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• 1) In prophase, the volume of the nucleus increases, and due to the spiralization of chromatin, chromosomes are formed. By the end of prophase, each chromosome is seen to consist of two chromatids. Gradually, the nucleoli and nuclear membrane dissolve, and the chromosomes are randomly located in the cytoplasm of the cell. In the cytoplasm of the cell there is a small granular body called the centriole. At the beginning of prophase, the centriole divides, and the daughter centrioles move to opposite ends of the cell. Thin filaments in the form of rays depart from each centriole, forming a star; a spindle arises between the centrioles, consisting of a number of protoplasmic filaments called spindle filaments. These filaments are built from a protein similar in properties to the contractile proteins of muscle fibers. They are arranged in the form of two cones, folded base to base, so that the spindle is narrow at the ends, or poles, near the centrioles, and wide in the center, or at the equator. The threads of the spindle stretch from the equator to the poles; they consist of the denser protoplasm of the nucleus. The spindle is a specific structure: with the help of a micromanipulator, a thin needle can be inserted into the cell and the spindle can be moved with it. Spindles isolated from dividing cells contain protein, mostly one kind of protein, as well as a small amount of RNA. As the centrioles separate and the spindle forms, the chromosomes in the nucleus shorten, become shorter and thicker. If earlier it could not be seen that they consist of two elements, now it is clearly noticeable.
• 2) Prometaphase begins with the rapid disintegration of the nuclear envelope into small fragments indistinguishable from fragments of the endoplasmic reticulum. Chromosomes on each side of the centromere in prometaphase form special structures called kinetochores. They attach to a special group of microtubules called kinetochore filaments or kinetochore microtubules. These filaments extend from both sides of each chromosome, run in opposite directions, and interact with the filaments of the bipolar spindle. In this case, the chromosomes begin to move intensively.
• 3) Metaphase. Chromatids are attached to spindle fibrils by kinetochores. Once connected to both centrosomes, the chromatids move toward the equator of the spindle until their centromeres line up along the equator of the spindle perpendicular to its axis. This allows the chromatids to move freely towards their respective poles. The placement of chromosomes characteristic of metaphase is very important for chromosome segregation, i.e. segregation of sister chromatids. If an individual chromosome “slows down” in its movement towards the spindle equator, the onset of anaphase is usually delayed as well. Metaphase ends with the separation of sister chromatids.
• 4) Anaphase usually lasts only a few minutes. Anaphase begins with a sudden splitting of each chromosome, which is caused by the separation of sister chromatids at their junction point at the centromere.

This kinetochore-separating cleavage is independent of other mitotic events and occurs even in chromosomes not attached to the mitotic spindle. It allows the polar forces of the spindle acting on the metaphase plate to start moving each chromatid towards the respective spindle poles at a rate of about 1 µm/min. If there were no spindle threads, then the chromosomes would be pushed in all directions, but due to the presence of these threads, one complete set of daughter chromosomes is collected at one pole, and the other at the other. During the movement to the poles, the chromosomes usually take a V-shape, with their top facing the pole. The centromere is located at the top, and the force that makes the chromosome move towards the pole is applied to the centromere. Chromosomes that have lost their centromere during mitosis do not move at all.

5) Telophase begins after the daughter chromosomes, consisting of one chromatid, have reached the poles of the cell. At this stage, the chromosomes despiralize again and acquire the same form as they had before the cell division began in the interphase (long thin filaments). A nuclear envelope arises around them, and a nucleolus is formed in the nucleus, in which ribosomes are synthesized. In the process of cytoplasm division, all organelles are distributed more or less evenly between daughter cells. This completes nuclear division, also called karyokinesis; then the cell body divides, or cytokinesis.

Table 2. Phases of mitosis

In most cases, the entire process of mitosis takes from 1 to 2 hours. In plants, division occurs through the formation of a so-called cell plate that separates the cytoplasm; it arises in the equatorial region of the spindle, and then grows in all directions, reaching the cell wall. The material of the cell plate is produced by the endoplasmic reticulum. Then each of the daughter cells forms a cytoplasmic membrane on its side of the cell plate, and, finally, cellulose cell walls are formed on both sides of the plate.

The frequency of mitoses in different tissues and in different species differs sharply. For example, in the human red bone marrow, where 10,000,000 red blood cells are formed every second, 10,000,000 mitoses should occur every second.

