This lesson allows you to independently study the topic " Life cycle cells." On it we'll talk about what's playing main role during cell division, which transfers genetic information from one generation to the next. You will also study the entire life cycle of a cell, which is also called the sequence of events that occurs from the moment a cell forms until it divides.
Topic: Reproduction and individual development organisms
Lesson: Cell Life Cycle
According to cell theory, new cells arise only by dividing previous mother cells. , which contain DNA molecules, play important role in the processes of cell division, since they ensure the transfer of genetic information from one generation to another.
Therefore, it is very important that the daughter cells receive the same amount of genetic material, and it is quite natural that before cell division the doubling of the genetic material, that is, the DNA molecule, occurs (Fig. 1).
What is the cell cycle? Cell life cycle- the sequence of events occurring from the moment of formation of a given cell until its division into daughter cells. According to another definition, the cell cycle is the life of a cell from the moment it appears as a result of the division of the mother cell until its own division or death.
During cell cycle the cell grows and changes so as to successfully perform its functions in a multicellular organism. This process is called differentiation. The cell then successfully performs its functions for a certain period of time, after which it begins to divide.
It is clear that all cells multicellular organism cannot be divided endlessly, otherwise all creatures, including humans, would be immortal.
Rice. 1. Fragment of a DNA molecule
This does not happen because there are “death genes” in the DNA that are activated under certain conditions. They synthesize certain enzyme proteins that destroy cell structures and organelles. As a result, the cell shrinks and dies.
This programmed cell death is called apoptosis. But in the period from the moment the cell appears and before apoptosis, the cell goes through many divisions.
The cell cycle consists of 3 main stages:
1. Interphase is a period of intensive growth and biosynthesis of certain substances.
2. Mitosis, or karyokinesis (nuclear division).
3. Cytokinesis (cytoplasm division).
Let's characterize the stages of the cell cycle in more detail. So, the first one is interphase. Interphase is the longest phase, a period of intense synthesis and growth. The cell synthesizes many substances necessary for its growth and the implementation of all its inherent functions. During interphase, DNA replication occurs.
Mitosis is the process of nuclear division in which chromatids are separated from each other and redistributed as chromosomes between daughter cells.
Cytokinesis is the process of separation of cytoplasm between two daughter cells. Usually, under the name mitosis, cytology combines stages 2 and 3, that is, cell division (karyokinesis) and cytoplasmic division (cytokinesis).
Let's characterize interphase in more detail (Fig. 2). Interphase consists of 3 periods: G 1, S and G 2. The first period, presynthetic (G 1) is the phase of intensive cell growth.
Rice. 2. The main stages of the cell life cycle.
Here the synthesis of certain substances occurs; this is the longest phase that follows cell division. In this phase, the accumulation of substances and energy necessary for the subsequent period occurs, that is, for the doubling of DNA.
According to modern concepts, in the G 1 period substances are synthesized that inhibit or stimulate next period cell cycle, namely the synthetic period.
The synthetic period (S) usually lasts from 6 to 10 hours, in contrast to the presynthetic period, which can last up to several days and involves DNA duplication as well as the synthesis of proteins, such as histone proteins, which can form chromosomes. By the end of the synthetic period, each chromosome consists of two chromatids connected to each other by a centromere. During the same period, the centrioles double.
The post-synthetic period (G 2) occurs immediately after chromosome doubling. It lasts from 2 to 5 hours.
During this same period, the energy necessary for the further process of cell division, that is, directly for mitosis, accumulates.
During this period, the division of mitochondria and chloroplasts occurs, and proteins are synthesized, which will subsequently form microtubules. Microtubules, as you know, form the spindle filament, and the cell is now ready for mitosis.
Before moving on to a description of cell division methods, let's consider the process of DNA duplication, which leads to the formation of two chromatids. This process occurs in the synthetic period. The doubling of a DNA molecule is called replication or reduplication (Fig. 3).
Rice. 3. The process of DNA replication (reduplication) (synthetic period of interphase). The helicase enzyme (green) unwinds the DNA double helix, and DNA polymerases (blue and orange) complete the complementary nucleotides.
