Cell life cycle: interphase (the period of preparation of the cell for division) and mitosis (division). Cell division

3.4. CELL CYCLE

An increase in the number of cells occurs by dividing the original cell. Cell division is usually preceded by reduplication of the chromosomal apparatus and DNA synthesis.

The time a cell exists from division to the next division or death is called the cell (life) cycle.

During life, cells grow, differentiate, perform certain functions, multiply, and die.

In the cell cycle, one can distinguish the mitotic cycle, which includes the preparation of cells for division and the division itself. There are periods in the life cycle when cells perform certain functions (Fig. 53).

Rice. 53. Scheme of the relationship between the mitotic cycle and the life cycle of the cell (from Tsanev and Markov, 1964). The inner circle represents the cycle of cell reproduction, which begins preparation for a new mitotic cycle immediately after the completion of division. A possible outcome of the mitotic cycle is shown; a - formation of two new (daughter) cells; b - division of the nucleus without division of the cell body - the formation of a multinucleated cell; c - mitosis occurs only up to the metaphase stage without chromosome divergence - polyploidy; d - DNA reduplication and increase in cell mass without entering mitosis - polyteny. The outer circle shows a differentiating cell with possible outcomes differentiation. 1 - cell death, 2 - final specialization with loss of the cell’s ability to mitotic division, 3 - cell entry into the division cycle without dedifferentiation, 4 - dedifferentiation followed by cell entry into the mitotic cycle. 2c and 4c - diploid and tetraploid amount of DNA, 2n and 4n - diploid and tetraploid set of chromosomes.

In the body of higher vertebrates, not all cells constantly divide. There are specialized cells that have lost the ability to divide (neutrophils, basophils, eosinophils, nerve cells). Other cells are capable of constantly dividing. They are found in renewing tissues (epithelial), in hematopoietic organs. For example, epithelial cells, hematopoietic cells bone marrow can constantly divide, replacing the dead.

Many cells that do not reproduce in normal conditions, begin to divide in the process of recovery after organ damage and reparative regeneration of organs and tissues.

Cells in the cell cycle contain different quantity DNA, depending on the stage of this cycle.

Male and female germ cells have a haploid set of chromosomes (n) and amount of DNA (c). During fertilization, these cells merge, resulting in the formation of a diploid cell with a 2n set of chromosomes and a 4c amount of DNA.

DNA duplication occurs during the synthetic period of interphase. Cells begin to divide only after this period.

3.4.1. PREPARING A CELL FOR DIVISION

In the cell cycle, one can distinguish mitosis itself and interphase, which includes the presynthetic (postmitotic) - G 1 period, the synthetic (S) period and the postsynthetic (premitotic) - G 2 period (Fig. 54).

Rice. 54. Mitotic cycle diploid cell(scheme). G 0 - period of cell life without processes of preparation for division; G 1 - presynthetic (postmitotic) period. Mitosis: P - prophase; M - metaphase, A - anaphase, T - telophase; n - haploid set of chromosomes; 2n - diploid set of chromosomes; 4n - tetroid set of chromosomes; c is the amount of DNA corresponding to the haploid set of chromosomes. Outside the circle, changes in chromosomes are schematically shown different periods cell life cycle.

Cell preparation for division occurs in interphase. The presynthetic period of interphase is the longest. It can last in eukaryotes from 10 hours to several days (Fig. 55).

Rice. 55. Cell cycle in eukaryotes.

In the presynthetic period (G 1), which occurs immediately after division, cells have a diploid (2n) set of chromosomes and 2c DNA genetic material. During this period, cell growth, protein and RNA synthesis begin. Cells are preparing for DNA synthesis (S-period). The activity of enzymes involved in energy metabolism increases (Fig. 56).

Rice. 56. DNA replication and chromosomes. 1 - The double helix unwinds and the base pairs are separated by the DNA enzyme helicase. 2 - Nucleotides are located opposite their complementary nucleotides (A - T, G - C) on the DNA template strand, hydrogen bonds are created, and the nucleotides are covalently bonded using the enzyme DNA polymerase. 3 - Two daughter strands of DNA are synthesized in various ways- one is immediately created as a continuous chain, and the other is synthesized in short sections, which are then linked together by DNA ligase. 4 - Inflow free nucleotides to create new DNA molecules along the unwound DNA template. 5 - Each copy of the DNA double helix is made up of one parent and one daughter strand, a process called semi-conservative replication.

