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In anaphase of mitosis, cells move apart towards the poles. Mitosis - indirect cell division

Lecture No. 10

Number of hours: 2

MITOSIS

1. Cell life cycle

2. Mitosis. Stages of mitosis, their duration and characteristics

3. Amitosis. Endoreproduction

1. Cell life cycle

The cells of a multicellular organism are extremely diverse in the functions they perform. In accordance with their specialization, cells have different durations life. So, after embryogenesis is completed, nerve cells stop dividing and function throughout the life of the organism. Cells of other tissues (bone marrow, epidermis, epithelium) small intestine) in the process of performing their function, they quickly die and are replaced by new ones as a result of cell division.Cell division underlies the development, growth and reproduction of organisms. Cell division also ensures self-renewal of tissues throughout the life of the body and restoration of their integrity after damage. There are two ways to divide somatic cells: amitosis And mitosis. Mostly common indirect division cells (mitosis). Reproduction by mitosis is called asexual reproduction, vegetative propagation or cloning.

Cell life cycle (cell cycle) is the existence of a cell from division until the next division or death. The duration of the cell cycle in reproducing cells is 10-50 hours and depends on the type of cells, their age, hormonal balance body, temperature and other factors. Details of the cell cycle vary among different organisms. In single-celled organisms life cycle coincides with the life of the individual. In continuously reproducing tissue cells, the cell cycle coincides with the mitotic cycle.

Mitotic cycle - a set of sequential and interconnected processes during the period of cell preparation for division and the period of division (Fig. 1). In accordance with the above definition, the mitotic cycle is divided into interphase And mitosis (Greek “mitos” - thread).

Interphase- the period between two cell divisions - is divided into phases G 1, S and G 2 (their duration is indicated below, typical for plant and animal cells.). In terms of duration, interphase makes up the majority of the cell's mitotic cycle. Most variable over time G 1 and G 2 periods.

G 1 (from English.grow- grow, increase). The duration of the phase is 4–8 hours. This phase begins immediately after the formation of the cell. During this phase, RNA and proteins are intensively synthesized in the cell, and the activity of enzymes involved in DNA synthesis increases. If the cell does not further divide, it enters the phase G 0 – period of rest. Taking into account the resting period, the cell cycle can last weeks or even months (liver cells).

S (from English)synthesis- synthesis).The duration of the phase is 6–9 hours. The cell mass continues to increase, and chromosomal DNA is doubled. The two helices of the old DNA molecule separate, and each becomes a template for the synthesis of new DNA strands. As a result, each of the two daughter molecules necessarily includes one old helix and one new one. However, the chromosomes remain single in structure, although doubled in mass, since two copies of each chromosome (chromatids) are still connected to each other along their entire length. After completing the phase S During the mitotic cycle, the cell does not immediately begin to divide.

G2.In this phase, the cell completes the process of preparation for mitosis: ATP accumulates, achromatin spindle proteins are synthesized, and centrioles double. The cell's mass continues to increase until it is approximately twice its original mass, and then mitosis occurs.

Rice. Mitotic cycle: M- mitosis, P - prophase, Mf - metaphase, A - anaphase, T- telophase, G 1 - presynthetic period, S - synthetic period, G 2 - postsynthetic

2. Mitosis. Stages of mitosis, their duration and characteristics. Mitosis is conditional divided into four phases: prophase, metaphase, anaphase and telophase.

Prophase.The two centrioles begin to diverge towards opposite poles of the nucleus. The nuclear membrane is destroyed; at the same time, special proteins combine to form microtubules in the form of threads. Centrioles, now located at opposite poles of the cell, have an organizing effect on microtubules, which as a result line up radially, forming a structure resembling appearance aster flower (“star”). Other filaments of microtubules extend from one centriole to another, forming a spindle. At this time, the chromosomes spiral and, as a result, thicken. They are clearly visible in light microscope, especially after dyeing. Reading genetic information from DNA molecules becomes impossible: RNA synthesis stops and the nucleolus disappears. In prophase, the chromosomes split, but the chromatids still remain attached in pairs at the centromere. Centromeres also have an organizing effect on the spindle filaments, which now stretch from centriole to centromere and from it to another centriole.

