periods of the cell cycle. cell cycle

This lesson allows you to independently study the topic "Cell Life Cycle". On it we will talk about what plays a major role in cell division, what transmits genetic information from one generation to another. You will also study the entire life cycle of a cell, which is also called the sequence of events that take place from the moment a cell is formed to its division.

Topic: Reproduction and individual development of organisms

Lesson: Life cycle of a cell

According to the cell theory, new cells arise only through the division of previous mother cells. , which contain DNA molecules, play an important role in the processes cell division, because they provide 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 there is a doubling of the genetic material, that is, the DNA molecule (Fig. 1).

What is the cell cycle? Cell life cycle- the sequence of events occurring from the moment of formation of a given cell to 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 to its own division or death.

During the cell cycle, the cell grows and changes in such a way as to successfully perform its functions in a multicellular organism. This process is called differentiation. Then the cell successfully performs its functions for a certain period of time, after which it proceeds to division.

It is clear that all cells of a multicellular organism cannot divide indefinitely, otherwise all beings, including humans, would be immortal.

Rice. 1. A 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 proteins-enzymes that destroy the structure of the cell, its 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 to apoptosis, the cell goes through many divisions.

The cell cycle consists of 3 main stages:

1. Interphase - a period of intensive growth and biosynthesis of certain substances.

2. Mitosis, or karyokinesis (nucleus fission).

3. Cytokinesis (division of the cytoplasm).

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 intensive 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 separate from each other and are redistributed in the form of chromosomes between daughter cells.

Cytokinesis is the process of division of the cytoplasm between two daughter cells. Usually under the name mitosis, cytology combines stages 2 and 3, that is, cell division (karyokinesis), and division of the cytoplasm (cytokinesis).

Let's characterize the 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.

This is where the synthesis of certain substances takes place, this is the longest phase that follows cell division. In this phase, there is an accumulation of substances and energy necessary for the next period, that is, for DNA doubling.

According to modern concepts, in the G 1 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 6 to 10 hours, in contrast to the pre-synthetic period, which can last up to several days and includes 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 this period, the centrioles double.

The postsynthetic period (G 2) occurs immediately after the doubling of the chromosomes. It lasts from 2 to 5 hours.

During the same period, the energy necessary for the further process of cell division, that is, directly for mitosis, is accumulated.

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 thread, and now the cell is ready for mitosis.

Before proceeding to a description of the methods of cell division, consider the process of DNA duplication, which leads to the formation of two chromatids. This process takes place in the synthetic period. The duplication 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 untwisted into two strands with the help of a special enzyme, helicase. Moreover, this is achieved by breaking the hydrogen bonds between complementary nitrogenous bases (A-T and G-C). Further, for each nucleotide of the dispersed DNA strands, the DNA polymerase enzyme adjusts its complementary nucleotide.

Thus, two double-stranded DNA molecules are formed, 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 for replication at the same time. Therefore, replication begins in separate sections of the DNA molecule, short fragments are formed, which are then sewn into a long thread using certain enzymes.

The duration of the cell cycle depends on the type of cell and on external factors such as temperature, the presence of oxygen, the presence of nutrients. For example, under favorable conditions, bacterial cells divide every 20 minutes, intestinal epithelial cells every 8-10 hours, and cells at the tips of onion roots 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 consist 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 are made up of cells. Exactly one year later, the zoologist Theodor Schwann (Fig. 5) came to the same conclusion, but only with regard to animal tissues.

Rice. 5. Matthias Schleiden (left) Theodor Schwann (right)

He concluded that animal tissues, like plant tissues, are made up of cells and that cells are the basis of life. Based on cellular data, scientists formulated a cellular theory.

Rice. 6. Rudolf Virchow

After 20 years, Rudolf Virchow (Fig. 6) expanded the cell theory and came to the conclusion that cells can arise from other cells. He wrote: “Where there is a cell, there must be a previous cell, just as animals come only from an animal, and plants only from a plant ... All living forms, whether they are animal or plant organisms, or their constituent parts, are dominated by the eternal law of continuous development.

The structure of chromosomes

As you know, chromosomes play a key role in cell division as they carry genetic information from one generation to the next. Chromosomes are made up of a DNA molecule bound to proteins by histones. 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 indistinguishable, but their chromosome material is stained with basic dyes and is called chromatin. Before cell division, chromosomes (Fig. 7) thicken and shorten, which allows them to be clearly seen in a light microscope.

Rice. 7. Chromosomes in prophase 1 of meiosis

In a dispersed, that is, stretched state, chromosomes participate in all biosynthesis processes or regulate biosynthesis 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 DNA strands are formed.

