Mitosis- This is the most common way of dividing eukaryotic cells. During mitosis, the genomes of each of the two resulting cells are identical to each other and coincide with the genome of the original cell.

Mitosis is the last and usually shortest stage of the cell cycle. With its end, the life cycle of the cell ends and the cycles of two newly formed cells begin.

The diagram illustrates the duration of the stages of the cell cycle. The letter M denotes mitosis. The highest rate of mitosis is observed in germ cells, the lowest in tissues with a high degree of differentiation, if their cells divide at all.

Although mitosis is considered independently of the interphase, consisting of the periods G 1, S and G 2, preparation for it occurs precisely in it. The most important point is the DNA replication that occurs in the synthetic (S) period. After replication, each chromosome already consists of two identical chromatids. They are close together along their entire length and connected at the centromere of the chromosome.

During interphase, chromosomes are located in the nucleus and are a tangle of thin, very long chromatin threads that are visible only under an electron microscope.

Mitosis has a number of successive phases, which may also be called stages or periods. In the classic simplified version of the consideration, four phases are distinguished. This prophase, metaphase, anaphase and telophase. Often more phases are distinguished: prometaphase(between prophase and metaphase), preprophase(characteristic of plant cells, precedes prophase).

Another process associated with mitosis is cytokinesis, which occurs mainly during the telophase period. We can say that cytokinesis is, as it were, an integral part of telophase, or both processes occur in parallel. Cytokinesis refers to the separation of the cytoplasm (but not the nucleus!) of the parent cell. Nuclear fission is called karyokinesis, and it precedes cytokinesis. However, during mitosis as such, nuclear division does not occur, because first one, the parent, disintegrates, then two new ones are formed, the daughter ones.

There are cases when karyokinesis occurs, but cytokinesis does not. In such cases, multinucleated cells are formed.

The duration of mitosis itself and its phases is individual and depends on the type of cell. Usually prophase and metaphase are the longest periods.

The average duration of mitosis is about two hours. Animal cells generally divide faster than plant cells.

When eukaryotic cells divide, a bipolar fission spindle is necessarily formed, consisting of microtubules and associated proteins. Thanks to it, an equal distribution of hereditary material between daughter cells occurs.

Below we will give a description of the processes that occur in the cell during various phases of mitosis. The transition to each subsequent phase is controlled in the cell by special biochemical control points, which “check” whether all the necessary processes have been completed correctly. If there are errors, division may or may not stop. In the latter case, abnormal cells appear.

Phases of mitosis

Prophase

In prophase, the following processes occur (mostly in parallel):

Chromosomes condense

The nucleoli disappear

The nuclear envelope disintegrates

Two spindle poles are formed

Mitosis begins with the shortening of chromosomes. The pairs of chromatids that make them up spiral, as a result of which the chromosomes become greatly shortened and thickened. Towards the end of prophase they can be seen under a light microscope.

The nucleoli disappear because the parts of the chromosomes that form them (nucleolar organizers) are already in a spiral form, therefore, they are inactive and do not interact with each other. In addition, nucleolar proteins disintegrate.

In the cells of animals and lower plants, the centrioles of the cell center diverge to the poles of the cell and protrude microtubule organizing centers. Although higher plants do not have centrioles, microtubules are also formed.

Short (astral) microtubules begin to diverge from each center of organization. A star-like structure is formed. It is not produced in plants. Their division poles are wider, microtubules emerge not from a small, but from a relatively wide region.

The breakdown of the nuclear envelope into small vacuoles marks the end of prophase.

On the right in the microphotograph microtubules are highlighted in green, chromosomes are highlighted in blue, and chromosome centromeres are highlighted in red.

It should also be noted that during the prophase of mitosis, fragmentation of the EPS occurs; it breaks up into small vacuoles; The Golgi apparatus breaks up into individual dictyosomes.

Prometaphase

The key processes of prometaphase occur mostly sequentially:

Chaotic arrangement and movement of chromosomes in the cytoplasm.

Connecting them with microtubules.

Movement of chromosomes to the equatorial plane of the cell.

The chromosomes end up in the cytoplasm and move randomly. Once at the poles, they have a better chance of attaching to the plus end of the microtubule. Eventually the filament attaches to the kinetochore.

Such a kinetochore microtubule begins to grow, which moves the chromosome away from the pole. At some point, another microtubule is attached to the kinetochore of the sister chromatid, growing from the other pole of division. She also begins to push the chromosome, but in the opposite direction. As a result, the chromosome becomes at the equator.