Metaphase.In metaphase, chromosome spiralization reaches its maximum, and shortened chromosomes rush to the equator of the cell, located at an equal distance from the poles. Formed equatorial, or metaphase, plate. At this stage of mitosis, the structure of the chromosomes is clearly visible, they are easy to count and study. individual characteristics. Each chromosome has a region of primary constriction - the centromere, to which the spindle thread and arms are attached during mitosis. At the metaphase stage, the chromosome consists of two chromatids, connected to each other only at the centromere.

Rice. 1. Mitosis of a plant cell. A - interphase;
B, C, D, D- prophase; E, F-metaphase; 3, I - anaphase; K, L, M-telophase

IN anaphase the viscosity of the cytoplasm decreases, the centromeres are separated, and from this moment the chromatids become independent chromosomes. The spindle threads attached to the centromeres pull the chromosomes towards the poles of the cell, while the chromosome arms passively follow the centromere. Thus, in anaphase, the chromatids of the chromosomes doubled in interphase precisely diverge to the poles of the cell. At this moment, the cell contains two diploid sets of chromosomes (4n4c).

Table 1. Mitotic cycle and mitosis

Phases		Process occurring in the cell
Interphase	Presynthetic period (G1)	Protein synthesis. RNA is synthesized on despiralized DNA molecules
	Synthetic period (S)	DNA synthesis is the self-duplication of a DNA molecule. Construction of the second chromatid into which the newly formed DNA molecule passes: bichromatid chromosomes are obtained
	Postsynthetic period (G2)	Protein synthesis, energy storage, preparation for division
Phases mitosis	Prophase	Bichromatid chromosomes spiral, nucleoli dissolve, centrioles separate, nuclear envelope dissolves, spindle filaments are formed
	Metaphase	The spindle strands are attached to the centromeres of the chromosomes, bichromatid chromosomes are concentrated at the equator of the cell
	Anaphase	Centromeres divide, single-chromatid chromosomes are stretched by spindle filaments to the cell poles
	Telophase	Single-chromatid chromosomes despiral, a nucleolus is formed, the nuclear membrane is restored, a partition between cells begins to form at the equator, and the spindle filaments dissolve

IN telophase chromosomes unwind and despiral. The nuclear envelope is formed from the membrane structures of the cytoplasm. At this time, the nucleolus is restored. This completes nuclear division (karyokinesis), then division of the cell body (or cytokinesis) occurs. When animal cells divide, a groove appears on their surface in the equatorial plane, gradually deepening and dividing the cell into two halves - daughter cells, each of which has a nucleus. 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 (i.e., grows from the inside out). The cell plate is formed from material supplied by the endoplasmic reticulum. Then each of the daughter cells forms on its side cell membrane and finally, cellulose cell walls are formed on both sides of the plate. Features of the course of mitosis in animals and plants are given in Table 2.

Table 2. Features of mitosis in plants and animals

plant cell

animal cell

There are no centrioles

No stars are formed

A cell plate is formed

During cytokinesis, no furrow is formed

Mitoses are predominantly

occur in meristems

Centrioles are present

Stars are formed

Cell plate is not formed

During cytokinesis, a furrow is formed

Mitoses occur

in various tissues of the body

Thus, from one cell two daughter cells are formed, in which the hereditary information exactly copies the information contained in the mother cell. Starting from the first mitotic division of a fertilized egg (zygote), all daughter cells resulting from mitosis contain the same set of chromosomes and the same genes. Therefore, mitosis is a method of cell division that involves the precise distribution of genetic material between daughter cells. As a result of mitosis, both daughter cells receive a diploid set of chromosomes.

The entire process of mitosis takes in most cases from 1 to 2 hours. Frequency of mitosis in different tissues and in different types different. For example, in red bone marrow In humans, where 10 million red blood cells are formed every second, 10 million mitoses should occur every second. And in nerve tissue mitoses are extremely rare: for example, in the central nervous system, cells generally stop dividing in the first months after birth; and in the red bone marrow, in the epithelial lining digestive tract and in the epithelium of the renal tubules they divide until the end of life.

Regulation of mitosis, the question of the trigger mechanism of mitosis.