In the S (synthetic) period, the replication of DNA molecules and the synthesis of proteins - histones, with which each DNA strand is connected, occurs. RNA synthesis increases according to the amount of DNA. During replication, two helices of the DNA molecule unwind, hydrogen bonds are broken, and each becomes a template for the reproduction of new DNA strands. The synthesis of new DNA molecules is carried out with the participation of enzymes. Each of the two daughter molecules necessarily includes one old and one new helix. The new molecules are identical to the old ones. This type of replication is called semi-conservative. In the S period, doubling of centrioles begins.

Each chromosome consists of two sister chromatids and contains 4c DNA. The number of chromosomes does not change (2n).

The duration of DNA synthesis - the S period of the mitotic cycle - lasts 6-12 hours in mammals.

In the post-synthetic period (G 2), RNA synthesis occurs, the ATP energy necessary for cell division is accumulated, the duplication of centrioles, mitochondria, and plastids is completed, the proteins from which the achromatin spindle is built are synthesized, and cell growth ends. Neither the DNA content (4c) nor the number of chromosomes (2n) changes (Fig. 57).

Rice. 57. Centrosomal cycle. In an interphase cell, the centrosome doubles to form the two poles of the mitotic spindle. In most animal (but not plant) cells, a pair of centrioles (shown as a pair of short black segments) is embedded in centrosome material (colored), from which microtubules grow. IN certain moment phase G 1 two centrioles diverge by several microns. During the S phase, a daughter centriole begins to form near each old centriole at a right angle to it. The growth of daughter centrioles usually ends in the G2 phase. Initially, both pairs of centrioles remain embedded in a single mass of centrosomal material, forming one centrosome. IN early phase Each pair of centrioles becomes part of a separate microtubule organizing center, from which a radial bundle of microtubules extends - a star. The two stars, which originally lay side by side near the nuclear shell, are now moving away from each other. In late prophase, bundles of polar microtubules belonging to the two stars and interacting with each other selectively elongate as the two centers diverge on the two sides of the nucleus. In this way, the mitotic spindle is quickly formed.

The duration of this period is 3-6 hours. Duration cell cycle different for different cells, but constant for a given tissue.

For example, in culture cancer cells In humans, the duration of the G 1 period is 8.5 hours, S - 6.2 hours, G 2 - 4.6 hours. The duration of mitosis is 0.6 hours. The entire cell cycle lasts 19.9 hours.

Preparing a cell for division

The ability of a cell to reproduce is one of the fundamental properties of living things. Cell division underlies embryogenesis and regeneration.

Regular changes in the structural and functional characteristics of a cell over time constitute the content life cycle of a cell (cell cycle). The cell cycle is the period of a cell's existence from the moment of its formation by dividing the mother cell until its own division or death.

An important component cell cycle is mitotic (proliferative) cycle- a complex of interconnected and time-coordinated events that occur in the process of preparing a cell for division and during division itself. In addition, in life cycle turns on cell execution period multicellular organism specific functions, as well as periods of rest. During periods of rest, the immediate fate of the cell is not determined: it can either begin preparation for mitosis, or begin specialization in a certain functional direction.

The duration of the mitotic cycle for most cells is from 10 to 50 hours. Its length varies significantly: for bacteria it is 20-30 minutes, for a slipper 1-2 times a day, for an amoeba about 1.5 days. The duration of the cycle is regulated by changing the duration of all its periods. Multicellular cells also have different abilities to divide. In early embryogenesis they divide frequently, and in the adult body they mostly lose this ability, as they become specialized. But even in an organism that has reached full development, many cells must divide to replace worn-out cells that are constantly sloughed off and, finally, new cells are needed to heal wounds.

Therefore, in some populations of cells, divisions must occur throughout life. Taking this into account, all cells can be divided into three categories:

1. In the body of higher vertebrates, not all cells constantly divide. There are specialized cells that have lost the ability to divide (neutrophils, basophils, eosinophils, nerve cells). By the time a child is born, nerve cells reach a highly specialized state, losing the ability to divide. During ontogenesis, their number continuously decreases. This circumstance also has one good side; if nerve cells divided, then higher nerve functions(memory, thinking) would be impaired.

