Life cycle of a cell: phases, periods. Life cycle of a virus in a host cell. Cell cycle - mitosis: description of phases G0, G1, G2, S Cell cycle of its stages
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.
Biological significance of cell division. New cells arise from the division of existing ones. If a single-celled organism divides, two new ones are formed from it. A multicellular organism also begins its development most often with a single cell. Through repeated divisions, a huge number of cells are formed, which make up the body. Cell division ensures the reproduction and development of organisms, and therefore the continuity of life on Earth.
Cell cycle- the life of a cell from the moment of its formation during the division of the mother cell until its own division (including this division) or death.
During this cycle, each cell grows and develops in such a way as to successfully perform its functions in the body. The cell then functions for a certain time, after which it either divides, forming daughter cells, or dies.
In different types of organisms, the cell cycle takes different times: for example, in bacteria it lasts about 20 minutes, ciliates slippers- from 10 to 20 hours. Cells of multicellular organisms divide frequently in the early stages of development, and then cell cycles lengthen significantly. For example, immediately after a person is born, brain cells divide a huge number of times: 80% of brain neurons are formed during this period. However, most of these cells quickly lose the ability to divide, and some survive until the natural death of the body without dividing at all.
The cell cycle consists of interphase and mitosis (Fig. 54).
Interphase- the interval of the cell cycle between two divisions. During the entire interphase, chromosomes are non-spiralized; they are located in the cell nucleus in the form of chromatin. As a rule, interphase consists of three periods: pre-synthetic, synthetic and postsynthetic.
Presynthetic period (G,)- the longest part of interphase. It can last in different types of cells from 2-3 hours to several days. During this period, the cell grows, the number of organelles increases, energy and substances are accumulated for the subsequent doubling of DNA. During the Gj period, each chromosome consists of one chromatid, i.e. the number of chromosomes ( P) and chromatids (With) matches. Set of chromosomes and chro-
matid (DNA molecules) of a diploid cell in the G r period of the cell cycle can be expressed by writing 2p2s.
In the synthetic period (S) DNA duplication occurs, as well as the synthesis of proteins necessary for the subsequent formation of chromosomes. IN During the same period, doubling of centrioles occurs.
DNA duplication is called replication. During replication, special enzymes separate the two strands of the original parent DNA molecule, breaking the hydrogen bonds between complementary nucleotides. Molecules of DNA polymerase, the main replication enzyme, bind to the separated strands. Then the DNA polymerase molecules begin to move along the mother chains, using them as templates, and synthesize new daughter chains, selecting nucleotides for them according to the principle of complementarity (Fig. 55). For example, if a section of the mother chain of DNA has the nucleotide sequence A C G T G A, then the section of the daughter chain will have the form THCACT. IN In connection with this, replication is referred to as matrix synthesis reactions. IN As a result of replication, two identical double-stranded DNA molecules are formed - IN each of them includes one chain of the original mother molecule and one newly synthesized daughter chain.
By the end of the S-period, each chromosome already consists of two identical sister chromatids connected to each other at the centromere. The number of chromatids in each pair of homologous chromosomes becomes four. Thus, the set of chromosomes and chromatids of a diploid cell at the end of the S-period (i.e. after replication) is expressed by the entry 2p4s.
Postsynthetic period (G 2) occurs after DNA doubling - At this time, the cell accumulates energy and synthesizes proteins for the upcoming division (for example, the protein tubulin for building microtubules, which subsequently form the division spindle). During the entire C 2 period, the set of chromosomes and chromatids in the cell remains unchanged - 2n4c.
Interphase ends and begins division, as a result of which daughter cells are formed. During mitosis (the main way eukaryotic cells divide), the sister chromatids of each chromosome separate from each other and end up in different daughter cells. Consequently, young daughter cells entering a new cell cycle have a set 2p2s.
Thus, the cell cycle covers the period of time from the emergence of a cell to its complete division into two daughter cells and includes interphase (G r, S-, C 2 periods) and mitosis (see Fig. 54). This sequence of periods of the cell cycle is characteristic of constantly dividing cells, for example, for cells of the germinal layer of the epidermis of the skin, red bone marrow, the mucous membrane of the gastrointestinal tract of animals, and cells of the educational tissue of plants. They are able to divide every 12-36 hours.