Kinetochores are protein formations at the centromeres of chromosomes. Each sister chromatid has its own kinetochore, which “matures” in prophase.

In addition to astral and kinetochore microtubules, there are those that go from one pole to the other, as if expanding the cell in a direction perpendicular to the equator.

Metaphase

A sign of the onset of metaphase is the arrangement of chromosomes along the equator, the so-called metaphase or equatorial plate. During metaphase, the number of chromosomes, their differences, and the fact that they consist of two sister chromatids connected at the centromere are clearly visible.

Chromosomes are held together by balanced tension forces on microtubules at different poles.

Anaphase

Sister chromatids separate, each moving towards its own pole.

The poles are moving away from each other.

Anaphase is the shortest phase of mitosis. It begins when the centromeres of the chromosomes split into two parts. As a result, each chromatid becomes an independent chromosome and is attached to a microtubule of one pole. The threads “pull” the chromatids to opposite poles. In fact, microtubules are disassembled (depolymerized), that is, they are shortened.

In anaphase of animal cells, not only the daughter chromosomes move, but also the poles themselves. Due to other microtubules they push apart, astral microtubules attach to the membranes and also “pull”.

Telophase

Chromosome movement stops

Chromosomes decondense

Nucleoli appear

The nuclear membrane is restored

Most microtubules disappear

Telophase begins when chromosomes stop moving, stopping at the poles. They despiral, become long and thread-like.

The spindle microtubules are destroyed from the poles to the equator, i.e., from their minus ends.

A nuclear envelope is formed around the chromosomes by the fusion of membrane vesicles into which the maternal nucleus and EPS broke up in prophase. At each pole, its own daughter nucleus is formed.

As chromosomes decoil, nucleolar organizers become active and nucleoli appear.

RNA synthesis resumes.

If the centrioles at the poles are not yet paired, then a pair is built near each one. Thus, at each pole, its own cell center is recreated, which will go to the daughter cell.

Typically, telophase ends with the separation of the cytoplasm, i.e., cytokinesis.

Cytokinesis

Cytokinesis can begin as early as anaphase. By the beginning of cytokinesis, cell organelles are distributed relatively evenly across the poles.

The separation of the cytoplasm of plant and animal cells occurs in different ways.

In animal cells, due to elasticity, the cytoplasmic membrane in the equatorial part of the cell begins to bulge inward. A furrow is formed which eventually closes. In other words, the mother cell divides by ligation.

In plant cells during telophase, the spindle filaments do not disappear at the equator. They move closer to the cytoplasmic membrane, their number increases, and they form phragmoplast. It consists of short microtubules, microfilaments, and parts of the EPS. Ribosomes, mitochondria, and the Golgi complex move here. Golgi vesicles and their contents at the equator form the median cell plate, cell walls and membrane of daughter cells.

Meaning and functions of mitosis

Mitosis ensures genetic stability: accurate reproduction of genetic material over a series of generations. The nuclei of new cells contain the same number of chromosomes as the parent cell contained, and these chromosomes are exact copies of the parent ones (unless, of course, mutations have occurred). In other words, the daughter cells are genetically identical to the mother cell.

However, mitosis also performs a number of other important functions:

growth of a multicellular organism,

asexual reproduction,

replacement of cells of various tissues in multicellular organisms,

In some species, regeneration of body parts can occur.

Mitosis (or karyokinesis, indirect division) is the main method of division of somatic cells of animals and plants, in which the distribution of genetic material between daughter cells occurs in such a way that they receive an identical set of chromosomes (and genes) from the mother cell. This maintains a constant diploid set of chromosomes in cells, characteristic of each species of animal and plant. The mitotic division of animal cell nuclei was first described in 1871 by A.O. Kovalevsky, and plant cell nuclei - in 1874 by I.D. Chistyakov.

The complex of processes when two new cells are formed from one parent is called the mitotic cycle. This cycle, in turn, consists of mitosis itself and interphase - the period between two cell divisions. The duration of mitosis is 30-60 minutes (in animal cells) and 2-3 hours (in plant cells); the duration of interphase in different types of cells can range from several hours to several years. During interphase, many processes take place that are necessary for normal cell division. The most important of them are the doubling of DNA and the synthesis of special histone proteins, which leads to the doubling of chromosomes and a change in the ratio of the mass of the nucleus and cytoplasm, the synthesis of ATP to ensure the process of energy division, and the synthesis of proteins necessary for the construction of the achromatin spindle. These processes are completed just before the start of mitosis.