There are four phases of mitosis: prophase, metaphase, anaphase and telophase. AT prophase clearly visible centrioles- formations located in the cell center and playing a role in the division of the daughter chromosomes of animals. (Recall that higher plants do not have centrioles in the cell center, which organizes the division of chromosomes). We will consider mitosis using the example of an animal cell, since the presence of a centriole makes the process of chromosome division more obvious. Centrioles divide and diverge to different poles of the cell. Microtubules extend from the centrioles, forming spindle fibers, which regulate the divergence of chromosomes to the poles of the dividing cell.
At the end of prophase, the nuclear membrane disintegrates, the nucleolus gradually disappears, the chromosomes spiralize and as a result shorten and thicken, and they can already be observed under a light microscope. They are even better seen at the next stage of mitosis - metaphase.
In metaphase, chromosomes are located in the equatorial plane of the cell. It is clearly seen that each chromosome, consisting of two chromatids, has a constriction - centromere. Chromosomes are attached by their centromeres to the spindle thread. After division of the centromere, each chromatid becomes an independent daughter chromosome.
Then comes the next stage of mitosis - anaphase, during which the daughter chromosomes (chromatids of one chromosome) diverge to different poles of the cell.
The next stage of cell division is telophase. It begins after the daughter chromosomes, consisting of one chromatid, have reached the poles of the cell. At this stage, the chromosomes despiralize again and acquire the same form as they had before the cell division began in the interphase (long thin filaments). A nuclear envelope arises around them, and a nucleolus is formed in the nucleus, in which ribosomes are synthesized. In the process of cytoplasm division, all organelles (mitochondria, Golgi complex, ribosomes, etc.) are distributed more or less evenly between daughter cells.
Thus, as a result of mitosis, two cells are obtained from one cell, each of which has a characteristic number and shape of chromosomes for a given type of organism, and, consequently, a constant amount of DNA.
The whole process of mitosis takes an average of 1-2 hours. Its duration is somewhat different for different types of cells. It also depends on the conditions of the external environment (temperature, light regime and other indicators).
The biological significance of mitosis lies in the fact that it ensures the constancy of the number of chromosomes in all cells of the body. All somatic cells are formed as a result of mitotic division, which ensures the growth of the organism. In the process of mitosis, the substances of the chromosomes of the mother cell are distributed strictly equally between the two daughter cells arising from it. As a result of mitosis, all cells of the body receive the same genetic information.

with identical genetic material.

Interphase

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. About 90% of a cell's time under normal conditions can be spent in interphase, which occurs in three main phases:

• Phase G1: period before DNA synthesis. In this phase, the cell increases in mass, preparing for division.
• S-phase: the period during which DNA synthesis occurs. In most cells, this stage occurs in a very short period of time.
• Phase G2: the cell continues to synthesize additional proteins to increase in size.

In the last part of interphase, the cell still has nucleoli. The nucleus is limited by the nuclear membrane, and are duplicated, but are in the form of chromatin. The two pairs of centrioles, formed from the replication of one pair, are located outside the nucleus.

After the G2 phase, mitosis occurs, which in turn consists of several stages and ends with cytokinesis (cell division).

Phases of mitosis:

Preprophase (in plant cells)

Preprophase is an additional phase during mitosis that does not occur in other eukaryotes such as animals or fungi. It precedes prophase and is characterized by two distinct events.

Changes that occur in preprophase:

• The formation of a preprophase band - a dense microtubular ring under.
• The beginning of the nucleation of microtubules in the nuclear envelope.

Prophase

In prophase, it condenses into discrete chromosomes. The nuclear membrane breaks, and the spindle of division is formed at opposite poles of the cell. Prophase (versus interphase) is the first true step of the mitotic process.

Changes that occur during prophase:

• Chromatin fibers turn into chromosomes, having two each connected to form a centromere. Fission fibers, consisting of microtubules and proteins, are formed in.
• In animal cells, fission fibers initially appear as structures called asters that surround each pair of centrioles.
• Two pairs of centrioles (formed from the replication of one pair in interphase) move away from each other towards opposite poles of the cell due to elongation of the microtubules formed between them.

prometaphase

Prometaphase is the phase of mitosis after prophase and preceding metaphase in eukaryotic somatic cells. Some sources attribute the processes occurring in prometaphase to the late prophase and the initial stage of metaphase.

Changes that occur in prometaphase:

• The nuclear envelope disintegrates.
• Polar fibers, which are microtubules that make up spindle fibers, travel from each pole to the equator of the cell.
• Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore filaments.
• The filaments of the kinetochore "interact" with the fission spindle.
• Chromosomes begin to migrate towards the center of the cell.

metaphase

In metaphase, fission fibers fully develop, and chromosomes align on the metaphase (equatorial) plate (the plane that is equidistant from the two poles).

Changes that occur in metaphase:

• The nuclear membrane completely disappears.
• In animal cells, the two pairs diverge in opposite directions towards the poles of the cell.
• The polar fibers (microtubules that make up the fibers of the spindle) continue to spread from the poles to the center. Chromosomes move randomly until they attach (via their kinetochores) to the polar fibers on either side of the centromeres.
• Chromosomes align on the metaphase plate at right angles to the spindle poles.
• Chromosomes are held on the metaphase plate by the equal forces of the polar fibers, which press on their centromeres.