In contrast, most cells of a multicellular organism take the path of specialization and, after passing through part of the Gj period, can move into the so-called rest period (Go-period). Cells in the G n period perform their specific functions in the body; metabolic and energy processes occur in them, but preparation for replication does not occur. Such cells, as a rule, permanently lose their ability to divide. Examples include neurons, cells in the lens of the eye, and many others.
However, some cells that are in the Gn period (for example, leukocytes, liver cells) can leave it and continue the cell cycle, going through all periods of interphase and mitosis. Thus, liver cells can again acquire the ability to divide after several months of being in a period of rest.
Cell death. The death (death) of individual cells or their groups constantly occurs in multicellular organisms, as well as the death of unicellular organisms. Cell death can be divided into two categories: necrosis (from the Greek. nekros- dead) and ap-ptosis, which is often called programmed cell death or even cell suicide.
Necrosis- death of cells and tissues in a living organism caused by the action of damaging factors. Necrosis can be caused by exposure to high and low temperatures, ionizing radiation, and various chemicals (including toxins released by pathogens). Necrotic cell death is also observed as a result of mechanical damage, disruption of blood supply and innervation of tissues, and allergic reactions.
In damaged cells, membrane permeability is disrupted, protein synthesis stops, other metabolic processes stop, the nucleus, organelles and, finally, the entire cell are destroyed. A feature of necrosis is that entire groups of cells are subject to such death (for example, during myocardial infarction, due to the cessation of oxygen supply, a section of the heart muscle containing many cells dies). Typically, dying cells are attacked by leukocytes, and an inflammatory reaction develops in the area of necrosis.
Apoptosis- programmed cell death, regulated by the body. During the development and functioning of the body, some of its cells die without direct damage. This process occurs at all stages of an organism’s life, even during the embryonic period.
In the adult body, planned cell death also occurs constantly. Millions of blood cells, skin epidermis, gastrointestinal mucosa, etc. die. After ovulation, some of the follicular cells of the ovary die, and after lactation, the cells of the mammary glands die. In the adult human body, 50–70 billion cells die every day as a result of apoptosis. During apoptosis, the cell breaks up into separate fragments surrounded by plasmalemma. Typically, fragments of dead cells are absorbed by white blood cells or neighboring cells without triggering an inflammatory response. Replenishment of lost cells is ensured by division.
Thus, apoptosis seems to interrupt the infinity of cell divisions. From their “birth” to apoptosis, cells go through a certain number of normal cell cycles. After each of them, the cell proceeds either to a new cell cycle or to apoptosis.
1. What is the cell cycle?
2. What is called interphase? What main events occur in the G r, S- and 0 2 periods of interphase?
3. Which cells are characterized by G 0 -nepnofl? What happens during this period?
4. How is DNA replication carried out?
5. Are the DNA molecules that make up homologous chromosomes the same? In the composition of sister chromatids? Why?
6. What is necrosis? Apoptosis? What are the similarities and differences between necrosis and apoptosis?
7. What is the significance of programmed cell death in the life of multicellular organisms?
8. Why do you think that in the vast majority of living organisms the main keeper of hereditary information is DNA, and RNA performs only auxiliary functions?
- § 1. Content of chemical elements in the body. Macro- and microelements
- § 2. Chemical compounds in living organisms. Inorganic substances
- § 10. History of the discovery of the cell. Creation of cell theory
- § 15. Endoplasmic reticulum. Golgi complex. Lysosomes
- § 24. General characteristics of metabolism and energy conversion
Chapter 1. Chemical components of living organisms
Chapter 2. Cell - structural and functional unit of living organisms
Chapter 3. Metabolism and energy conversion in the body
Chapter 4. Structural organization and regulation of functions in living organisms
This lesson allows you to independently study the topic “The Life Cycle of a Cell”. Here we will talk about what plays a major role in cell division, which transmits 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 of organisms
Lesson: Cell Life Cycle
1. Cell cycle
According to cell theory, new cells arise only by dividing previous mother cells. Chromosomes, which contain DNA molecules, play an important role in the processes of cell division, since they ensure the transmission 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 the cell cycle, a cell grows and changes 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 of a multicellular organism cannot divide 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.
