It is considered the initial phase of mitosis. Mitosis, cell cycle

Cell division is a biological process that 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 ensures that the genetic information of the parent cell is evenly distributed among the daughter cells.

The period of cell life between two mitoses is called interphase. It is ten times longer than mitosis. A number of very important processes occur in it prior to cell division: ATP and protein molecules are synthesized, each chromosome doubles, forming two sister chromatids held together by a common centromere, and 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.

Growth, individual development, and constant renewal of tissues of multicellular organisms are determined by the processes of mitotic cell division.

All changes that occur during the process of 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 gametes), during which the diploid set of chromosomes is restored. The fertilized egg is called a zygote.

During the process of fertilization, various variants of the connection of gametes can be observed. For example, the fusion of both gametes that have the same alleles of one or more genes produces a homozygote, the offspring of which retain all the characteristics in their 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, in genetics (the science of the laws of heredity and variability of organisms), concepts such as dominant and recessive traits, genotype and phenotype, etc. have been firmly established. Dominant traits are dominant, recessive traits are inferior or disappear in subsequent generations. In genetics, these traits are designated by letters of the Latin alphabet: dominant ones are designated in capital letters, recessive - lowercase. 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.

In 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, the redistribution of genetic material (recombination of genes) occurs in the offspring or the exchange of alleles and their combination in new variations, which determines the appearance of a new individual.

Shortly after fertilization, DNA synthesis occurs, the chromosomes double, and the first division of the zygote nucleus occurs, which is carried out by mitosis and represents the beginning of the development of a new organism.

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 chromosome threads become clearly visible in it, which at this time are already spiralized.

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 exact distribution of genetic material occurs, and at each pole there is the same number of chromosomes as the original cell had before they doubled.

The movement of chromatids to the poles occurs due to contraction of the trailing threads and 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.

Mitosis goes through several phases, which are characterized by the location and behavior of chromosomes

Some transitions between phases correspond to cell cycle events and represent irreversible transitions

Mitosis occurs when two separate and dissimilar processes complete. During the first process, sometimes called karyokinesis (Greek: karyo - nucleus; kinesis - division), the replicated chromosomes are distributed into two separate daughter nuclei. In the second process, called cytokinesis, the cytoplasm is divided between these two nuclei and two separate daughter cells are formed. Historically, nuclear division is divided into several phases, depending on the structure and position of the chromosomes.

Division into phases of such a complex event as mitosis, is useful because some transitions cause irreversible changes in the cell. Most of these changes are associated with the activation or inactivation of certain enzymes. Sometimes changes can be accompanied by the destruction of specific proteins that play an important role in division. It is also useful to view mitosis as a series of events because the chromosomes and spindle change their behavior between phases. This suggests that each phase has its own specific molecular mechanism. For a detailed look at the phases of mitosis, we will use the figure below.

The first visible sign upcoming division is the appearance of condensed chromosomes in the nucleus. This is the beginning of a phase of mitosis called prophase. In cold-blooded animals, whose cells contain large chromosomes (for example, salamanders, grasshoppers), prophase lasts several hours; in warm-blooded animals with small chromosomes (for example, mice, humans), it lasts less than 15 minutes. At a certain point in prophase, biochemical changes occur in the cell, which transfer it to a state committed to mitosis. Until the point of irreversibility is reached, chromosome condensation can be interrupted by physical or chemical influences that damage the cell.

Prophase is usually characterized by the appearance centrosomes. In many cells, in the cytoplasm, two organelles become visible in the form of small dots surrounded by a light area. As we will see later, centrosomes play an important role in the formation of the spindle: they not only determine its poles, but also participate in the nucleation of microtubules from which this spindle is created structure.

Cells enter mitosis when some cells are phosphorylated proteins and dephosphorylation of other. These processes are carried out by enzymes called kinases and phosphatases. The most important kinase complex for mitosis is cyclin B/CDK1. This complex serves as the main regulator of division, since when it is introduced into the cell, mitosis is induced. (The Nobel Prize in Physiology or Medicine was awarded in 2001 for the discovery of this complex and studies of the mechanisms of its regulation.) By the end of prophase, cyclin B/CDK1 accumulates in the nucleus in an inactive form. Soon after this, another enzyme, cdc25 phosphatase, begins to enter the nucleus, which activates cyclin B/CDK1.

The activated complex phosphorylates many nuclear proteins, including those that provide the structural integrity of the shell surrounding the core. As a result, these proteins lose contact with the nuclear membrane, causing the nucleus to swell and rupture the surrounding membrane.

The rupture of the nuclear membrane marks the beginning of the next phase of mitosis - prometaphase. During this phase, the chromosomes interact with the two centrosomes and their associated microtubules to form the spindle. As chromosomes attach to the spindle, they make a series of complex movements called congressions. During congression, chromosomes move toward and away from the spindle poles. Each chromosome moves independently, first to one pole, then to the other, often changing direction several times before the end of the movement.