During replication, part of the maternal DNA molecule is unraveled into two strands with the help of a special enzyme - helicase. Moreover, this is achieved by breaking hydrogen bonds between complementary nitrogenous bases (A-T and G-C). Next, for each nucleotide of the diverged DNA strands, the DNA polymerase enzyme adjusts a complementary nucleotide to it.
This creates two double-stranded DNA molecules, each of which includes one strand of the parent molecule and one new daughter strand. These two DNA molecules are absolutely identical.
It is impossible to unwind the entire large DNA molecule at the same time for replication. Therefore, replication begins in separate sections of the DNA molecule, short fragments are formed, which are then stitched into a long strand using certain enzymes.
The length of the cell cycle depends on the cell type and external factors such as temperature, oxygen availability, presence nutrients. For example, bacterial cells under favorable conditions divide every 20 minutes, intestinal epithelial cells every 8-10 hours, and onion root tip cells divide every 20 hours. And some cells nervous system never share.
The emergence of cell theory
In the 17th century, the English physician Robert Hooke (Fig. 4), using a homemade light microscope, saw that cork and other plant tissues consisted of small cells separated by partitions. He called them cells.
Rice. 4. Robert Hooke
In 1738, the German botanist Matthias Schleiden (Fig. 5) came to the conclusion that plant tissues consist of cells. Exactly a year later, zoologist Theodor Schwann (Fig. 5) came to the same conclusion, but only regarding animal tissues.
Rice. 5. Matthias Schleiden (left) Theodor Schwann (right)
He concluded that animal tissues, like plant tissues, are composed of cells and that cells are the basis of life. Based on cellular data, scientists formulated the cell theory.
Rice. 6. Rudolf Virchow
20 years later, Rudolf Virchow (Fig. 6) expanded the cell theory and came to the conclusion that cells can arise from other cells. He wrote: “Where a cell exists, there must be a previous cell, just as animals come only from an animal, and plants only from a plant... All living forms, whether animal or plant organisms, or their constituent parts, are dominated by the eternal law of continuous development."
Chromosome structure
As you know, chromosomes play a key role in cell division because they pass genetic information from one generation to the next. Chromosomes consist of a DNA molecule bound to histone proteins. Ribosomes also contain a small amount of RNA.
In dividing cells, chromosomes are presented in the form of long thin threads, evenly distributed throughout the entire volume of the nucleus.
Individual chromosomes are not distinguishable, but their chromosomal material is stained with basic dyes and is called chromatin. Before cell division, the chromosomes (Fig. 7) thicken and shorten, which allows them to be clearly seen under a light microscope.
Rice. 7. Chromosomes in prophase 1 of meiosis
In a dispersed, that is, stretched state, chromosomes participate in all biosynthetic processes or regulate biosynthetic processes, and during cell division this function is suspended.
In all forms of cell division, the DNA of each chromosome is replicated so that two identical, double polynucleotide strands of DNA are formed.
Rice. 8. Chromosome structure
These chains are surrounded by a protein shell and at the beginning of cell division they look like identical threads lying side by side. Each thread is called a chromatid and is connected to the second thread by a non-staining region called a centromere (Fig. 8).
Homework
1. What is the cell cycle? What stages does it consist of?
2. What happens to the cell during interphase? What stages does interphase consist of?
3. What is replication? What is its biological significance? When does it happen? What substances are involved in it?
4. How it started cell theory? Name the scientists who participated in its formation.
5. What is a chromosome? What is the role of chromosomes in cell division?
1. Technical and humanitarian literature ().
2. Unified collection of Digital Educational Resources ().
3. Unified collection of Digital Educational Resources ().
4. Unified collection of Digital Educational Resources ().
Bibliography
1. Kamensky A. A., Kriksunov E. A., Pasechnik V. V. General biology 10-11 grade Bustard, 2005.
2. Biology. Grade 10. General biology. Basic level / P. V. Izhevsky, O. A. Kornilova, T. E. Loshchilina and others - 2nd ed., revised. - Ventana-Graf, 2010. - 224 pp.
