell nucleus occurs, doubling of centrioles also occurs (if they exist, of course).
G2 phase, during which preparation for mitosis occurs.
In differentiated cells that no longer divide, there may be no G1 phase in the cell cycle. Such cells are in the G0 resting phase.
The period of cell division (phase M) includes two stages:
mitosis (division of the cell nucleus);
cytokinesis (cytoplasm division).
In turn, mitosis is divided into five stages; in vivo, these six stages form a dynamic sequence.
The description of cell division is based on light microscopy data in combination with microcine photography and on the results of light and electron microscopy of fixed and stained cells.
The regular sequence of changes in periods of the cell cycle occurs through 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-derived, epidermal, and nerve growth factors, by binding to their receptors, trigger an intracellular signaling cascade, ultimately leading to the transcription of cyclin genes 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 cell cycle. Cyclin is a regulatory component of the cyclin-cyclin-dependent kinase complex. The kinase is the catalytic component of this complex. Kinases are not active without cyclins. Different cyclins are synthesized at different stages of the cell cycle. Thus, the content of cyclin B in frog oocytes reaches a maximum at the time of mitosis, when the entire cascade of phosphorylation reactions catalyzed by the cyclin B/cyclin-dependent kinase complex is launched. By the end of mitosis, cyclin is rapidly destroyed by proteinases.
To determine the completion of each phase of the cell cycle, it requires the presence of checkpoints. If the cell “passes” the checkpoint, then it continues to “move” through the cell cycle. If some 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 the obstacles are removed, preventing the cell from passing through the checkpoint. There are at least four checkpoints in the cell cycle: a checkpoint in G1, which checks for intact DNA before entering S phase, a checkpoint in S phase, which checks for correct DNA replication, a checkpoint in G2, which checks for lesions missed when passing previous verification points, or obtained at subsequent stages of the cell cycle. In the G2 phase, the completeness of DNA replication is detected and cells in which the DNA is under-replicated do not enter mitosis. At the spindle assembly checkpoint, it is checked that all kinetochores are attached to microtubules.
An increase in the synthesis of the p53 protein leads to the induction of the synthesis of the p21 protein, a cell cycle inhibitor.
Disruption of normal cell cycle regulation is the cause of most solid tumors. In the cell cycle, as already mentioned, passing 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 cell cycle checkpoints. 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 S phase. With mutations leading to the loss of p53 protein genes, or with their changes, blockage of the cell cycle does not occur, the cells enter mitosis, which leads to the appearance of mutant cells, most of which are non-viable, others give rise to malignant cells.
Cyclins are a family of proteins that are activators of cyclin-dependent protein kinases (CDKs), key enzymes involved in the regulation of the eukaryotic cell cycle. Cyclins get their name due to the fact that their intracellular concentration changes periodically as cells pass through the cell cycle, reaching a maximum at certain stages of the cycle.
The catalytic subunit of cyclin-dependent protein kinase is partially activated by interaction with a cyclin molecule, which forms the regulatory subunit of the enzyme. The formation of this heterodimer becomes possible after cyclin reaches a critical concentration. In response to a decrease in cyclin concentration, the enzyme is inactivated. For complete activation of cyclin-dependent protein kinase, specific phosphorylation and dephosphorylation of certain amino acid residues in the polypeptide chains of this complex must occur. One of the enzymes that carries out such reactions is CAK kinase (CAK - CDK activating kinase).
Cyclin-dependent kinase
Cyclin-dependent kinases (CDK) are a group of proteins regulated by cyclin and cyclin-like molecules. Most CDKs are involved in cell cycle phase transitions; they also regulate transcription and processing of mRNA. CDKs are serine/threonine kinases that phosphorylate corresponding protein residues. Several CDKs are known, each of which is activated by one or more cyclins and other similar molecules after reaching their critical concentration, and for the most part CDKs are homologous, differing primarily in the configuration of the cyclin binding site. In response to a decrease in the intracellular concentration of a particular cyclin, the corresponding CDK is reversibly inactivated. If CDKs are activated by a group of cyclins, each of them, as if transferring protein kinases to each other, maintains CDKs in the activated state for a long time. Such waves of CDK activation occur during the G1 and S phases of the cell cycle.
CDK1; cyclin A, cyclin B
CDK2; cyclin A, cyclin E
CDK4; cyclin D1, cyclin D2, cyclin D3
CDK5; CDK5R1, CDK5R2
CDK6; cyclin D1, cyclin D2, cyclin D3
CDK7; cyclin H
CDK8; cyclin C
CDK9; cyclin T1, cyclin T2a, cyclin T2b, cyclin K
CDK11 (CDC2L2); cyclin L
Amitosis (or direct cell division) occurs less frequently in somatic cells of eukaryotes than mitosis. It was first described by the German biologist R. Remak in 1841, the term was proposed by a histologist. V. 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. 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. It is currently 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.
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