Mitosis consists of 4 phases – prophase , metaphases , anaphase And telophases .

The beginning prophase can be considered an increase in the volume of the nucleus and the spiralization of chromosomes, which become visible under a light microscope. Each chromosome consists of two identical halves (sister chromatids), which are connected to each other at the centromere. In prophase, cell polarization occurs - the centrioles of the cell center diverge to opposite ends of the cell and the formation of a division spindle (achromatin spindle) begins. In angiosperm cells there is no cell center, but despite this, the formation of the division spindle also begins at opposite poles of the cell. At the end of prophase, the nucleolus disappears, the nuclear membrane dissolves, and the chromosomes are located in the cytoplasm of the cell.

IN metaphase The formation of the fission spindle is completed, its threads go from pole to pole, and some of them join the centromeres of the chromosomes. Maximum spiralization of chromosomes occurs, which are located in the equatorial plane of the cell, forming a metaphase plate. At this time, it is clearly visible that each chromosome consists of 2 chromatids, so the study and counting of chromosomes is carried out precisely in this phase of division.

IN anaphase each of the chromosomes in the centromere region is split into chromatids, forming two daughter chromosomes, which, due to the contraction of the spindle threads, begin to move to the poles of the cell. As a result, a diploid set of single-stranded chromosomes is concentrated at each pole of the cell.

IN telophase processes occur that are opposite to those that took place in prophase: chromosomes despiral, nucleoli are formed, and the nuclear membrane is formed. As a result, two nuclei are formed with the same set of chromosomes that the nucleus of the mother cell had. After the separation of the nuclei, the process of division of the cytoplasm begins, which occurs due to constriction (in animal cells) or the formation of a plate in the middle of the equatorial plane (in plant cells).

Biological significance of mitosis in that there is an exact distribution of genetic material between daughter cells, this ensures constancy karyotype cells (chromosomal set) and genetic continuity between cell generations. Growth, development, restoration of tissues and organs of plants and animals occurs due to mitotic cell division.

• 1) 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. In the cytoplasm of the cell there is a small granular body called a centriole. At the beginning of prophase, the centriole divides and daughter centrioles move to opposite ends of the cell. From each centriole, thin threads extend in the form of rays, forming a star; between the centrioles a spindle arises, consisting of a number of protoplasmic threads called spindle filaments. These threads are built from a protein similar in its 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 spindle threads extend from the equator to the poles; they consist of denser protoplasm of the nucleus. The spindle is a specific structure: using a micromanipulator, you can insert a thin needle into the cell and move the spindle with it. Spindles isolated from dividing cells contain protein, mostly one type of protein, but also a small amount of RNA. As the centrioles separate and the spindle forms, the chromosomes in the nucleus contract, becoming shorter and thicker. If previously it might not have been visible that they consist of two elements, now this is clearly noticeable.
• 2) Prometaphase begins with the rapid disintegration of the nuclear membrane into small fragments, indistinguishable from fragments of the endoplasmic reticulum. In prometaphase, special structures called kinetochores are formed in the chromosomes on each side of the centromere. They attach to a special group of microtubules called kinetochore filaments or kinetochore microtubules. These strands extend from both sides of each chromosome, run in opposite directions, and interact with the strands of the bipolar spindle. At the same time, 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 spindle equator until their centromeres line up along the spindle equator perpendicular to its axis. This allows the chromatids to move unimpeded to their respective poles. The arrangement of chromosomes characteristic of metaphase is very important for chromosome segregation, i.e. sister chromatid divergence. If an individual chromosome “hesitates” in its movement towards the spindle equator, the onset of anaphase is usually delayed. Metaphase ends with the separation of sister chromatids.
• 4) Anaphase usually lasts only a few minutes. Anaphase begins with the sudden splitting of each chromosome, which is caused by the separation of sister chromatids at the point of their junction at the centromere.

This cleavage separating kinetochores is independent of other mitotic events and occurs even on chromosomes not attached to the mitotic spindle. It allows spindle polar forces acting on the metaphase plate to begin moving each chromatid towards the corresponding spindle poles at a speed of about 1 µm/min. If there were no spindle threads, then the chromosomes would be pushed apart in all directions, but thanks to the presence of these threads, one complete set of daughter chromosomes is assembled at one pole, and the other at the other. During movement towards the poles, chromosomes usually take on a V-shape, with their apex facing the pole. The centromere is located at the apex, and the force that causes the chromosome to move toward 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 despiral again and take on the same appearance as they had before the start of cell division in interphase (long thin threads). A nuclear envelope appears around them, and a nucleolus is formed in the nucleus, in which ribosomes are synthesized. During the division of the cytoplasm, all organelles are distributed more or less evenly between daughter cells. This completes nuclear division, also called karyokinesis; Then cell division, or cytokinesis, occurs.