Rice. 8. The structure of the chromosome

These chains are surrounded by a protein coat 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 area, which is called the 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 are the stages of interphase?

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. A single collection of Digital Educational Resources ().

3. A single collection of Digital Educational Resources ().

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Bibliography

1. Kamensky A. A., Kriksunov E. A., Pasechnik V. V. General biology 10-11 class 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 pages.

3. Belyaev D.K. Biology grade 10-11. General biology. A basic level of. - 11th ed., stereotype. - M.: Education, 2012. - 304 p.

4. Biology grade 11. 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 class. General biology. A basic level of. - 6th ed., add. - Bustard, 2010. - 384 p.

The biological significance of cell division. New cells arise as a result of the division of existing ones. If a unicellular organism divides, then 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 hence the continuity of life on Earth.

cell cycle- the life of a cell from the moment of its formation in the process of division of the mother cell to 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. Further, the cell functions for a certain time, after which it either divides, forming daughter cells, or dies.

At various kinds organisms, the cell cycle takes a different time: for example, in bacteria it lasts about 20 minutes ciliates shoes- from 10 to 20 hours. Cells of multicellular organisms on early stages development divide frequently, and then the cell cycles are significantly lengthened. For example, immediately after the birth of a person, brain cells divide a huge number of times: 80% of brain neurons are formed during this period. However, most of these cells quickly lose their ability to divide, and some survive until the natural death of the organism without dividing at all.

The cell cycle consists of interphase and mitosis (Fig. 54).

Interphase- cell cycle interval between two divisions. During the entire interphase, the chromosomes are not spiralized; they are located in the cell nucleus in the form of chromatin. As a rule, the interphase consists of three periods: pre-synthetic, synthetic and postsynthetic.

Presynthetic period (G,) is the longest part of the 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 in it, energy and substances accumulate for the subsequent duplication of DNA. During the Gj-period, each chromosome consists of one chromatid, i.e. the number of chromosomes ( P) and chromatids (from) matches. A set of chromosomes and chromo-

matid (DNA molecules) of a diploid cell in the G r period of the cell cycle can be expressed by writing 2p2s.

In synthetic period (S) DNA duplication occurs, as well as the synthesis of proteins necessary for the subsequent formation of chromosomes. IN the same period there is a doubling of centrioles.

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 enzyme of replication, bind to the separated chains. Then the DNA polymerase molecules begin to move along the parent 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 parent DNA chain has the nucleotide sequence A C G T G A, then the section of the daughter chain will look like TGCAC. IN In connection with this, replication is referred to as matrix synthesis reactions. IN replication produces two identical double-stranded DNA molecules IN each of them includes one chain of the original parent 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 record 2p4s.

Postsynthetic period (G 2) occurs after DNA duplication. At this time, the cell accumulates energy and synthesizes proteins for the upcoming division (for example, tubulin protein 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, resulting in the formation of daughter cells. During mitosis (the main method of cell division in eukaryotes), the sister chromatids of each chromosome separate from each other and enter 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 appearance of a cell to its complete division into two daughter ones and includes interphase (Gr, S-, C2-periods) and mitosis (see Fig. 54). Such a sequence of periods of the cell cycle is typical for constantly dividing cells, for example, for cells of the germ layer of the epidermis of the skin, red bone marrow, mucous membrane of the gastrointestinal tract of animals, cells of the educational tissue of plants. They are able to divide every 12-36 hours.

In contrast, most of the cells of a multicellular organism embark on the path of specialization and, after passing through part of the Gj period, can move into the so-called rest period (Go-period). Cells that are in the G n -period perform their specific functions in the body, they undergo metabolic and energy processes, but there is no preparation for replication. Such cells, as a rule, permanently lose the ability to divide. Examples include neurons, cells of 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, having gone through all periods of interphase and mitosis. So, liver cells can again acquire the ability to divide after several months of being in a dormant period.

Cell death. The death (death) of individual cells or their groups is constantly encountered 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 apoptosis, which is often called programmed cell death or even cell suicide.

Necrosis- the death of cells and tissues in a living organism, caused by the action of damaging factors. The causes of necrosis can be exposure to high and low temperatures, ionizing radiation, various chemicals (including toxins released by pathogens). Necrotic cell death is also observed as a result of their mechanical damage, impaired blood supply and tissue innervation, and allergic reactions.

In damaged cells, membrane permeability is disturbed, 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 undergo such death (for example, in myocardial infarction, a section of the heart muscle containing many cells dies due to a cessation of oxygen supply). Usually, dying cells are attacked by leukocytes, and an inflammatory reaction develops in the necrosis zone.

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 the life of the organism, even in the embryonic period.