The factors that induce a cell to undergo mitosis are not precisely known. But it is believed that the factor of the ratio of the volumes of the nucleus and cytoplasm (nuclear-plasma ratio) plays a major role. According to some data, dying cells produce substances that can stimulate cell division. The protein factors responsible for the transition to the M phase were initially identified based on cell fusion experiments. The fusion of a cell at any stage of the cell cycle with a cell in the M phase leads to the entry of the nucleus of the first cell into the M phase. This means that in a cell in the M phase there is a cytoplasmic factor capable of activating the M phase. Later, this factor was secondarily discovered in experiments on the transfer of cytoplasm between frog oocytes at different stages of development, and was called the “maturation promoting factor” MPF (maturation promoting factor). Further study of MPF showed that this protein complex determines all M-phase events. The figure shows that nuclear membrane breakdown, chromosome condensation, spindle assembly, and cytokinesis are regulated by MPF.

Mitosis is inhibited by high temperature, high doses ionizing radiation, the action of plant poisons. One such poison is called colchicine. With its help, you can stop mitosis at the stage of the metaphase plate, which allows you to count the number of chromosomes and give each of them an individual characteristic, i.e., carry out karyotyping.

4. Amitosis. Endoreproduction

Amitosis (from the Greek a - negative particle and mitosis) -direct division of the interphase nucleus by ligation without transformation of chromosomes. During amitosis, uniform divergence of chromatids to the poles does not occur. And this division does not ensure the formation of genetically equivalent nuclei and cells. Compared to mitosis, amitosis is a shorter and more economical process. Amitotic division can occur in several ways. The most common type of amitosis is the lacing of the nucleus into two parts. This process begins with the division of the nucleolus. The constriction deepens, and the core is divided in two. After this, the separation of the cytoplasm begins, but this does not always happen. If amitosis is limited only to nuclear division, then this leads to the formation of bi- and multinucleated cells. During amitosis, budding and fragmentation of nuclei can also occur.

A cell that has undergone amitosis is subsequently unable to enter the normal mitotic cycle.

Amitosis occurs in cells of various tissues of plants and animals. In plants, amitotic division occurs quite often in the endosperm, in specialized root cells and in storage tissue cells. Amitosis is also observed in highly specialized cells with weakened viability or degenerating, under various pathological processes, such as malignant growth, inflammation, etc.

The main process in preparing a cell for mitosis is DNA replication and chromosome duplication. But DNA synthesis and mitosis are not directly related, because final DNA synthesis is not direct cause cell entry into mitosis. Therefore, in some cases, cells do not divide after chromosome doubling; the nucleus and all cells increase in volume and become polyploid. This phenomenon - reduplication of chromosomes, without division, developed in the process of evolution as a method to ensure the growth of organs without increasing the number of cells. All cases where chromosome reduplication or DNA replication occurs, but mitosis does not occur, are called endoreproductions. The cells become polyploid. How constant process endoreproduction is observed in liver and epithelial cells urinary tract mammals. When endomitosis chromosomes become visible after reduplication, but the nuclear membrane is not destroyed.

If dividing cells are cooled for a while ortreat them with some substance that destroys microtubulesspindles (for example, colchicine), then cell division will stopXia. In this case, the spindle will disappear, and the chromosomes without divergence tothe poles will continue the cycle of their transformations: they will beginto swell, to be covered with a nuclear membrane. This arises due tounions of all undiverged sets of chromosomes largenew kernels. They will naturally contain a 4n number at the beginningchromatid and, accordingly, 4c the amount of DNA. A-priory,it is no longer a diploid, but a tetraploid cell. Such polyplo idnyecells can go out of stage gi go to the S-period and, if remove colchicine, divide again mitotically, givingdescendants with 4 n number of chromosomes. As a result, you can getpolyploid cell lines different ploidy values.This technique is often used to produce polyploid plants.

As it turned out, in many organs and tissues of normal diploidy organisms of animals and plants contain cellswith large nuclei, the amount of DNA in which is many times greater2 p. When such cells divide, it is clear that the number of chromosomesthey also have a multiple increase in comparison with ordinary diploside cells. These cells are the result of somaticskoy polyploidy. This phenomenon is often called endoreproduct tion- - the appearance of cells with increased DNA content.The appearance of such cells occurs as a result of the lackin general or incompleteness of individual stages of mitosis. ExistingThere are several points in the process of mitosis, the blockade of whichwill lead to its stop and to the appearance of polyploid cells.A block may occur during the transition from the C 2 period to the properbut mitosis, arrest can occur in prophase and metaphase, inIn the latter case, there is often a violation of the integrity of thefission retena. Finally, cytotomy abnormalities may alsostop fission, which will lead to the appearance of binuclear and polyploid cells.