2. Another category of cells is also highly specialized, but due to their constant exfoliation, they are replaced by new ones and this function is performed by cells of the same line, but not yet specialized and have not lost the ability to divide. These cells are called renewing cells. An example is the constantly renewed cells of the intestinal epithelium, hematopoietic cells. Even cells bone tissue capable of being formed from unspecialized ones (this can be observed during reparative regeneration bone fractures). Populations of unspecialized cells that retain the ability to divide are usually called stem cells.

3. The third category of cells is an exception, when highly specialized cells under certain conditions can enter the mitotic cycle. It's about about cells that have a long lifespan and where cell division occurs rarely after complete growth. An example is hepatocytes. But if 2/3 of the liver is removed from an experimental animal, then in less than two weeks it is restored to its previous size. The same are the cells of the glands that produce hormones: in normal conditions only a few of them are able to reproduce, and under altered conditions most of them can begin to divide.

Based on the two main events of the mitotic cycle, it is distinguished reproductive And dividing phases corresponding interphase And mitosis classical cytology.

IN initial segment interphase (in eukaryotes 8-10 hours) (postmitotic, presynthetic, or G 1 period) the organizational features of the interphase cell are restored, and the formation of the nucleolus, which began in telophase, is completed. A significant (up to 90%) amount of protein enters the nucleus from the cytoplasm. In the cytoplasm, in parallel with the reorganization of the ultrastructure, protein synthesis intensifies. This promotes cell mass growth. If the daughter cell is to enter the next mitotic cycle, the syntheses become directed: chemical precursors of DNA, enzymes that catalyze the DNA reduplication reaction are formed, and a protein is synthesized that begins this reaction. Thus, the preparation processes are carried out next period interphase - synthetic. Cells have a diploid set of chromosomes 2n and 2c genetic material DNA (genetic formula of the cell).

IN synthetic or S-period (6-10 h) the amount of hereditary material in the cell doubles. With few exceptions reduplication(DNA doubling is sometimes referred to as replication, leaving term reduplication to denote the duplication of chromosomes.) DNA is carried out in a semi-conservative manner. It consists in the divergence of a DNA coil into two chains, followed by the synthesis of a complementary chain near each of them. As a result, two identical coils appear. DNA molecules, complementary to the maternal ones, are formed in separate fragments along the length of the chromosome, and not simultaneously (asynchronously) in different areas one chromosome, as well as on different chromosomes. Then sections (replication units - replicons) newly formed DNA is “stitched” into one macromolecule. A human cell contains more than 50,000 replicons. The length of each of them is about 30 microns. Their number changes during ontogenesis. The meaning of DNA reduplication by replicons becomes clear from the following comparisons. The DNA synthesis rate is 0.5 µm/min. In this case, reduplication of a DNA strand of one human chromosome about 7 cm long would take about three months. The regions of chromosomes where synthesis begins are called initiation points. Perhaps they are the sites of attachment of interphase chromosomes to the inner membrane of the nuclear membrane. One might think that the DNA of individual fractions, which will be discussed below, is reduplicated in a strictly defined phase of the S-period. Thus, most rRNA genes duplicate DNA at the beginning of the period. Reduplication is triggered by a signal entering the nucleus from the cytoplasm, the nature of which is not clear. DNA synthesis in a replicon is preceded by RNA synthesis. In a cage past S-period interphase, chromosomes contain double the amount of genetic material. Along with DNA, RNA and protein are intensively formed in the synthetic period, and the number of histones strictly doubles.

Approximately 1% DNA animal cell located in mitochondria. A small part of mitochondrial DNA is reduplicated in the synthetic period, while the main part is reduplicated in the postsynthetic period of interphase. At the same time, it is known that the lifespan of mitochondria in liver cells, for example, is 10 days. Considering that under normal conditions hepatocytes rarely divide, it should be assumed that reduplication of mitochondrial DNA can occur regardless of the stages of the mitotic cycle. Each chromosome consists of two sister chromatids ( 2n), contains DNA 4c.