In an adult organism, planned cell death also constantly occurs. Blood cells, epidermis of the skin, mucous membrane of the gastrointestinal tract, etc., die in millions. After ovulation, part of the follicular cells of the ovary dies, after lactation, the cells of the mammary glands. 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 the plasmalemma. Usually, fragments of dead cells are taken up by leukocytes or neighboring cells without triggering an inflammatory response. Replenishment of lost cells is provided by division.

Thus, apoptosis, as it were, interrupts 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 goes either to a new cell cycle or to apoptosis.

1. What is the cell cycle?

2. What is called interphase? What main events take place in the G r , S- and 0 2 -periods of the interphase?

3. What 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? As part 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?

Chapter 1. Chemical components of living organisms

• § 1. The content of chemical elements in the body. Macro- and microelements
• § 2. Chemical compounds in living organisms. inorganic substances

Chapter 2. Cell - structural and functional unit of living organisms

• § 10. The history of the discovery of the cell. Creation of the cell theory
• § 15. Endoplasmic reticulum. Golgi complex. Lysosomes

Chapter 3

• § 24. General characteristics of metabolism and energy conversion

Chapter 4. Structural organization and regulation of functions in living organisms

Cell life cycle, or cell cycle, is the period of time during which it exists as a unit, i.e. the period of the cell's life. It lasts from the moment the cell appears as a result of the division of its mother and until the end of its division itself, when it "breaks up" into two daughters.

There are times when the cell does not divide. Then its life cycle is the period from the appearance of a cell to death. Usually cells of a number of tissues of multicellular organisms do not divide. For example, nerve cells and red blood cells.

It is customary in the life cycle of eukaryotic cells to distinguish a number of specific periods, or phases. They are characteristic of all dividing cells. The phases are designated G 1 , S, G 2 , M. From the G 1 phase, a cell can go to the G 0 phase, remaining in which it does not divide and in many cases differentiates. At the same time, some cells can return from G 0 to G 1 and go through all stages of the cell cycle.

The letters in phase abbreviations are the first letters of English words: gap (gap), synthesis (synthesis), mitosis (mitosis).

The cells are illuminated with a red fluorescent indicator in the G1 phase. The remaining phases of the cell cycle are green.

Period G 1 - presynthetic– begins as soon as the cell has appeared. At this moment, it is smaller in size than the mother, it has few substances, the number of organelles is not enough. Therefore, in G 1, cell growth, the synthesis of RNA, proteins, and the construction of organelles take place. Usually G 1 is the longest phase of the cell life cycle.

S - synthetic period. Its most important distinguishing feature is the duplication of DNA by replication. Each chromosome becomes composed of two chromatids. During this period, the chromosomes are still despiralized. In chromosomes, in addition to DNA, there are many histone proteins. Therefore, in the S-phase, histones are synthesized in large quantities.

IN postsynthetic period - G 2 The cell prepares for division, usually by mitosis. The cell continues to grow, ATP synthesis is actively going on, centrioles can double.

Next, the cell enters phase of cell division - M. This is where the division of the cell nucleus takes place. mitosis followed by division of the cytoplasm cytokinesis. The completion of cytokinesis marks the end of the life cycle of a given cell and the beginning of two new cell cycles.

Phase G0 sometimes referred to as the "rest" period of the cell. The cell "leaves" the normal cycle. During this period, the cell may begin to differentiate and never return to the normal cycle. The G0 phase can also include senescent cells.

The transition to each subsequent phase of the cycle is controlled by special cellular mechanisms, the so-called checkpoints - checkpoints. In order for the next phase to begin, everything must be ready for this in the cell, the DNA must not contain gross errors, etc.

Phases G 0 , G 1 , S, G 2 together form interphase - I.

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

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

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

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

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

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

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

cell cycle= Interphase + Mitosis + Cytokinesis

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

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

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

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

Major events during interphase

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

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

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

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

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

Cell life cycle: interphase

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

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

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

Periods of the cell life cycle (interphases)

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

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

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

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

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

Generation time duration

In other words, this is how long the life cycle of a cell lasts. The duration of the generation time, as well as individual periods, takes different meanings in various cells. This can be seen from the table below.

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

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

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

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

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

cell division

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

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

Cell cycle phases

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

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

Methods of cell division

Out of just two:

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

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

Indirect cell division

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

Biological significance of indirect cell division

It is as follows:

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

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

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

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

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

Mechanism of mitosis

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

Prophase

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

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

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

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

metaphase

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

Anaphase

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

Telophase

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

Mechanisms of entry of the virus into the cell

There are only two of them:

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

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

Intracellular replication of the virus

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

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

Conclusion

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