With a natural blockade of mitosis at its very beginning, with transition G 2 - prophase, cells begin the next cyclereplication, which will lead to a progressive increase inamount of DNA in the nucleus. In this case, no morphologicallogical features of such kernels, in addition to their large sizes.When the nuclei are enlarged, mitoti chromosomes are not detected in them chetical type. Often this type of endoreproduction without mitotic condensationsation of chromosomes occurs in invertebrate animals, revealing It is also found in vertebrates and plants.In invertebrates, as a result of a block of mitosis, the degree of polyploidy can reach enormous values. So, in giantneurons of the mollusk tritonia, the nuclei of which reach the size up to 1 mm (!), contains more than 2-10 5 haploid sets of DNA.Another example of a giant polyploid cell isresulting from DNA reduplication without the entry of cellscurrent into mitosis, can serve as a silk gland cellsilkworm. Its core has a bizarre branchingform and may contain huge quantities DNA. GiganticAscaris esophageal gland cells can contain up to 100,000c DNA.

A special case endoreproduction is an increaseploidy reduction by polythenia. When poured in S -period during replication of DIC new toblack chromosomes continue to remain despiralizedcondition, but are located near each other, do not diverge anddo not undergo mitotic condensation. In suchIn a truly interphase form, the chromosomes again enter the next replication cycle, double again and do not diverge. Bygradually as a result of replication and nondisjunction of chromosomalthreads, a multifilamentous, polytene chromoso structure is formedwe have an interphase nucleus. The last circumstance is necessary underdraw a line, since such giant polytene chromosomes are neitherwhen they do not participate in mitosis, moreover, this is truly interphasenal chromosomes involved in the synthesis of DNA and RNA.They differ sharply from mitotic chromosomes in size.frames: several times thicker than mitotic chromosomes due tothat they consist of a bundle of multiple unseparated chromatid - 1000 times the volume of Drosophila polytene chromosomes “more mitotic. They are 70-250 times longer than mitotic due to the fact that in the interphase state the chromosomes are less condensed densified (coiled) than mitotic chromosomes.In addition, Diptera have their total number equal in cells haploid due to the fact that during polytenization there is a volume formation, conjugation of homologous chromosomes. So, in DrosophilaThere are 8 chromosomes in a diploid somatic cell, and in a giant cellcage salivary gland - 4. There are giant polyploid nuclei with polytene chromosomes in some larvae of dipteran insects in the cellkah salivary glands, intestines, malpighian vessels, adipose bodies, etc. Polytene chromosomes in the macronucleus infuso are described ria stilonychia. This type of endoreproduction has been best studied in insects.It has been calculated that in Drosophila, in the cells of the salivary glandsup to 6-8 cycles of reduplication may occur, leading tototal cell ploidy equal to 1024. In some chironomids(their larva is called a bloodworm) ploidy in these cells is up toreaches 8000-32000. In cells, polytene chromosomes beginbe visible after reaching polyteny at 64-128 p, before thatsuch nuclei do not differ in anything except size from surrounding onesdiploid nuclei.

Polytene chromosomes also differ in their structure: they structurally heterogeneous along the length, consist of disks, betweenkovy areas and poufs. Location drawingdisks is strictly characteristic of each chromosome and differseven in closely related animal species. The disks are areas of condensed chromium matina. Discs may vary in thickness. Their total number in polytene chromosomes of chironomids reaches 1.5-2.5 thousand.Drosophila has about 5 thousand discs.The discs are separated by interdiscal spaces, which, like discs, consist of chromatin fibrils, only looser packed. On the polytene chromosomes of dipterans, swellings are often visible,poufs. It turned out that poufs appear in places of some diskov due to their decondensation and loosening. Reveals in poufsRNA is present and synthesized there.The pattern of arrangement and alternation of discs on polytene chromosomes is constant and does not depend on either the organ or age animal. This is a good illustration of the sameness the quality of genetic information in every cell of the body.Puffs are temporary formations on chromosomes, and during the development of the organism there is a certain sequence in their appearance and disappearance on the genetically various areas chromosomes. This afterbirthThe effectiveness varies for different fabrics. It has now been proven thatthe formation of puffs on polytene chromosomes is an expressiongene activity: RNA necessary for for carrying out protein synthesis on different stages insect development. IN natural conditions in Diptera they are especially active inin relation to RNA synthesis, the two largest puffs, the so-calledwashed rings by Balbiani, who described them 100 years ago.