Anaphase

In anaphase, paired chromosomes () separate and begin to move towards opposite ends (poles) of the cell. The spindle fibers, not associated with chromatids, stretch and lengthen the cell. At the end of anaphase, each pole contains a complete compilation of chromosomes.

Changes that occur in anaphase:

• Paired ones in each individual chromosome begin to move apart.
• Once the paired sister chromatids are separated from each other, each is considered a "complete" chromosome. They are called daughter chromosomes.
• With the help of the division spindle, they move to the poles at opposite ends of the cell.
• The daughter chromosomes first migrate to the centromere, and the kinetochore filaments become shorter than the chromosomes near the poles.
• In preparation for telophase, the two poles of the cell also move away from each other during anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.
• The process of cytokinesis begins (separation of the cytoplasm of the original cell), which ends after telophase.

Telophase

In telophase, chromosomes reach the nuclei of new daughter cells.

Changes that occur in telophase:

• The polar fibers continue to lengthen.
• Nuclei begin to form at opposite poles.
• The nuclear membranes of new nuclei are formed from the remains of the nuclear membrane of the mother cell and pieces of the endomembrane system.
• The nucleolus appears.
• The chromatin fibers of the chromosomes are unwound.
• After these changes, telophase and mitosis are basically completed, and the genetic content of one cell is divided into two parts.

cytokinesis

Cytokinesis is the division of the cytoplasm of a cell. It begins before the end of mitosis in anaphase and ends shortly after telophase. At the end of cytokinesis, two genetically identical daughter cells are formed.

daughter cells

At the end of mitosis and cytokinesis, the chromosomes are distributed equally between the two daughter cells. These cells are identical, with each containing a complete set of chromosomes.

Cells produced through mitosis are different from cells produced through . Meiosis produces four daughter cells. These cells are containing half the number of chromosomes from the original cell. undergo meiosis. When germ cells divide during fertilization, haploid cells become diploid cells.

Cell reproduction is one of the most important biological processes, it is a necessary condition for the existence of all living things. Reproduction is carried out by dividing the original cell.

Cell- this is the smallest morphological unit of the structure of any living organism, capable of self-production and self-regulation. The time of its existence from division to death or subsequent reproduction is called the cell cycle.

Tissues and organs are made up of various cells that have their own period of existence. Each of them grows and develops to ensure the vital activity of the organism. The duration of the mitotic period is different: blood and skin cells enter the process of division every 24 hours, and neurons are capable of reproduction only in newborns, and then completely lose their ability to reproduce.

There are 2 types of division - direct and indirect. Somatic cells reproduce indirectly; gametes or germ cells are characterized by meiosis (direct division).

Mitosis - indirect division

Mitotic cycle

The mitotic cycle includes 2 consecutive stages: interphase and mitotic division.

Interphase(rest stage) - preparation of the cell for further division, where duplication of the source material is performed, followed by its uniform distribution among the newly formed cells. It includes 3 periods:

• • Presynthetic(G-1) G - from the English gar, that is, a gap, preparations are underway for the subsequent synthesis of DNA, the production of enzymes. The inhibition of the first period was experimentally carried out, as a result of which the cell did not enter the next phase.
  • Synthetic(S) - the basis of the cell cycle. Replication of chromosomes and centrioles of the cell center occurs. Only after that the cell can proceed to mitosis.
  • Postsynthetic(G-2) or pre-mitotic period - there is an accumulation of mRNA, which is needed for the onset of the actual mitotic stage. In the G-2 period, proteins (tubulins) are synthesized - the main component of the mitotic spindle.

After the end of the premitotic period, mitotic division. The process includes 4 phases:

1. Prophase- during this period, the nucleolus is destroyed, the nuclear membrane (nucleolema) dissolves, centrioles are located at opposite poles, forming an apparatus for division. It has two subphases:
   • early- thread-like bodies (chromosomes) are visible, they are not yet clearly separated from each other;
   • late- separate parts of chromosomes are traced.
2. metaphase- begins from the moment of destruction of the nucleolema, when the chromosomes lie randomly in the cytoplasm and only begin to move towards the equatorial plane. All pairs of chromatids are connected to each other at the centromere.
3. Anaphase- at one moment all the chromosomes are separated and move to opposite points of the cell. This is a short and very important phase, since it is in it that the exact division of the genetic material takes place.
4. Telophase- chromosomes stop, the nuclear membrane, the nucleolus, is formed again. A constriction is formed in the middle, it divides the body of the mother cell into two daughter cells, completing the mitotic process. In the newly formed cells, the G-2 period begins again.

Meiosis - direct division

There is a special process of reproduction that occurs only in germ cells (gametes) - this meiosis (direct division). A distinctive feature for him is the absence of interphase. Meiosis from one original cell produces four, with a haploid set of chromosomes. The whole process of direct division includes two successive stages, which consist of prophase, metaphase, anaphase and telophase.