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 the so-called cell plate, which separates the cytoplasm; it arises in the equatorial region of the spindle and then grows in all directions, reaching the cell wall. The cell plate material is produced by the endoplasmic reticulum. Each of the daughter cells then 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 varies dramatically. For example, in human red bone marrow, where 10,000,000 red blood cells are produced every second, 10,000,000 mitoses should occur every second.

The sequence of phases of the mitotic cycle is presented in Fig. 4.

Rice. 4. Phases of mitosis

Prophase. In prophase, the nucleus enlarges, and the chromosomal strands, which at this time are already spiralized, become clearly visible.

Each chromosome, after reduplication in interphase, consists of two sister chromatids connected by one centromere. At the end of prophase, the nuclear envelope and nucleoli usually disappear. Sometimes the nucleolus disappears in the next phase of mitosis. On preparations you can always find early and late prophases and compare them with each other. The changes are clearly visible: the nucleolus and the nuclear membrane disappear. Chromosome strands are more clearly visible in late prophase, and it is often possible to notice that they are duplicated. In prophase, there is also a separation of the centrioles, which form the two poles of the cell.

Prometaphase begins with the rapid disintegration of the nuclear membrane into small fragments, indistinguishable from fragments of the endoplasmic reticulum (Fig. 5). In prometaphase, special structures called kinetochores are formed in the chromosomes on each side of the centromere. They attach to a special group of microtubules called kinetochore filaments or kinetochore microtubules. These strands extend from both sides of each chromosome, run in opposite directions, and interact with the strands of the bipolar spindle. At the same time, the chromosomes begin to move intensively.

Rice. 5. Prometaphase (the figure of the mother star is built) in a pigment-free cell. Iron hematoxylin staining according to Heidenhain. Average magnification

Metaphase. After the nuclear membrane disappears, it is clear that the chromosomes have reached maximum spiralization, become shorter and move towards the equator of the cell, located in the same plane. Centrioles located at the cell poles complete the formation of the spindle, and its threads join the chromosomes in the centromere region. The centromeres of all chromosomes are located in the same equatorial plane, and the arms can be located higher or lower. This position of chromosomes is convenient for counting them and studying their morphology.

Anaphase begins with the contraction of the filaments of the spindle, due to which it can be located higher or lower. All this is convenient for counting the number of chromosomes, studying their morphology and dividing centromeres. In anaphase of mitosis, the centromeric region of each of the two-chromatid chromosomes is split, leading to the separation of sister chromatids and their transformation into independent chromosomes (the formal ratio of the number of chromosomes and DNA molecules is 4n4c).

This is how the genetic material is distributed accurately, and each pole ends up with the same number of chromosomes as the original cell had before they doubled.

The movement of chromatids to the poles occurs due to the contraction of the trailing threads and the elongation of the supporting threads of the mitotic spindle.

Telophase. After the completion of chromosome divergence to the poles of the mother cell, two daughter cells are formed in telophase, each of which receives a full set of single-chromatid chromosomes of the mother cell (formula 2n2c for each of the daughter cells).

In telophase, chromosomes at each pole undergo despiralization, i.e. a process opposite to what occurs in prophase. The contours of the chromosomes lose their clarity, the mitotic spindle is destroyed, the nuclear membrane is restored and nucleoli appear. The separation of cell nuclei is called karyokinesis (Fig. 6).

Then, a cell wall is formed from the phragmoplast, which divides the entire contents of the cytoplasm into two equal parts. This process is called cytokinesis. This is how mitosis ends.

Rice. 6. Phases of mitosis in various plants

Rice. 7. Distribution of homologous chromosomes and the genes they contain during the mitotic cycle in a hypothetical organism (2n = 2) generations and genetic continuity of life in the case of asexual reproduction of organisms.

Basic terms and concepts: anaphase; daughter cell; interphase; mother (parent) cell; metaphase; mitosis (period M); mitotic (cell) cycle; post-synthetic period (G 2); presynthetic period (G 1); prophase; sister chromatids; synthetic period (S); telophase; chromatid; chromatin; chromosome; centromere.

The cell reproduces by division. There are two methods of division: mitosis and meiosis.