In the end these movement lead to the congregation of all chromosomes in a plane or "plate" at the equator of the spindle halfway between the poles. For most cells, prometaphase is the longest phase of mitosis because it continues until all chromosomes are at the equator. This can take anywhere from a few minutes in embryonic cells to several hours in highly flattened tissue cells.

When all chromosomes gathered at the equator of the spindle, it is believed that the cell enters the metaphase of mitosis. Depending on the cell type, the duration of metaphase may vary. Curiously, the complex chain of events leading a cell to metaphase is reversible. If the spindle in metaphase or prometaphase cells is disrupted by treating them with toxic substances (for example, colcemid or nocodazole), or exposing them to low temperatures or high blood pressure, causing depolymerization of microtubules, then with subsequent reformation of the structure, as soon as the toxic effects stop, the chromosomes repeat the process of congression. Disruption of the spindle at metaphase prevents cell progression through the cycle and is often used experimentally to produce "metaphase-locked" cells.

In fact these cells are in prometaphase, since their condensed chromosomes are distributed throughout the cytoplasm.

Metaphase ends when the two sister chromatids of each chromosome begin to separate and anaphase of mitosis begins. Although each chromosome is replicated before mitosis, its two chromatids usually become visible only shortly before the end of metaphase. In the video, the process of chromatid separation appears to be instantaneous and occurring in all chromosomes simultaneously. In reality, it lasts several minutes and lasts for different times for different chromosomes. The separation of chromatids at the beginning of anaphase represents another point of irreversibility in mitosis: it coincides with the destruction of the “glue” proteins that hold the chromatids together and the master regulatory kinase that causes the cell to enter mitosis.

After separation sister chromatids they diverge towards the spindle poles. This movement is achieved by a combination of two various mechanisms. In anaphase A, the distance between each chromatid and the pole to which it is attached shortens. At the same time, the two spindle poles themselves diverge, pulling apart the groups of chromosomes attached to them. This process is called spindle elongation or anaphase B. As two groups of chromosomes diverge, the spindle begins to collapse and new structures are formed between them, which in animal cells are called interzonal region structures.

Final phase, telophase(from the Greek telo - end), begins when the chromosomes form nuclei at the poles. In cases where, upon the onset of telophase, neighboring anaphase chromosomes do not touch each other (as in large cells), each chromosome forms its own small nucleus. Then they merge to form one large nucleus. Telophase also begins the events that cause the cell to divide into two. First, a constriction forms on the surface of the cell, in the same plane in which the chromosomes line up in metaphase. In this position, the constriction is located in the middle between the two new nuclei and encircles the equatorial region.

After formation, the constriction gradually contracts, separating cell into two approximately equal parts during cytokinesis. As the constriction contracts, the interzonal structures come together to form a strong ligament called the residual body. This is the last structure connecting the two cells. Events that occur in telophase require activation of the cyclin B/CDK1 complex and indicate that the cell is exiting mitosis.

Viewing mitosis as a sequence of events and examining photographs of living or recorded cell preparations, it may seem that it is a static discrete process. In reality, however, mitosis is a continuous and highly dynamic process. This can be fully verified by video recording of dividing cells. The first frame of such a shooting is shown in the figure below.

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:

1. Doubling information at the gene level. Here, mother cells distribute genetic information among themselves. At this stage, chromosomes change.
2. 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 different processes take place, 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. At the initial stage in the prophase of mitosis, the nucleolus enlarges, as a result of which molecules are formed. 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, achromatin division is formed, 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 the prophase of mitosis, the nuclear membrane disintegrates, as a result of which spiral chromosomes end up in the cytoplasm. Particles of the disintegrated nuclear membrane form small membrane vesicles.

In the prophase of mitosis, the following happens: the 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 have 56. The human body has 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. When studying cell mitosis in various multicellular organisms, poisons can be used 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.

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 DNA duplication and the synthesis of special histone proteins, which leads to chromosome duplication and a change in the ratio of the mass of the nucleus and cytoplasm, ATP synthesis to ensure the energy division process, and the synthesis of proteins necessary to build 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 region of the centromere splits into chromatids, forming two daughter chromosomes, which, due to the contraction of the spindle fibers, begin to move to the poles of the cell. As a result, a diploid set of single-stranded chromosomes is concentrated in each pole of the cell.

IN telophase processes occur that are opposite to those that took place in prophase: chromosomes are despiralized, nucleoli are formed, and the nuclear envelope 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 isolation of the nuclei, the process of division of the cytoplasm begins, which occurs due to the 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 accurate distribution of genetic material between daughter cells, this ensures constancy karyotype cells (chromosomal set) and genetic continuity between generations of cells. Growth, development, restoration of tissues and organs of plants and animals occurs due to mitotic cell division.