2. Stages of the cell cycle
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).
3. Interphase
Let's characterize interphase in more detail (Fig. 2). Interphase consists of 3 periods: G1, S and G2. The first period, presynthetic (G1) is a 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 G1 period substances are synthesized that inhibit or stimulate the next period of the 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 (G2) 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.
4. DNA duplication process
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 duration of the cell cycle depends on the type of cell and on external factors such as temperature, oxygen availability, and nutrient availability. 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 of the nervous system never divide.
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 did the cell theory originate? 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.
5. Internet portal Schooltube.
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.
Human body height is caused by an increase in the size and number of cells, the latter being ensured by the process of division, or mitosis. Cell proliferation occurs under the influence of extracellular growth factors, and the cells themselves undergo a repeating sequence of events known as the cell cycle.
There are four main phases: G1 (presynthetic), S (synthetic), G2 (postsynthetic) and M (mitotic). This is followed by separation of the cytoplasm and plasma membrane, resulting in two identical daughter cells. The phases Gl, S and G2 are part of the interphase. Chromosome replication occurs during the synthetic phase, or S phase.
Majority cells are not subject to active division; their mitotic activity is suppressed during the GO phase, which is part of the G1 phase.
M-phase duration is 30-60 minutes, while the entire cell cycle takes place in about 20 hours. Depending on age, normal (non-tumor) human cells undergo up to 80 mitotic cycles.
Processes cell cycle are controlled by sequentially repeated activation and inactivation of key enzymes called cyclin-dependent protein kinases (CDPKs), as well as their cofactors, cyclins. In this case, under the influence of phosphokinases and phosphatases, phosphorylation and dephosphorylation of special cyclin-CZK complexes occur, which are responsible for the onset of certain phases of the cycle.
In addition, on the relevant stages similar to CZK proteins cause compaction of chromosomes, rupture of the nuclear envelope and reorganization of cytoskeletal microtubules in order to form a fission spindle (mitotic spindle).
G1 phase of the cell cycle
G1 phase- an intermediate stage between the M and S phases, during which the amount of cytoplasm increases. In addition, at the end of the G1 phase there is a first checkpoint where DNA repair and environmental conditions are checked (whether they are favorable enough for the transition to the S phase).
In case nuclear DNA damaged, the activity of the p53 protein increases, which stimulates the transcription of p21. The latter binds to a specific cyclin-CZK complex, responsible for transferring the cell to the S-phase, and inhibits its division at the Gl-phase stage. This allows repair enzymes to correct damaged DNA fragments.
If pathologies occur p53 protein replication of defective DNA continues, which allows dividing cells to accumulate mutations and contributes to the development of tumor processes. This is why the p53 protein is often called the “guardian of the genome.”
G0 phase of the cell cycle
Cell proliferation in mammals is possible only with the participation of cells secreted by other cells. extracellular growth factors, which exert their effect through cascade signal transduction of proto-oncogenes. If during the G1 phase the cell does not receive appropriate signals, then it exits the cell cycle and enters the G0 state, in which it can remain for several years.
The G0 block occurs with the help of proteins - suppressors of mitosis, one of which is retinoblastoma protein(Rb protein) encoded by normal alleles of the retinoblastoma gene. This protein attaches to skew regulatory proteins, blocking the stimulation of transcription of genes necessary for cell proliferation.
Extracellular growth factors destroy the block by activation Gl-specific cyclin-CZK complexes, which phosphorylate the Rb protein and change its conformation, as a result of which the connection with regulatory proteins is broken. At the same time, the latter activate the transcription of the genes they encode, which trigger the process of proliferation.
S phase of the cell cycle
Standard quantity DNA double helices in each cell, the corresponding diploid set of single-stranded chromosomes is usually designated as 2C. The 2C set is maintained throughout G1 phase and doubles (4C) during S phase, when new chromosomal DNA is synthesized.