3. Belyaev D.K. Biology 10-11 grade. General biology. A basic level of. - 11th ed., stereotype. - M.: Education, 2012. - 304 p.
4. Biology 11th grade. General biology. Profile level / V. B. Zakharov, S. G. Mamontov, N. I. Sonin and others - 5th ed., stereotype. - Bustard, 2010. - 388 p.
5. Agafonova I. B., Zakharova E. T., Sivoglazov V. I. Biology 10-11 grade. General biology. A basic level of. - 6th ed., add. - Bustard, 2010. - 384 p.
The period of life of a cell from the moment of its birth as a result of division of the mother cell until the next division or death is called life (cellular) cycle of a cell.
| The cell cycle of cells capable of reproducing includes two stages: - INTERPHASE (stage between divisions, interkinesis); - DIVISION PERIOD (mitosis). In interphase, the cell prepares for division - the synthesis of various substances, but the main thing is the doubling of DNA. In terms of duration, it makes up most of the life cycle. Interphase consists of 3 periods: 1) Presynthetic - G1 (ji one) - occurs immediately after the end of division. The cell grows, accumulates various substances (rich in energy), nucleotides, amino acids, enzymes. Preparing for DNA synthesis. A chromosome contains 1 DNA molecule (1 chromatid). 2) Synthetic – S material is duplicated – DNA molecules are replicated. Proteins and RNA are intensively synthesized. The number of centrioles doubles. |
3) Postsynthetic G2 – premitotic, RNA synthesis continues. Chromosomes contain 2 copies of themselves - chromatids, each of which carries 1 DNA molecule (double-stranded). The cell is ready to divide; the chromosome is sporalized.
Amitosis - direct division
Mitosis – indirect division
Meiosis – reduction division
Amitosis– occurs rarely, especially in senescent cells or when pathological conditions(tissue repair), the nucleus remains in the intephase state, chromosomes are not sporalized. The nucleus is divided by constriction. The cytoplasm may not divide, then binucleate cells are formed.
MITOSIS- a universal method of division. In the life cycle it is only a small part. The cycle of cat intestinal epithemal cells is 20–22 hours, mitosis is 1 hour. Mitosis consists of 4 phases.
1) PROPHASE - shortening and thickening of chromosomes occurs (spiralization); they are clearly visible. Chromosomes consist of 2 chromatids (doubling during interphase). The nucleolus and nuclear membrane disintegrate, the cytoplasm and karyoplasm mix. The divided cell centers diverge along the long axis of the cell towards the poles. A fission spindle (consisting of elastic protein filaments) is formed.
2) METOPHASE - chromosomes are located in the same plane along the equator, forming a metaphase plate. The spindle consists of 2 types of threads: some connect cell centers, the second (their number = number of chromosomes is 46) are attached, one end to the centrosome (cellular center), the other to the centromere of the chromosome. The centromere also begins to divide into 2. The chromosomes (at the end) are split at the centromere.
3) ANAPHASE – the shortest phase of mitosis. The spindle strands begin to shorten and the chromatids of each chromosome move away from each other towards the poles. Each chromosome consists of only 1 chromatid.
4) TELOPHASE - chromosomes are concentrated in the corresponding cell centers, despiralize. Nucleoli and the nuclear membrane are formed, and a membrane is formed that separates sister cells from each other. Sister cells separate.
Biological significance Mitosis is that as a result, each daughter cell receives exactly the same set of chromosomes, and therefore exactly the same genetic information that the mother cell possessed.
7. MEIOSIS – DIVISION, MATURATION OF GERMS CELLS
The essence of sexual reproduction is the fusion of two nuclei of germ cells (gametes) of sperm (husband) and egg (wives). During development, germ cells undergo mitotic division, and during maturation, meiotic division. Therefore, mature germ cells contain a haploid set of chromosomes (p): P + P = 2P (zygote). If the gametes had 2n (diploid), then the descendants would have a tetraploid (2n+2n) = 4n number of chromosomes, etc. The number of chromosomes in parents and offspring remains constant. The number of chromosomes is halved by meiosis (gametogenesis). It consists of 2 consecutive divisions:
Reductive
Equational (equalizing)
without interphase between them.