In other cases of endoreproduction polyploid cells cartdisappear as a result of violations of the division apparatus - spindle:In this case, mitotic condensation of chromosomes occurs. This the phenomenon is called endomitosis, because condensation of chromomosome and their changes occur inside the nucleus, without disappearingnuclear shell.For the first time, the phenomenon of endomitosis was well studied in cells:various tissues of the water bug - Guerria. At the beginning of endomiAs a result, chromosomes condense, causing them to become homogeneousclearly distinguishable inside the nucleus, then the chromatids separate, stretch out. These stages, according to the state of the chromosomes, can correspond to promote prophase and metaphase of normal mitosis. Then the chromosomesin such nuclei disappear, and the nucleus takes the form of an ordinary interphase core, but its size increases in accordance with the increasedetermination of ploidy. After the next DNA reduplication, this cycle of endomitosis is repeated. As a result, there may bepolyploid (32 p) and even giant nuclei.A similar type of endomitosis has been described during the development of macronucleiowls in some ciliates and in a number of plants.

Endoreproduction result: polyploidy and increase in cell size.

The value of endoreproduction: cell activity is not interrupted. So, for example, intion nerve cells would lead to their temporary shutdownfunctions; endoreproduction allows without interruption in functionin order to increase cell mass and thereby increase the volumeThis is the amount of work performed by one cell.

increasing cell productivity.

Mitosis is conventionally divided into four phases: prophase, metaphase, anaphase and telophase.

Prophase. The two centrioles begin to diverge towards opposite poles of the nucleus. The nuclear membrane is destroyed; at the same time, special proteins combine to form microtubules in the form of threads. The centrioles, now located at opposite poles of the cell, have an organizing effect on the microtubules, which as a result line up radially, forming a structure reminiscent in appearance of an aster flower (“star”). Other filaments of microtubules extend from one centriole to another, forming a spindle. At this time, the chromosomes spiral and, as a result, thicken. They are clearly visible in a light microscope, especially after staining. Reading genetic information from DNA molecules becomes impossible: RNA synthesis stops and the nucleolus disappears. In prophase, the chromosomes split, but the chromatids still remain attached in pairs at the centromere. Centromeres also have an organizing effect on the spindle filaments, which now stretch from centriole to centromere and from it to another centriole.

Metaphase. In metaphase, chromosome spiralization reaches its maximum, and shortened chromosomes rush to the equator of the cell, located at an equal distance from the poles. Formed equatorial, or metaphase, plate. At this stage of mitosis, the structure of the chromosomes is clearly visible, they are easy to count and study their individual characteristics. Each chromosome has a region of primary constriction - the centromere, to which the spindle thread and arms are attached during mitosis. At the metaphase stage, the chromosome consists of two chromatids, connected to each other only at the centromere.

Rice. 1. Mitosis of a plant cell. A - interphase;
B, C, D, D- prophase; E, F-metaphase; 3, I - anaphase; K, L, M-telophase

Table 1. Mitotic cycle and mitosis

Phases	Process occurring in the cell
Interphase	Presynthetic period (G1)	Protein synthesis. RNA is synthesized on despiralized DNA molecules
Synthetic period (S)	DNA synthesis is the self-duplication of a DNA molecule. Construction of the second chromatid into which the newly formed DNA molecule passes: bichromatid chromosomes are obtained
Postsynthetic period (G2)	Protein synthesis, energy storage, preparation for division
Phases mitosis	Prophase	Bichromatid chromosomes spiral, nucleoli dissolve, centrioles separate, nuclear envelope dissolves, spindle filaments are formed
Metaphase	The spindle strands are attached to the centromeres of the chromosomes, bichromatid chromosomes are concentrated at the equator of the cell
Anaphase	Centromeres divide, single-chromatid chromosomes are stretched by spindle filaments to the cell poles
Telophase	Single-chromatid chromosomes despiral, a nucleolus is formed, the nuclear membrane is restored, a partition between cells begins to form at the equator, and the spindle filaments dissolve

IN telophase chromosomes unwind and despiral. The nuclear envelope is formed from the membrane structures of the cytoplasm. At this time, the nucleolus is restored. This completes nuclear division (karyokinesis), then division of the cell body (or cytokinesis) occurs. When animal cells divide, a groove appears on their surface in the equatorial plane, gradually deepening and dividing the cell into two halves - daughter cells, each of which has a nucleus. 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 (i.e., grows from the inside out). The cell plate is formed from material supplied by the endoplasmic reticulum. Each of the daughter cells then forms a cell membrane on its side and finally cellulose cell walls are formed on both sides of the plate. Features of the course of mitosis in animals and plants are given in Table 2.

