Cell life cycle: phases, periods. The life cycle of the virus in the host cell

The life cycle of a cell includes the beginning of its formation and the end of existence as an independent unit. To begin with, the 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 under consideration is equivalent to the cellular.

Cell life cycle: interphase

This is the period between the two mitotic cell divisions. Reproduction of chromosomes proceeds similarly to the reduplication (semi-conservative replication) of DNA molecules. In the interphase, the nucleus of the cell is surrounded by a special two-membrane membrane, and the chromosomes are untwisted, and are not noticeable with ordinary light microscopy.

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

In the cell life cycle, interphase is accompanied by three periods. Consider each of them in more detail.

in the cell life cycle, interphase is accompanied

Periods of the cell life cycle (interphase)

The first one is calledre-synthetic. The result of previous mitosis is an increase in the number of cells. Here the transcription of newly-made RNA molecules (informational) proceeds, as well as the molecules of the remaining RNAs are systematized, proteins are synthesized in the nucleus and cytoplasm. Some substances of the cytoplasm are gradually split with the formation of ATP, its molecules are endowed with macroergic bonds, they transfer energy to where it is not enough. At the same time the cell increases, in size it reaches the parent. This period lasts a long time in specialized cells, during which they carry out their special functions.

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

DNA molecules begin to bind to the protein, causing the chromosomes to thicken. At the same time, a reproduction of centrioles is observed, with the result that 2 pairs appear. The new centriole in all pairs is placed relatively old at an angle of 90 °. Subsequently, each pair in the period of the next mitosis moves back to the cell poles.

cell life cycle mitosis

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

Duration of generation time

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

Period

Generation time

Type of cell population

presynthetic interphase period

synthetic interphase period

post-synthetic interphase period

mitosis

528

30

4,6

3,8

585,6

dermal epithelium

4,75

4,5

1

0,5

11,0

duodenal ulcer

9,5

7,5

≈0,75

1

18,75

small intestine

9

9

0,5

3

21,5

liver cells of a 3 week animal

So, the shortest cell life cycle is in the cambial ones. It happens that the third period falls completely - the postsynthetic one. For example, in a 3-week-old rat in the cells of its liver, it decreases to half an hour, 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 is not too far gone, it can go through the full life cycle of the cell with the formation of 2 new cells in the mitosis. In this situation, the first period may increase significantly. For example, generation time in the cells of the cutaneous epithelium of the mouse, namely 585.6 hours, is in the first period — presynthetic, and in the cells of the periosteum of the baby rat 102 hours out of 114.

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

If part of the liver is removed, most of its cells will switch to full living, first synthetic, then the postsynthetic period, and at the end - the mitotic process. So, for various cell populations, the reversibility of such a G0-period has already been proved. In other situations, the degree of specialization increases so much that, under typical conditions, the 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 in an ordinary light microscope.life cycle cell division

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

Cell division

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

The life cycle (cell division) is the period of existence of the unit in question (it starts from the moment of its appearance through the division of the mother cell), including the division itself.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:

  1. Division. Here the mitotic division proceeds.
  2. Growth. After division, the cell increases in volume. It reaches a certain size.
  3. Peace. At this time, further fate is not yet determined: the cell is preparing either for division or for specialization.
  4. Differentiation. At the end of the growth phase, the cell acquires the corresponding structural, functional features.
  5. Maturity. A cell performs certain functions based on its specialization.
  6. Aging. Weakening of cellular vital functions. Further - either division or death.phase of the cell life cycle

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

Cell division methods

From only two:

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

Now we will learn in more detail 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 consists in the following:

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 accurate replication of maternal DNA, which is why the genes of the daughter cells include identical hereditary information. They are genetically identical with the parent cell. So, we can say that mitosis provides the identity of the transmission of hereditary information to the daughter cells from the parent.

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

3A large number of animals, plants reproduces it asexually by means of mitotic cell division, therefore mitosis is the basis of vegetative reproduction.

4. It is mitosis that provides complete regeneration of lost parts, as well as cell replacement, which proceeds 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.the life cycle of the virus in the host cell

Mitosis mechanism

The division of the cytoplasm and nucleus - 2 independent processes that occur continuously, sequentially. But for the convenience of studying the events occurring during the period of division, it is artificially delimited 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 nucleus. As a result, spiralization occurs compaction, shortening of chromosomes. In a later prophase, the structure of chromosomes is already clearly visible: 2 chromatids, which are connected by a centromere.The movement of the chromosomes to the equator of the cell begins.

A spindle of division is formed from the cytoplasmic material in the prophase (late), 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, from the centrioles begin to appear 2-type filaments of the spindle, more precisely:

  • supporting, which connect the cell poles;
  • chromosomal (pulling) that intersect 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. The chromosomes are first aligned in the equatorial plane, forming a metaphase plate. At the same time, chromosome centromeres are strictly located in the equatorial plane. The spindle filaments join the chromosome centromeres, and some of them pass from one pole to the other without attaching.

Anaphase

Its beginning is considered to be the division of centromeres of chromosomes. As a result, chromatids are transformed into two separate daughter chromosomes.Further, the latter begin to diverge towards the cell poles. They, as a rule, at this time take a special V-shape. This discrepancy is accomplished by accelerating the spindle threads. At the same time, the elongation of the support threads proceeds, the result of which is the separation of the poles from each other.

Telophase

Here the chromosomes are assembled at the cell poles, then they are distillized. Next is the destruction of the spindle division. Around the chromosomes formed the nuclear membrane of the daughter cells. So karyokinesis is completed, cytokinesis is subsequently performed.

Mechanisms of entry of the virus into the cell

There are only two of them:

1. By fusing viral supercapsid and cell membranes. As a result, nucleocapsid is released into the cytoplasm. Subsequently, the implementation of the properties of the virus genome is observed.

2. Through pinocytosis (receptor-mediated endocytosis). Here is the binding of the virus in the place of the bordered fossa with receptors (specific). The latter is drawn into the inside of the cell, and then transformed into a so-called fringed vial. It, in turn, contains the absorbed virion, merges with a temporary intermediate vesicle, which is called the endosome.somatic cell life cycle

Intracellular virus multiplication

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 energy generation systems, it embodies its own reproduction, sacrificing, as a rule, cell life.

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

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