PROPHASE 1 IS DIFFERENT FROM PROPHASE OF MITOSIS.
1. Leptonema (thin filaments) in the nucleus, a diploid set (2p) of long thin chromosomes 46 pcs.
2. Zygonema – homologous chromosomes (paired) – 23 pairs in humans are conjugated (zipper) “fitting” gene to gene are connected along the entire length 2p – 23 pcs.
3.Pachynema (thick filaments) homologue. chromosomes are closely connected (bivalent). Each chromosome consists of 2 chromatids, i.e. bivalent - from 4 chromatids.
4.Diplonema (double strands) conjugation of chromosomes repel each other. There is a twisting, and sometimes an exchange of broken parts of chromosomes - a crossover (crossing over) - this sharply increases hereditary variability, new combinations of genes.
5. Diakinesis (movement into the distance) - prophase ends, the chromosomes are speralized, the nuclear membrane disintegrates and the second phase begins - metaphase of the first division.
Metaphase 1 – bivalents (tetrads) lie along the equator of the cell, the spindle is formed (23 pairs).
Anaphase 1 - not just one chromatid, but two chromosomes move to each pole. The connection between homologous chromosomes is weakened. Paired chromosomes move away from each other to different poles. A haploid set is formed.
Telophase 1 - a single, haploid set of chromosomes is assembled at the spindle poles, in which each type of chromosome is represented not by a pair, but by the 1st chromosome consisting of 2 chromatids; the cytoplasm is not always divided.
Meiosis 1- division leads to the formation of cells carrying a haploid set of chromosomes, but the chromosomes consist of 2 chromatids, i.e. have double the amount of DNA. Therefore, the cells are already ready for the 2nd division.
Meiosis 2 division (equivalent). All stages: prophase 2, metaphase 2, anaphase 2 and telophase 2. Proceeds as mitosis, but haploid cells divide.
As a result of division, the maternal double-stranded chromosomes split to form single-stranded daughter chromosomes. Each cell (4) will have a haploid set of chromosomes.
THAT. as a result of 2 methotic divisions occurs:
Hereditary variability increases due to different combinations of chromosomes in daughter sets
The number of possible combinations of chromosome pairs = 2 to the power of n (the number of chromosomes in a haploid set is 23 - humans).
The main purpose of meiosis is to create cells with a haploid set of chromosomes - this is achieved due to the formation of pairs of homologous chromosomes at the beginning of the 1st meiotic division and the subsequent divergence of homologues into different daughter cells. The formation of male germ cells is spermatogenesis, and the formation of female germ cells is oogenesis.
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 period G 2 continues active process transcription of various RNAs and the process of protein synthesis, mainly tubulin proteins, which are necessary for the division spindle. 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, fast recovery liver parenchyma cells.
Stem cells are also in the G 0 period, 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 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 pole, 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.
Material from Wikipedia - the free encyclopedia
Cell cycle- this is the period of existence of a cell from the moment of its formation through division of the mother cell until its own division or death.
Duration of the cell cycle of eukaryotes
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 the 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). 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:
- 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:
- G 1-phase (from English. gap- interval), or the initial growth phase, during which the synthesis of mRNA, proteins, and other cellular components occurs;
- S-phase (from English. synthesis- synthesis), during which DNA replication of the cell nucleus occurs, doubling of centrioles also occurs (if they exist, of course).
- G 2 phase, during which preparation for mitosis occurs.
In differentiated cells that no longer divide, there may be no G 1 phase in the cell cycle. Such cells are in the resting phase G0.
The period of cell division (phase M) includes two stages:
- karyokinesis (division of the cell nucleus);
- cytokinesis (cytoplasm division).
In turn, mitosis is divided into five stages.