Table 2. Features of mitosis in plants and animals

The entire process of mitosis takes in most cases from 1 to 2 hours. The frequency of mitosis varies between tissues and species. For example, in human red bone marrow, where 10 million red blood cells are formed every second, 10 million mitoses should occur every second. And in nervous tissue, mitoses are extremely rare: for example, in the central nervous system, cells generally stop dividing in the first months after birth; and in the red bone marrow, in the epithelial lining of the digestive tract and in the epithelium of the renal tubules, they divide until the end of life.

Regulation of mitosis, the question of the trigger mechanism of mitosis.

Mitosis is inhibited by high temperature, high doses of ionizing radiation, and the action of plant poisons. One such poison is called colchicine. With its help, you can stop mitosis at the stage of the metaphase plate, which allows you to count the number of chromosomes and give each of them an individual characteristic, i.e., carry out karyotyping.

Amitosis (from the Greek a - negative particle and mitosis)-direct division of the interphase nucleus by ligation without transformation of chromosomes. During amitosis, uniform divergence of chromatids to the poles does not occur. And this division does not ensure the formation of genetically equivalent nuclei and cells. Compared to mitosis, amitosis is a shorter and more economical process. Amitotic division can occur in several ways. The most common type of amitosis is the lacing of the nucleus into two parts. This process begins with the division of the nucleolus. The constriction deepens, and the core is divided in two. After this, the separation of the cytoplasm begins, but this does not always happen. If amitosis is limited only to nuclear division, then this leads to the formation of bi- and multinucleated cells. During amitosis, budding and fragmentation of nuclei can also occur.

A cell that has undergone amitosis is subsequently unable to enter the normal mitotic cycle.

Amitosis occurs in cells of various tissues of plants and animals. In plants, amitotic division occurs quite often in the endosperm, in specialized root cells and in storage tissue cells. Amitosis is also observed in highly specialized cells with weakened viability or degenerating, during various pathological processes such as malignant growth, inflammation, etc.

What are mitosis and meiosis and what phases do they have? cells with some differences. During meiosis, four daughter nuclei are formed from the mother nucleus, in which the number of chromosomes is reduced by half. Mitosis also occurs, but in this type only two daughter cells are formed with the same chromosomes as the parents.

So is meiosis? These are biological division procedures that produce cells with specific chromosomes. Reproduction by mitosis occurs in multicellular, complex living organisms.

Stages

Mitosis occurs in two stages:

Doubling information at the gene level. Here, mother cells distribute genetic information among themselves. At this stage, the chromosomes change.
Mitotic stage. It consists of time periods.

Cell formation occurs in several stages.

Phases

Mitosis is divided into several phases:

telophase;
anaphase;
metaphase;
prophase.

These phases occur in a certain sequence and have their own characteristics.

In any complex multicellular organism, mitosis most often involves cell division according to an undifferentiated type. During mitosis, the mother cell divides into daughter cells, usually two. One of them becomes a stem and continues division, and the second stops dividing.

Interphase

Interphase is the cell's preparation for division. Typically this stage lasts up to twenty hours. At this time, many of the most different processes, during which cells prepare for mitosis.

During this period, protein division occurs and the number of organelles in the DNA structure increases. By the end of division, the genetic molecules double, but the number of chromosomes does not change. Identical DNAs are spliced and are two chromatids in one molecule. The resulting chromatids are identical and sister.

After the completion of interphase, mitosis proper begins. It consists of prophase, metaphase, anaphase and telophase.

Prophase

The first phase of mitosis is prophase. It lasts about an hour. It is conventionally divided into several stages. On initial stage In prophase of mitosis, the nucleolus enlarges, resulting in the formation of molecules. By the end of the phase, each chromosome already consists of two chromatids. The nucleoli and nuclear membranes dissolve, all the elements in the cell are in disarray. Further, in the prophase of mitosis, the formation of achromatic division occurs, some of the threads pass through the entire cell, and some are connected to the central elements. During this process, the content of the genetic code remains unchanged.