The length of time from the end of the synthetic period to the beginning of mitosis is postsynthetic (premitotic), or G 2 -period interphase ( 2n and 4c) (3-6 hours). It is characterized by intense synthesis of RNA and especially protein. The doubling of the mass of the cytoplasm compared to the beginning of interphase is completed. This is necessary for the cell to enter mitosis. Some of the proteins produced (tubulins) are subsequently used to build spindle microtubules. The synthetic and postsynthetic periods are directly related to mitosis. This allows us to isolate them during a special period of interphase - preprophase.

There are three ways of cell division: mitosis, amitosis, meiosis.

Cell life cycle

Patterns of cell existence over time

The ability of a cell to reproduce is one of the fundamental properties of living things. Cell division underlies embryogenesis and regeneration.

Regular changes in the structural and functional characteristics of a cell over time constitute the content life cycle of a cell (cell cycle). The cell cycle is the period of cell existence from the moment of its formation through the division of the mother cell until its own division or death.

An important component of the cell cycle is mitotic (proliferative) cycle- a complex of interconnected and time-coordinated events that occur in the process of preparing a cell for division and during the division itself. At the same time, the life cycle includes cell execution period multicellular organism specific functions, as well as periods of rest. During periods of rest, the immediate fate of the cell is not determined: it can either begin preparation for mitosis, or begin specialization in a certain functional direction.

Consequently, in some populations of cells divisions must occur throughout life. Taking this into account, all cells can be divided into three categories:

1. In the body of higher vertebrates, not all cells constantly divide. There are specialized cells that have lost the ability to divide (neutrophils, basophils, eosinophils, nerve cells). By the time a child is born, nerve cells reach a highly specialized state, losing the ability to divide. During the process of ontogenesis, their number continuously decreases. This circumstance also has one good side; if nerve cells divided, then higher nervous functions (memory, thinking) would be disrupted.

2. Another category of cells is also highly specialized, but due to their constant exfoliation, they are replaced by new ones and this function is performed by cells of the same line, but not yet specialized and have not lost the ability to divide. These cells are called renewing cells. An example is the constantly renewed cells of the intestinal epithelium, hematopoietic cells. Even bone tissue cells can be formed from unspecialized ones (this can be observed during the reparative regeneration of bone fractures). Populations of unspecialized cells that retain the ability to divide are usually called stem cells.

3. The third category of cells is an exception, when highly specialized cells under certain conditions can enter the mitotic cycle. We are talking about cells that have a long lifespan and where, after complete growth, cell division occurs rarely. An example is hepatocytes. But if 2/3 of the liver is removed from an experimental animal, then in less than two weeks it is restored to its previous size. The cells of the glands that produce hormones are the same: under normal conditions, only a few of them are able to reproduce, and under changed conditions, most of them can begin to divide.

Based on the two main events of the mitotic cycle, it is distinguished reproductive And dividing phases corresponding interphase And mitosis classical cytology.

During the initial period of interphase (in eukaryotes 8-10 hours) (postmitotic, presynthetic, or G 1 period) the organizational features of the interphase cell are restored, and the formation of the nucleolus, which began in telophase, is completed. A significant (up to 90%) amount of protein enters the nucleus from the cytoplasm. In the cytoplasm, in parallel with the reorganization of the ultrastructure, protein synthesis intensifies. This promotes cell mass growth. If the daughter cell is about to enter the next mitotic cycle, the syntheses become directed: chemical precursors of DNA, enzymes that catalyze the DNA reduplication reaction are formed, and a protein is synthesized that begins this reaction. However, the processes of preparation for the next interphase period - the synthetic one - are being implemented. Cells have a diploid set of chromosomes 2n and 2c genetic material DNA (genetic formula of the cell).