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.
Cell cycle regulation
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 factors 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 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. On different stages During the cell cycle, different cyclins are synthesized. 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.
Cell cycle checkpoints
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 Obstacles that prevented the cell from passing through the checkpoint have been removed. 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 DNA is under-replicated do not enter mitosis. At the spindle assembly checkpoint, it is checked that all kinetochores are attached to microtubules.
Cell cycle disorders and tumor formation
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. For tumor cells Characteristic 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.
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Literature
- Kolman, J., Rehm, K., Wirth, Y., (2000). ‘Visual biochemistry’,
- Chentsov Yu. S., (2004). ‘Introduction to Cell Biology’. M.: ICC "Akademkniga"
- Kopnin B.P., ‘Mechanisms of action of oncogenes and tumor suppressors’
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An excerpt characterizing the Cell Cycle
“Residents of Moscow!Your misfortunes are cruel, but His Majesty the Emperor and King wants to stop their course. Terrible examples have taught you how he punishes disobedience and crime. Strict measures are taken to stop the disorder and restore everyone's safety. The paternal administration, elected from among yourself, will constitute your municipality or city government. It will care about you, about your needs, about your benefit. Its members are distinguished by a red ribbon, which will be worn over the shoulder, and the city head will have a white belt on top of it. But, except during their office, they will only have a red ribbon around their left hand.
The city police were established according to the previous situation, and through its activities a better order exists. The government appointed two general commissars, or chiefs of police, and twenty commissars, or private bailiffs, stationed in all parts of the city. You will recognize them by the white ribbon they will wear around their left arm. Some churches of different denominations are open, and divine services are celebrated in them without hindrance. Your fellow citizens return daily to their homes, and orders have been given that they should find in them help and protection following misfortune. These are the means that the government used to restore order and alleviate your situation; but in order to achieve this, it is necessary that you unite your efforts with him, so that you forget, if possible, your misfortunes that you have endured, surrender to the hope of a less cruel fate, be sure that an inevitable and shameful death awaits those who dare to your persons and your remaining property, and in the end there was no doubt that they would be preserved, for such is the will of the greatest and fairest of all monarchs. Soldiers and residents, no matter what nation you are! Restore public trust, the source of happiness of the state, live like brothers, give mutual help and protection to each other, unite to refute the intentions of evil-minded people, obey the military and civil authorities, and soon your tears will stop flowing.”
Regarding the food supply of the troops, Napoleon ordered all troops to take turns going to Moscow a la maraude [looting] to procure provisions for themselves, so that in this way the army would be provided for the future.
On the religious side, Napoleon ordered ramener les popes [bring back the priests] and resume services in the churches.
In terms of trade and food for the army, the following was posted everywhere:
Proclamation
“You, calm Moscow residents, artisans and working people, whom misfortunes have removed from the city, and you, absent-minded farmers, whom unfounded fear still detains in the fields, listen! Silence returns to this capital, and order is restored in it. Your fellow countrymen come out boldly from their shelters, seeing that they are respected. Any violence committed against them and their property is immediately punished. His Majesty the Emperor and King protects them and among you does not consider anyone to be his enemies, except those who disobey his commands. He wants to end your misfortunes and return you to your courts and your families. Comply with his charitable intentions and come to us without any danger. Residents! Return with confidence to your homes: you will soon find ways to satisfy your needs! Craftsmen and hardworking craftsmen! Come back to your handicrafts: houses, shops, security guards are waiting for you, and for your work you will receive the payment due to you! And you, peasants, finally come out of the forests where you hid in horror, return without fear to your huts, in the exact assurance that you will find protection. Storehouses have been established in the city, where peasants can bring their excess supplies and land plants. The government has taken the following measures to ensure they free sale: 1) Counting from this date, peasants, farmers and those living in the vicinity of Moscow can, without any danger, bring their supplies to the city, no matter what their family, in two designated storage areas, that is, on Mokhovaya and in Okhotny Ryad. 2) These foodstuffs will be purchased from them at such a price as the buyer and seller agree upon; but if the seller does not receive the fair price he demands, then he will be free to take them back to his village, which no one can prevent him from doing under any circumstances. 3) Every Sunday and Wednesday are assigned weekly for large trading days; why a sufficient number of troops will be stationed on Tuesdays and Saturdays on all major roads, at such a distance from the city, to protect those carts. 4) The same measures will be taken so that there are no obstacles on the way back to the peasants with their carts and horses. 5) The funds will be immediately used to restore normal trading. Residents of the city and villages, and you, workers and artisans, no matter what nation you are! You are called upon to fulfill the paternal intentions of His Majesty the Emperor and the King and to contribute with him to the general welfare. Bring respect and trust to his feet and do not hesitate to unite with us!”