The number of chromosomes does not change in prophase of mitosis. What else happens? In prophase of mitosis, the nuclear membrane disintegrates, resulting in spiral chromosomes ending up in the cytoplasm. Particles of the disintegrated nuclear membrane form small membrane vesicles.

In the prophase of mitosis, the following happens: an animal cell becomes round, but in plants it does not change shape.

Metaphase

After prophase comes metaphase. In this phase, chromosome spiralization reaches its peak. The shortened chromosomes begin to move towards the center of the cell. During movement, they are located equally in both parts. Here the metaphase plate is formed. When examining a cell, chromosomes are clearly visible. It is during metaphase that they are easy to count.

After the formation of the metaphase plate, the set of chromosomes inherent in this cell type is analyzed. This occurs by blocking chromosome segregation using alkaloids.

Each organism has its own set of chromosomes. For example, corn has 20, and garden strawberries- 56.V human body Fewer chromosomes than berries, only 46.

Anaphase

All processes occurring in prophase of mitosis end and anaphase begins. During this process, all chromosomal connections are broken and begin to move in opposite directions from each other. In anaphase, related chromosomes become independent. They end up in different cells.

The phase ends with the divergence of chromatids to the poles of the cell. Also here the distribution of hereditary information between daughter and mother cells occurs.

Telophase

Chromosomes are located at the poles. Under a microscope, they become difficult to see, as a nuclear shell forms around them. The fission spindle is completely destroyed.

In plants, the membrane forms in the center of the cell, gradually spreading to the poles. It divides the mother cell into two parts. Once the membrane has fully grown, a cellulose wall appears.

Features of mitosis

Cell division may be inhibited due to high temperatures, exposure to poisons, radiation. While studying cell mitosis in different multicellular organisms You can use poisons that inhibit mitosis at the metaphase stage. This allows you to study chromosomes in detail and carry out karyotoping.

Mitosis in the table

Consider the phases of cell division in the table below.

The process of the stages of mitosis can also be traced in the table.

Mitosis in animals and plants

The features of this process can be described in a comparative table.

So, we examined the process of cell division in animal organisms and plants, as well as their features and differences.

There are four phases of mitosis: prophase, metaphase, anaphase and telophase. IN prophase clearly visible centrioles- formations located in the cell center and playing a role in the division of daughter chromosomes of animals. (Remember that higher plants There are no centrioles in the cell center, which organizes chromosome division). We will look at mitosis using the example animal cell, since the presence of a centriole makes the process of chromosome division more visual. Centrioles divide and move to different poles of the cell. Microtubules extend from the centrioles, forming filaments of the spindle, which regulates 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 spiral and, as a result, shorten and thicken, and they can already be observed under a light microscope. They are even better visible at the next stage of mitosis - metaphase.
In metaphase, chromosomes are located in the equatorial plane of the cell. It is clearly visible that each chromosome, consisting of two chromatids, has a constriction - centromere. Chromosomes are attached to the spindle filament by their centromeres. After centromere division, each chromatid becomes an independent daughter chromosome.
Then comes next stage mitosis - anaphase, during which 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 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 process of cytoplasmic division, all organelles (mitochondria, Golgi complex, ribosomes, etc.) are distributed more or less evenly between daughter cells.
Thus, as a result of mitosis, one cell turns into two, each of which has a characteristic number and shape of chromosomes for a given type of organism, and therefore a constant amount of DNA.
The entire 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 external environment(temperature, light conditions and other indicators).
The biological significance of mitosis is 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. During 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 in the body receive the same genetic information.

Cell division - biological process, which underlies the reproduction and individual development of all living organisms.

The most widespread form of cell reproduction in living organisms is indirect division, or mitosis (from the Greek “mitos” - thread). Mitosis consists of four successive phases. Mitosis provides uniform distribution genetic information of the parent cell between daughter cells.

The period of cell life between two mitoses is called interphase. It is ten times longer than mitosis. In it a series of very important processes, preceding cell division: ATP and protein molecules are synthesized, each chromosome doubles, forming two sister chromatids held together by a common centromere, the number of main organelles of the cell increases.

Mitosis

There are four phases in the process of mitosis: prophase, metaphase, anaphase and telophase.