IN synthetic or S-period (6-10 h) the amount of hereditary material in the cell doubles. With few exceptions reduplication(DNA doubling is sometimes referred to as replication, leaving term reduplication to denote the duplication of chromosomes.) DNA is carried out in a semi-conservative manner. It consists in the divergence of a DNA coil into two chains, followed by the synthesis of a complementary chain near each of them. As a result, two identical coils appear. DNA molecules, complementary to the maternal ones, are formed in separate fragments along the length of the chromosome, and not simultaneously (asynchronously) in different parts of the same chromosome, as well as in different chromosomes. Then sections (replication units - replicons) newly formed DNA is “stitched” into one macromolecule. A human cell contains more than 50,000 replicons. The length of each of them is about 30 microns. Their number changes during ontogenesis. The meaning of DNA reduplication by replicons becomes clear from the following comparisons. The DNA synthesis rate is 0.5 µm/min. In this case, reduplication of a DNA strand of one human chromosome about 7 cm long would take about three months. The regions of chromosomes where synthesis begins are called initiation points. Perhaps they are the sites of attachment of interphase chromosomes to the inner membrane of the nuclear membrane. One might think that the DNA of individual fractions, which will be discussed below, is reduplicated in a strictly defined phase of the S-period. Thus, most rRNA genes duplicate DNA at the beginning of the period. Reduplication is triggered by a signal entering the nucleus from the cytoplasm, the nature of which is not clear. DNA synthesis in a replicon is preceded by RNA synthesis. In a cell that has gone through the S-period of interphase, the chromosomes contain twice the amount of genetic material. Along with DNA, RNA and protein are intensively formed in the synthetic period, and the number of histones strictly doubles.

Approximately 1% of the DNA of an animal cell is found in mitochondria. A small part of mitochondrial DNA is reduplicated in the synthetic period, while the main part is reduplicated in the postsynthetic period of interphase. At the same time, it is known that the lifespan of mitochondria in liver cells, for example, is 10 days. Considering that under normal conditions hepatocytes rarely divide, it should be assumed that reduplication of mitochondrial DNA can occur regardless of the stages of the mitotic cycle. Each chromosome consists of two sister chromatids ( 2n), contains DNA 4c.

The length of time from the end of the synthetic period to the beginning of mitosis is postsynthetic (premitotic), or G 2 -period interphase ( 2n and 4c) (3-6 hours). It is characterized by intense synthesis of RNA and especially protein. The doubling of the mass of the cytoplasm compared to the beginning of interphase is completed. This is extremely important for the cell to enter mitosis. Some of the proteins produced (tubulins) are subsequently used to build spindle microtubules. The synthetic and postsynthetic periods are directly related to mitosis. This allows us to isolate them during a special period of interphase - preprophase.

There are three ways of cell division: mitosis, amitosis, meiosis.

Remember!

How, according to cell theory, is there an increase in the number of cells?

New daughter cells are formed by dividing the mother cell, so the process of reproduction of an organism is cellular in nature.

Do you think life expectancy is the same? different types cells in a multicellular organism? Justify your opinion.

No, the duration depends on the structure and functions performed

Review questions and assignments

1. What is the life cycle of a cell?

The cellular or cell life cycle is the life of a cell from its appearance until division or death. The cell cycle is conventionally divided into two periods: long - interphase, and relatively short - division itself.

2. How does DNA duplication occur in the mitotic cycle? Explain the biological meaning of this process.

DNA duplication occurs in the synthetic phase of interphase. Each DNA molecule turns into two identical daughter DNA molecules. This is necessary so that during cell division each daughter cell receives its own copy of DNA. The DNA helicase enzyme breaks hydrogen bonds between nitrogenous bases, the double strand of DNA unwinds into two single strands. Then the enzyme DNA polymerase completes each single strand into a double strand according to the principle of complementarity. Each daughter DNA contains one chain from the maternal DNA and one newly synthesized one - this is the principle of semi-conservatism. According to the principle of antiparallelism, DNA chains lie with opposite ends to each other. DNA can only extend at the 3" end, so at each replication fork only one of the two strands is synthesized continuously. The second strand (lagging) grows in the 5" direction with the help of short (100-200 nucleotides) Okazaki fragments, each of which grows in 3" direction, and then, with the help of the DNA ligase enzyme, joins the previous strand. The replication speed in eukaryotes is 50-100 nucleotides per second. Each chromosome has many replication origins, from each of which 2 replication forks diverge; due This entire replication takes about an hour, called DNA duplication. complex process its self-reproduction. Thanks to the property of DNA molecules to self-double, reproduction is possible, as well as the transmission of heredity by an organism to its offspring, because complete data on the structure and functioning are encoded in the genetic information of organisms. DNA is the basis of the hereditary materials of most micro- and macroorganisms. Correct name the process of DNA doubling - replication (reduplication).