Before the start of prophase, the germ cells double the initial material, thus, it becomes tetraploid.

Prophase 1:

1. Leptotena- chromosomes are visible in the form of thin threads, they are shortened.
2. Zygoten- the stage of conjugation of homologous chromosomes, as a result, bivalents are formed. Conjugation is an important moment of meiosis, the chromosomes are as close as possible to each other in order to carry out crossing over.
3. Pachytene- there is a thickening of chromosomes, their increasing shortening, there is a crossing over (the exchange of genetic information between homologous chromosomes, this is the basis of evolution and hereditary variability).
4. Diploten- the stage of doubled strands, the chromosomes of each bivalent diverge, keeping the connection only in the area of ​​​​the decussation (chiasm).
5. diakinesis- DNA begins to condense, chromosomes become very short and diverge.

Prophase ends with the destruction of the nucleolema and the formation of the spindle.

Metaphase 1: bivalents are located in the middle of the cell.

Anaphase 1: Doubled chromosomes move to opposite poles.

Telophase 1: the division process is completed, the cells receive 23 bivalents.

Without subsequent doubling of the material, the cell enters into second phase division.

Prophase 2: all the processes that were in prophase 1 are repeated again, namely the condensation of chromosomes, which are randomly located between the organelles.

Metaphase 2: two chromatids connected at the intersection (univalents) are located in the equatorial plane, creating a plate called metaphase.

Anaphase 2:- the univalent is divided into separate chromatids or monads, and they go to different poles of the cell.

Telophase 2: the division process is completed, the nuclear envelope is formed, and each cell receives 23 chromatids.

Meiosis is an important mechanism in the life of all organisms. As a result of this division, we get 4 haploid cells that have half of the desired set of chromatids. During fertilization, two gametes form a complete diploid cell, retaining its inherent karyotype.

It is difficult to imagine our existence without meiotic division, otherwise all organisms with each subsequent generation would receive double sets of chromosomes.

Among all the interesting and rather complex topics in biology, it is worth highlighting two processes of cell division in the body - meiosis and mitosis. At first it may seem that these processes are the same, since in both cases cell division occurs, but in fact there is a big difference between them. First of all, you need to deal with mitosis. What is this process, what is the interphase of mitosis and what role do they play in the human body? More about this and will be discussed in this article.

The complex biological process that is accompanied by cell division and the distribution of chromosomes between these cells - all this can be said about mitosis. Thanks to him, chromosomes containing DNA are evenly distributed between the daughter cells of the body.

There are 4 main phases of the mitosis process. All of them are interconnected, since the phases smoothly pass from one to another. The prevalence of mitosis in nature is due to the fact that it is he who participates in the process of division of all cells, including muscle, nerve, and so on.

Briefly about interphase

Before entering the state of mitosis, the cell that divides goes into the period of interphase, that is, it grows. The duration of interphase can take more than 90% of the total time of cell activity in the normal mode..

Interphase is divided into 3 main periods:

• phase G1;
• S-phase;
• phase G2.

All of them pass in a certain sequence. Let's consider each of these phases separately.

Interphase - main components (formula)

Phase G1

This period is characterized by the preparation of the cell for division. It increases in volume for the next phase of DNA synthesis.

S-phase

This is the next stage in the process of interphase, in which the cells of the body divide. As a rule, the synthesis of most cells occurs for a short period of time. After cell division, the cells do not increase in size, but the last phase begins.

Phase G2

The final stage of interphase, during which cells continue to synthesize proteins, while increasing in size. During this period, the cell still has nucleoli. Also in the last part of the interphase, duplication of chromosomes occurs, and the surface of the nucleus at this time is covered with a special shell that has a protective function.

On a note! At the end of the third phase, mitosis occurs. It also includes several stages, after which cell division occurs (this process in medicine is called cytokinesis).

Stages of mitosis

As noted earlier, mitosis is divided into 4 stages, but sometimes there may be more. Below are the main ones.

Table. Description of the main phases of mitosis.

Important! The duration of the complete process of mitosis, as a rule, is no more than 1.5-2 hours. The duration may vary depending on the type of cell being divided. Also, the duration of the process is influenced by external factors, such as light conditions, temperature, and so on.

What biological role does mitosis play?

Now let's try to understand the features of mitosis and its importance in the biological cycle. Primarily, it provides many vital processes of the organism, among which - embryonic development.

Mitosis is also responsible for the restoration of tissues and internal organs of the body after various types of damage, resulting in regeneration. In the process of functioning, cells gradually die off, but with the help of mitosis, the structural integrity of tissues is constantly maintained.

Mitosis ensures the preservation of a certain number of chromosomes (it corresponds to the number of chromosomes in the mother cell). read on our website.

Video - Features and types of mitosis