I. Prophase is the longest phase of mitosis. In it, chromosomes, consisting of two sister chromatids held together by the centromere, spiral and as a result thicken. By the end of prophase, the nuclear membrane and nucleoli disappear and chromosomes are dispersed throughout the cell. In the cytoplasm, towards the end of prophase, the centrioles extend to the stripes and form the spindle.
II. Metaphase - chromosomes continue to spiral, their centromeres are located along the equator (in this phase they are most visible). The spindle threads are attached to them.
III. Anaphase - centromeres divide, sister chromatids separate from each other and, due to the contraction of spindle filaments, move to opposite poles of the cell.
IV. Telophase - the cytoplasm divides, chromosomes unwind, nucleoli and nuclear membranes are formed again. After this, a constriction is formed in the equatorial zone of the cell, separating two sister cells.

So from one initial cell (maternal) two new ones are formed - daughter ones, having a chromosome set that is in quantity and quality, in terms of the content of hereditary information, morphological, anatomical and physiological characteristics completely identical to the parents.

Height, individual development, the constant renewal of tissues of multicellular organisms is determined by the processes of mitotic cell division.

All changes that occur during mitosis are controlled by the neuroregulation system, i.e. nervous system, hormones of the adrenal glands, pituitary gland, thyroid gland and etc.

Meiosis (from the Greek “meiosis” - reduction) is a division in the maturation zone of germ cells, accompanied by a halving of the number of chromosomes. It also consists of two sequential divisions, which have the same phases as mitosis. However, the duration of individual phases and the processes occurring in them differ significantly from the processes occurring in mitosis.

These differences are mainly as follows. In meiosis, prophase I is longer. It is where the conjugation (connection) of chromosomes and the exchange of genetic information occurs. (In the figure above, prophase is marked with numbers 1, 2, 3, conjugation is shown with number 3). In metaphase, the same changes occur as in metaphase of mitosis, but with a haploid set of chromosomes (4). In anaphase I, the centromeres holding the chromatids together do not divide, and one of the homologous chromosomes moves to the poles (5). In telophase II, four cells with a haploid set of chromosomes are formed (6).

The interphase before the second division in meiosis is very short, during which DNA is not synthesized. Cells (gametes) formed as a result of two meiotic divisions contain a haploid (single) set of chromosomes.

The full set of chromosomes - diploid 2n - is restored in the body during fertilization of the egg, during sexual reproduction.

Sexual reproduction is characterized by the exchange of genetic information between females and males. It is associated with the formation and fusion of special haploid germ cells - gametes, formed as a result of meiosis. Fertilization is the process of fusion of an egg and a sperm (female and male gamete), in which the diploid set of chromosomes is restored. The fertilized egg is called a zygote.

During the fertilization process you can observe various options gamete connections. For example, when both gametes that have the same alleles of one or more genes merge, a homozygote is formed, the offspring of which retain all the characteristics in pure form. If the genes in the gametes are represented by different alleles, a heterozygote is formed. Hereditary rudiments corresponding to various genes are found in her offspring. In humans, homozygosity is only partial, for individual genes.

The basic patterns of transmission of hereditary properties from parents to descendants were established by G. Mendel in the second half of the 19th century. Since that time, such concepts as dominant and recessive traits, genotype and phenotype, etc. have been firmly established in genetics (the science of the laws of heredity and variability of organisms). Dominant traits- predominant, recessive - inferior, or disappearing in subsequent generations. In genetics, these traits are denoted by letters of the Latin alphabet: dominant are denoted in capital letters, recessive are denoted in lowercase letters. In the case of homozygosity, each of a pair of genes (alleles) reflects either dominant or recessive traits, which manifest their effect in both cases.

U heterozygous organisms the dominant allele is located on one chromosome, and the recessive allele, suppressed by the dominant, is in the corresponding region of another homologous chromosome. During fertilization, a new combination of the diploid set is formed. Consequently, the formation of a new organism begins with the fusion of two germ cells (gametes) resulting from meiosis. During meiosis, a redistribution of genetic material (gene recombination) occurs in descendants or an exchange of alleles and their combination in new variations, which determines the appearance of a new individual.

Soon after fertilization, DNA synthesis occurs, chromosomes are doubled, and the first division of the zygote nucleus occurs, which occurs through mitosis and represents the beginning of the development of a new organism.