Mitosis(from the Greek mitos - thread), or indirect cell division, is a continuous process, as a result of which there is first a doubling, and then an even distribution of the hereditary material contained in the chromosomes between the two resulting cells. This is its biological significance. Nuclear division entails division of the entire cell. This process is called cytokinesis (from the Greek cytos - cell).

The state of the cell between two mitoses is called interphase, or interkinesis, and all changes that occur in it during preparation for mitosis and during the period of division are called the mitotic, or cell cycle.

Different cells have different mitotic cycles. Most of the time the cell is in a state of interkinesis; mitosis lasts a relatively short time. In the general mitotic cycle, mitosis itself takes 1/25-1/20 of the time, and in most cells it lasts from 0.5 to 2 hours.

The thickness of the chromosomes is so small that when examining the interphase nucleus with a light microscope, they are not visible; it is only possible to distinguish chromatin granules in the knots of their twisting. An electron microscope made it possible to detect chromosomes in a non-dividing nucleus, although at this time they are very long and consist of two strands of chromatids, the diameter of each of which is only 0.01 microns. Consequently, the chromosomes in the nucleus do not disappear, but take the form of long and thin threads that are almost invisible.

During mitosis, the nucleus goes through four successive phases: prophase, metaphase, anaphase and telophase.

Prophase(from the Greek about - before, phase - manifestation). This is the first phase of nuclear division, during which structural elements appear inside the nucleus that look like thin double threads, which led to the name of this type of division - mitosis. As a result of the spiralization of chromonemas, chromosomes in prophase become denser, shortened and become clearly visible. By the end of prophase, it can be clearly observed that each chromosome consists of two chromatids closely touching each other. Subsequently, both chromatids are connected by a common area - the centromere and begin to gradually move towards the cell equator.

In the middle or at the end of prophase, the nuclear envelope and nucleoli disappear, the centrioles double and move towards the poles. A fission spindle begins to form from the material of the cytoplasm and nucleus. It consists of two types of threads: supporting and pulling (chromosomal). The supporting threads form the basis of the spindle; they stretch from one pole of the cell to the other. Traction threads connect the centromeres of the chromatids to the poles of the cell and subsequently ensure the movement of chromosomes towards them. The mitotic apparatus of the cell is very sensitive to various external influences. When exposed to radiation, chemicals and high temperatures, the cell spindle can be destroyed, and all sorts of irregularities in cell division occur.

Metaphase(from the Greek meta - after, phase - manifestation). In metaphase, chromosomes become highly compacted and acquire a specific shape characteristic of a given species. The daughter chromatids in each pair are separated by a clearly visible longitudinal cleft. Most chromosomes become double-armed. At the point of inflection - the centromere - they are attached to the spindle thread. All chromosomes are located in the equatorial plane of the cell, their free ends are directed towards the center of the cell. Chromosomes are best observed and counted at this time. The cell spindle is also very clearly visible.

Anaphase(from the Greek ana - up, phase - manifestation). In anaphase, following the division of the centromeres, the chromatids, which have now become separate chromosomes, begin to separate to opposite poles. In this case, the chromosomes have the form of various hooks, with their ends facing the center of the cell. Since two completely identical chromatids arose from each chromosome, the number of chromosomes in both resulting daughter cells will be equal to the diploid number of the original mother cell.

The process of centromere division and movement to different poles of all newly formed paired chromosomes is characterized by exceptional synchrony.

At the end of anaphase, the chromonemal threads begin to unwind, and the chromosomes that have moved to the poles are no longer visible so clearly.

Telophase(from Greek telos - end, phase - manifestation). In telophase, the despiralization of chromosome threads continues, and the chromosomes gradually become thinner and longer, approaching the state in which they were in prophase. A nuclear envelope is formed around each group of chromosomes and a nucleolus is formed. At the same time, cytoplasmic division is completed and a cell septum appears. Both new daughter cells enter interphase.

The entire process of mitosis, as already noted, takes no more than 2 hours. Its duration depends on the type and age of the cells, as well as on the external conditions in which they are located (temperature, light, air humidity, etc.). High temperatures, radiation, various drugs and plant poisons (colchicine, acenaphthene, etc.) negatively affect the normal course of cell division.

Mitotic cell division is distinguished by a high degree of accuracy and perfection. The mechanism of mitosis was created and improved over many millions of years of evolutionary development of organisms. In mitosis, one of the most important properties of the cell as a self-governing and self-reproducing living biological system finds its manifestation.

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