In order to raise the morale of the troops and the people, reviews were constantly held and awards were given out. The emperor rode horseback through the streets and consoled the residents; and despite all the concern state affairs, himself visited the theaters established on his orders.
In terms of charity, the best valor of the crowned people, Napoleon also did everything that depended on him. On charitable institutions he ordered the inscription Maison de ma mere [My Mother's House], uniting by this act the tender filial feeling with the greatness of the virtue of the monarch. He visited the Orphanage and, letting the orphans he saved kiss his white hands, graciously talked with Tutolmin. Then, according to Thiers’ eloquent account, he ordered that the salaries of his troops be distributed in Russian, made by him, with counterfeit money. Relevant l"emploi de ces moyens par un acte digue de lui et de l"armee Francaise, il fit distribuer des secours aux incendies. Mais les vivres etant trop precieux pour etre donnes a des etrangers la plupart ennemis, Napoleon aima mieux leur fournir de l "argent afin qu"ils se fournissent au dehors, et il leur fit distribuer des roubles papiers. [Elevating the use of these measures to an action worthy of him and the French army, he ordered the distribution of benefits to the burnt. But, since food supplies were too expensive to give to people of a foreign land and for the most part hostile, Napoleon considered it best to give them money so that they could obtain food for themselves on the side; and he ordered that they be provided with paper rubles.]
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 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.
Periods and phases of the cell cycle
Scheme of the main phases of the cell cycle
The two main periods of the eukaryotic cell cycle include interphase and mitosis:
Interphase
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:
- Phase G1: the period of time before DNA synthesis. During this phase, the cell increases in size and number in preparation for division. in this phase they are diploid, meaning they have two sets of chromosomes.
- S-phase: stage of the cycle during which DNA is synthesized. Most cells have a narrow window of time during which DNA synthesis occurs. The chromosome content doubles in this phase.
- Phase G2: the period after DNA synthesis but before the onset of mitosis. The cell synthesizes additional proteins and continues to grow in size.
Phases of mitosis
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.
- Prophase: at this stage, changes occur both in the cytoplasm and in the dividing cell. condenses into discrete chromosomes. Chromosomes begin to migrate to the center of the cell. The nuclear envelope breaks down and spindle fibers form at opposite poles of the cell.
- Prometaphase: mitosis phase in eukaryotes somatic cells after prophase and before metaphase. In prometaphase, the nuclear membrane breaks down into numerous “membrane vesicles”, and the chromosomes inside form protein structures called kinetochores.
- Metaphase: at this stage, the nuclear one completely disappears, a spindle is formed, and the chromosomes are located on the metaphase plate (a plane that is equally distant from the two poles of the cell).
- Anaphase: at this stage, the paired chromosomes () separate and begin to move towards opposite ends (poles) of the cell. The fission spindle, which is not connected to the spindle, extends and lengthens the cell.
- Telophase: At this stage, the chromosomes reach new nuclei, and the genetic content of the cell is divided equally into two parts. Cytokinesis (eukaryotic cell division) begins before the end of mitosis and ends shortly after 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 groove that pinches 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 at this stage for a very long time. long period time until signals are received to move through the cell cycle.
Cells that contain genetic mutations, are permanently placed in 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.
Cell cycle and meiosis
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.