Starting from the end S-phase and until M phase (including G2 phase), each visible chromosome contains two tightly bound DNA molecules called sister chromatids. Thus, in human cells, from the end of the S-phase to the middle of the M-phase, there are 23 pairs of chromosomes (46 visible units), but 4C (92) double helices of nuclear DNA.
In progress mitosis identical sets of chromosomes are distributed among two daughter cells in such a way that each of them contains 23 pairs of 2C DNA molecules. It should be noted that the G1 and G0 phases are the only phases of the cell cycle during which 46 chromosomes in cells correspond to a 2C set of DNA molecules.
G2 phase of the cell cycle
Second check Point, where cell size is tested, is at the end of the G2 phase, located between S phase and mitosis. In addition, at this stage, before moving on to mitosis, the completeness of replication and DNA integrity are checked. Mitosis (M-phase)
1. Prophase. The chromosomes, each consisting of two identical chromatids, begin to condense and become visible inside the nucleus. At the opposite poles of the cell, a spindle-like apparatus begins to form around two centrosomes from tubulin fibers.
2. Prometaphase. The nuclear membrane divides. Kinetochores form around the centromeres of chromosomes. Tubulin fibers penetrate into the nucleus and concentrate near the kinetochores, connecting them with fibers emanating from the centrosomes.
3. Metaphase. The tension of the fibers causes the chromosomes to line up midway between the spindle poles, thereby forming the metaphase plate.
4. Anaphase. Centromere DNA, shared between sister chromatids, is duplicated, and the chromatids separate and move apart closer to the poles.
5. Telophase. The separated sister chromatids (which from this point on are considered chromosomes) reach the poles. A nuclear membrane appears around each group. The compacted chromatin dissipates and nucleoli form.
6. Cytokinesis. The cell membrane contracts and a cleavage furrow is formed in the middle between the poles, which over time separates the two daughter cells.
Centrosome cycle
In G1 phase time a pair of centrioles linked to each centrosome separates. During the S and G2 phases, a new daughter centriole is formed to the right of the old centrioles. At the beginning of the M phase, the centrosome divides, and two daughter centrosomes move toward the cell poles.
Cell cycle
The cell cycle consists of mitosis (M phase) and interphase. In interphase, phases G 1, S and G 2 are successively distinguished.
STAGES OF THE CELL CYCLE
Interphase
G 1 follows the telophase of mitosis. During this phase, the cell synthesizes RNA and proteins. The duration of the phase ranges from several hours to several days.
G 2 cells can exit the cycle and are in phase G 0 . In phase G 0 cells begin to differentiate.
S. During the S phase, protein synthesis continues in the cell, DNA replication occurs, and centrioles separate. In most cells, the S phase lasts 8-12 hours.
G 2 . In the G 2 phase, the synthesis of RNA and protein continues (for example, the synthesis of tubulin for microtubules of the mitotic spindle). Daughter centrioles reach the size of definitive organelles. This phase lasts 2-4 hours.
MITOSIS
During mitosis, the nucleus (karyokinesis) and cytoplasm (cytokinesis) divide. Phases of mitosis: prophase, prometaphase, metaphase, anaphase, telophase.
Prophase. Each chromosome consists of two sister chromatids connected by a centromere; the nucleolus disappears. Centrioles organize the mitotic spindle. A pair of centrioles is part of the mitotic center, from which microtubules extend radially. First, the mitotic centers are located near the nuclear membrane, and then diverge, and a bipolar mitotic spindle is formed. This process involves pole microtubules, which interact with each other as they elongate.
Centriole is part of the centrosome (the centrosome contains two centrioles and a pericentriole matrix) and has the shape of a cylinder with a diameter of 15 nm and a length of 500 nm; the cylinder wall consists of 9 triplets of microtubules. In the centrosome, the centrioles are located at right angles to each other. During the S phase of the cell cycle, centrioles are duplicated. In mitosis, pairs of centrioles, each consisting of an original and a newly formed one, diverge to the cell poles and participate in the formation of the mitotic spindle.