3. What is the preparation of a cell for mitosis?

The stage of cell preparation for division is called interphase. It is divided into several periods. The presynthetic period (G1) is the longest period of the cell cycle, occurring after cell division (mitosis). Number of chromosomes and

DNA content - 2n2c. U different types cells, the G1 period can last from several hours to several days. During this period, proteins, nucleotides and all types of RNA are actively synthesized in the cell, mitochondria and proplastids (in plants) are divided, ribosomes and all single-membrane organelles are formed, the volume of the cell increases, energy is accumulated, and preparations for DNA reduplication are underway. The synthetic period (S) is the most important period in the life of a cell, during which DNA doubling (reduplication) occurs. The duration of the S period is from 6 to 10 hours. At the same time, there is active synthesis of histone proteins that make up the chromosomes and their migration into the nucleus. By the end of the period, each chromosome consists of two sister chromatids connected to each other at the centromere. Thus, the number of chromosomes does not change (2n), but the amount of DNA doubles (4c). The post-synthetic period (G2) occurs after the completion of chromosome duplication. This is the period of preparation of the cell for division. It lasts 2-6 hours. At this time, energy is actively accumulating for the upcoming division, microtubule proteins (tubulins) and regulatory proteins that trigger mitosis are synthesized.

4. Describe the phases of mitosis sequentially.

The process of mitosis is usually divided into four main phases: prophase, metaphase, anaphase and telophase. Since it is continuous, the change of phases is carried out smoothly - one imperceptibly passes into the other. In prophase, the volume of the nucleus increases, and due to the spiralization of chromatin, chromosomes are formed. By the end of prophase, it is clear that each chromosome consists of two chromatids. The nucleoli and nuclear membrane gradually dissolve, and the chromosomes appear randomly located in the cytoplasm of the cell. Centrioles diverge towards the poles of the cell. An achromatin fission spindle is formed, some of the threads of which go from pole to pole, and some are attached to the centromeres of the chromosomes. The content of genetic material in the cell remains unchanged (2n4c). In metaphase, chromosomes reach maximum spiralization and are arranged in an orderly manner at the equator of the cell, so they are counted and studied during this period. The content of genetic material does not change (2n4c). In anaphase, each chromosome “splits” into two chromatids, which from this point on are called daughter chromosomes. The spindle strands attached to the centromeres contract and pull the chromatids (daughter chromosomes) toward opposite poles of the cell. The content of genetic material in the cell at each pole is represented by a diploid set of chromosomes, but each chromosome contains one chromatid (4n4c). In telophase, the chromosomes located at the poles despiral and become poorly visible. Around the chromosomes at each pole, a nuclear membrane is formed from membrane structures of the cytoplasm, and nucleoli are formed in the nuclei. The fission spindle is destroyed. At the same time, the cytoplasm is dividing. Daughter cells have a diploid set of chromosomes, each of which consists of one chromatid (2n2c).

It consists in the fact that mitosis ensures the hereditary transmission of characteristics and properties in a series of cell generations during the development of a multicellular organism. Thanks to the accurate and uniform distribution Chromosomes during mitosis, all cells of a single organism are genetically identical. Mitotic division cells underlies all forms of asexual reproduction in both unicellular and multicellular organisms. Mitosis determines the most important phenomena of life: growth, development and restoration of tissues and organs and asexual reproduction organisms.

Think! Remember!

1. Explain why the completion of mitosis - division of the cytoplasm occurs differently in animal and plant cells.

Since in plant and animal organisms different cells and fabrics. For example, cells of specialized plant tissues (integumentary, mechanical, conductive) are not capable of division. Therefore, the plant must have tissues whose sole function is the formation of new cells. The possibility of plant growth depends only on them. These are educational tissues, or meristems (from the Greek meristos - divisible).

2. What plant tissue cells actively divide and give rise to all other plant tissues?

Educational tissues, or meristems, consist of small thin-walled large-nucleated cells containing proplastids, mitochondria and small vacuoles, practically indistinguishable under a light microscope. Meristems ensure plant growth and the formation of all other types of tissues. Their cells divide by mitosis. After each division, one of the sister cells retains the properties of the mother, while the other soon stops dividing and begins initial stages differentiation, further forming cells of a certain tissue.