Prometaphase. The nuclear envelope disintegrates into small fragments. In the region of centromeres, kinetochores appear, functioning as centers for organizing kinetochore microtubules. The departure of kinetochores from each chromosome in both directions and their interaction with the pole microtubules of the mitotic spindle is the reason for the movement of chromosomes.
Metaphase. Chromosomes are located in the equator region of the spindle. A metaphase plate is formed in which each chromosome is held by a pair of kinetochores and associated kinetochore microtubules directed to opposite poles of the mitotic spindle.
Anaphase– divergence of daughter chromosomes to the poles of the mitotic spindle at a speed of 1 µm/min.
Telophase. The chromatids approach the poles, the kinetochore microtubules disappear, and the pole ones continue to elongate. The nuclear envelope is formed and the nucleolus appears.
Cytokinesis– division of the cytoplasm into two separate parts. The process begins in late anaphase or telophase. The plasmalemma is retracted between the two daughter nuclei in a plane perpendicular to the long axis of the spindle. The cleavage furrow deepens, and a bridge remains between the daughter cells - a residual body. Further destruction of this structure leads to complete separation of daughter cells.
Regulators of cell division
Cell proliferation, which occurs through mitosis, is tightly regulated by a variety of molecular signals. The coordinated activity of these multiple cell cycle regulators ensures both the transition of cells from phase to phase of the cell cycle and the precise execution of the events of each phase. The main reason for the appearance of proliferatively uncontrolled cells is mutations in genes encoding the structure of cell cycle regulators. Regulators of the cell cycle and mitosis are divided into intracellular and intercellular. Intracellular molecular signals are numerous, among them, first of all, cell cycle regulators themselves (cyclins, cyclin-dependent protein kinases, their activators and inhibitors) and tumor suppressors should be mentioned.
MEIOSIS
During meiosis, haploid gametes are formed.
First meiotic division
The first division of meiosis (prophase I, metaphase I, anaphase I and telophase I) is reduction.
ProphaseI goes through several stages successively (leptotene, zygotene, pachytene, diplotene, diakinesis).
Leptotene – chromatin condenses, each chromosome consists of two chromatids connected by a centromere.
Zygotene– homologous paired chromosomes come closer and come into physical contact ( synapsis) in the form of a synaptonemal complex that ensures the conjugation of chromosomes. At this stage, two adjacent pairs of chromosomes form a bivalent.
Pachytena– chromosomes thicken due to spiralization. Separate sections of conjugated chromosomes intersect with each other and form chiasmata. Happening here crossing over- exchange of sections between paternal and maternal homologous chromosomes.
Diplotena– separation of conjugated chromosomes in each pair as a result of longitudinal splitting of the synaptonemal complex. The chromosomes are split along the entire length of the complex, with the exception of the chiasmata. In the bivalent, 4 chromatids are clearly distinguishable. Such a bivalent is called a tetrad. Unwinding sites appear in the chromatids where RNA is synthesized.
Diakinesis. The processes of chromosome shortening and splitting of chromosome pairs continue. Chiasmata move to the ends of chromosomes (terminalization). The nuclear membrane is destroyed and the nucleolus disappears. The mitotic spindle appears.
MetaphaseI. In metaphase I, the tetrads form the metaphase plate. In general, paternal and maternal chromosomes are randomly distributed on one side or the other of the equator of the mitotic spindle. This pattern of chromosome distribution underlies Mendel's second law, which (along with crossing over) ensures genetic differences between individuals.
AnaphaseI differs from anaphase of mitosis in that during mitosis sister chromatids move towards the poles. During this phase of meiosis, intact chromosomes move to the poles.
TelophaseI no different from the telophase of mitosis. Nuclei with 23 conjugated (doubled) chromosomes are formed, cytokinesis occurs, and daughter cells are formed.
Second division of meiosis.
The second division of meiosis - equational - proceeds in the same way as mitosis (prophase II, metaphase II, anaphase II and telophase), but much faster. Daughter cells receive a haploid set of chromosomes (22 autosomes and one sex chromosome).