Cell life cycle

Patterns of cell existence over time

The ability of a cell to reproduce is one of the fundamental properties of living things. Cell division underlies embryogenesis and regeneration.

An important component of the cell cycle is mitotic (proliferative) cycle- a complex of interconnected and time-coordinated events that occur in the process of preparing a cell for division and during division itself. In addition, the life cycle includes cell execution period multicellular organism specific functions, as well as periods of rest. During periods of rest, the immediate fate of the cell is not determined: it can either begin preparation for mitosis, or begin specialization in a certain functional direction.

Therefore, in some populations of cells, divisions must occur throughout life. Taking this into account, all cells can be divided into three categories:

2. Another category of cells is also highly specialized, but due to their constant exfoliation, they are replaced by new ones and this function is performed by cells of the same line, but not yet specialized and have not lost the ability to divide. These cells are called renewing cells. An example is the constantly renewed cells of the intestinal epithelium, hematopoietic cells. Even bone tissue cells can be formed from unspecialized ones (this can be observed during the reparative regeneration of bone fractures). Populations of unspecialized cells that retain the ability to divide are usually called stem cells.

3. The third category of cells is an exception, when highly specialized cells under certain conditions can enter the mitotic cycle. We are talking about cells that have a long lifespan and where, after growth is complete, cell division occurs rarely. An example is hepatocytes. But if 2/3 of the liver is removed from an experimental animal, then in less than two weeks it is restored to its previous size. The cells of the glands that produce hormones are the same: under normal conditions, only a few of them are able to reproduce, and under altered conditions, most of them can begin to divide.

Based on the two main events of the mitotic cycle, it is distinguished reproductive And dividing phases corresponding interphase And mitosis classical cytology.

During the initial period of interphase (in eukaryotes 8-10 hours) (postmitotic, presynthetic, or G 1 period) the organizational features of the interphase cell are restored, and the formation of the nucleolus, which began in telophase, is completed. A significant (up to 90%) amount of protein enters the nucleus from the cytoplasm. In the cytoplasm, in parallel with the reorganization of the ultrastructure, protein synthesis intensifies. This promotes cell mass growth. If the daughter cell is to enter the next mitotic cycle, the syntheses become directed: chemical precursors of DNA, enzymes that catalyze the DNA reduplication reaction are formed, and a protein is synthesized that begins this reaction. Thus, the processes of preparation for the next period of interphase - synthetic - are carried out. Cells have a diploid set of chromosomes 2n and 2c genetic material DNA (genetic formula of the cell).

IN synthetic or S-period (6-10 h) the amount of hereditary material in the cell doubles. With few exceptions reduplication(DNA doubling is sometimes referred to as replication, leaving term reduplication to denote the duplication of chromosomes.) DNA is carried out in a semi-conservative manner. It consists in the divergence of a DNA coil into two chains, followed by the synthesis of a complementary chain near each of them. As a result, two identical coils appear. DNA molecules, complementary to the maternal ones, are formed in separate fragments along the length of the chromosome, and not simultaneously (asynchronously) in different parts of the same chromosome, as well as in different chromosomes. Then sections (replication units - replicons) newly formed DNA is “stitched” into one macromolecule. A human cell contains more than 50,000 replicons. The length of each of them is about 30 microns. Their number changes during ontogenesis. The meaning of DNA reduplication by replicons becomes clear from the following comparisons. The DNA synthesis rate is 0.5 µm/min. In this case, reduplication of a DNA strand of one human chromosome about 7 cm long would take about three months. The regions of chromosomes where synthesis begins are called initiation points. Perhaps they are the sites of attachment of interphase chromosomes to the inner membrane of the nuclear membrane. One might think that the DNA of individual fractions, which will be discussed below, is reduplicated in a strictly defined phase of the S-period. Thus, most rRNA genes duplicate DNA at the beginning of the period. Reduplication is triggered by a signal entering the nucleus from the cytoplasm, the nature of which is not clear. DNA synthesis in a replicon is preceded by RNA synthesis. In a cell that has gone through the S-period of interphase, the chromosomes contain twice the amount of genetic material. Along with DNA, RNA and protein are intensively formed in the synthetic period, and the number of histones strictly doubles.

There are three ways of cell division: mitosis, amitosis, meiosis.