During the process of cell division, the telophase of mitosis is characterized. Stages (phases) of mitosis

Cell reproduction is one of the most important biological processes and is a necessary condition existence of all living things. Reproduction occurs by dividing the original cell.

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

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

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

Mitosis - indirect division

Mitotic cycle

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

Interphase(resting stage) - preparation of the cell for further division, where the original material is duplicated, followed by its uniform distribution between the newly formed cells. It includes 3 periods:

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

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

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

Meiosis - direct division

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

Before the onset of prophase, the germ cells double their initial material, thus becoming tetraploid.

Prophase 1:

1. Leptotene- chromosomes are visible in the form of thin threads, they shorten.
2. Zygotene- the stage of conjugation of homologous chromosomes, as a result, bivalents are formed. Conjugation important point Meiosis, chromosomes come as close to each other as possible to accomplish crossing over.
3. Pachytena- chromosomes thicken, they become increasingly shortened, crossing over occurs (exchange of genetic information between homologous chromosomes, this is the basis of evolution and hereditary variability).
4. Diplotena– stage of doubled strands, the chromosomes of each bivalent diverge, maintaining connection only in the region of the cross (chiasma).
5. Diakinesis— The DNA begins to condense, the chromosomes become very short and separate.

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

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

Anaphase 1: duplicated chromosomes move to opposite poles.

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

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

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

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

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

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

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

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

All cells of our body are formed from one original cell (zygote) through numerous divisions. Scientists have found that the number of such divisions is limited. The amazing accuracy of cell reproduction is ensured by mechanisms fine-tuned over billions of years of evolution. If a failure occurs in the cell division system, the organism becomes unviable. In this lesson you will learn how cell reproduction occurs. After watching the lesson, you can independently study the topic “Cell division. Mitosis”, get acquainted with the mechanism of cell division. You will learn how the process of cell division (karyogenesis and cytogenesis), which is called “mitosis,” occurs, what phases it includes and what role it plays in the reproduction and life of organisms.

Topic: Cellular level

Lesson: Cell division. Mitosis

1. Introduction

Lesson topic: “Cell division. Mitosis".

American biologist and Nobel Prize laureate H. J. Miller wrote: “Every second in our body, hundreds of millions of inanimate, but very disciplined little ballerinas converge, disperse, line up and scatter in different directions, like dancers at a ball performing complex steps of the ancient dance. This oldest dance on Earth is the Dance of Life. In such dances, the cells of the body replenish their ranks, and we grow and exist.”

One of the main characteristics of living things—self-reproduction—is determined at the cellular level. During mitotic division, two daughter cells are formed from one parent cell, which ensures continuity of life and transmission of hereditary information.

The life of a cell from the beginning of one division to the next division is called the cell cycle (Fig. 1).

The period between cell divisions is called interphase.

Rice. 1. Cell cycle (counterclockwise - from top to bottom)

3. Stages of cell division

Eukaryotic cell division can be divided into two stages. First, nuclear division occurs (karyogenesis), followed by cytoplasmic division (cytogenesis).

Rice. 2. The relationship between interphase and mitosis in the life of a cell

Interphase

Interphase was discovered in the 19th century when scientists studied cell morphology. The instrument for studying cells was a light microscope, and the most obvious changes in the structure of cells occurred during division. The state of the cell between two divisions is called “interphase” - an intermediate phase.

The most important processes in the life of a cell (such as transcription, translation and replication) occur during interphase.

The cell spends 1 to 3 hours dividing, and interphase can last from 20 minutes to several days.

Interphase (in Fig. 3 - I) consists of several intermediate phases:

Rice. 3. Phases of the cell cycle

G1 phase (initial growth phase - presynthetic): transcription, translation and synthesis of proteins occur;

S-phase (synthetic phase): DNA replication occurs;

G2 phase (postsynthetic phase): the cell prepares for mitotic division.

Differentiated cells that are no longer dividing do not enter the G2 phase and may remain quiescent in the G0 phase.

Before nuclear division begins, chromatin (which, in fact, contains hereditary information) is condensed and converted into chromosomes, which are visible in the form of threads. Hence the name of cell division: “mitosis,” which translated means “thread.”

4. Mitosis. Phases of mitosis

Mitosis is an indirect cell division in which two daughter cells with the same set of chromosomes as the mother cell are formed from one parent cell.

This process ensures cell enlargement, growth and regeneration of organisms.

In single-celled organisms, mitosis ensures asexual reproduction.

The process of division by mitosis takes place in 4 phases, during which copies of hereditary information (sister chromosomes) are evenly distributed between cells (Fig. 2).

	Prophase. Chromosomes spiral. Each chromosome consists of two chromatids. The nuclear envelope dissolves, the centrioles divide and move towards the poles. The spindle begins to form - a system of protein filaments consisting of microtubules, some of which are attached to the chromosomes, some stretch from the centriole to the other.
	Metaphase. Chromosomes are located in the equatorial plane of the cell.
	Anaphase. The chromatids that make up chromosomes move to the poles of the cell and become new chromosomes.
	Telophase. Despiralization of chromosomes begins. Formation of the nuclear membrane, cell septum, formation of two daughter cells.

Rice. 4. Phases of mitosis: prophase, metaphase, anaphase, telophase

5. Prophase

The first phase of mitosis is prophase. Before division begins, during the synthetic period of interphase, the number of carriers of hereditary information - DNA transcription - doubles.

The DNA then combines with histone proteins and spirals as much as possible, forming chromosomes. Each chromosome consists of two sister chromatids united by a centromere (see video). Chromatids are fairly exact copies of each other - the genetic material (DNA) of the chromatids is copied during the synthetic period of interphase.

The amount of DNA in cells is designated 4c: after replication in the synthetic period of interphase, it becomes twice as large as the number of chromosomes, which is designated 2n.

In prophase, the nuclear membrane and nucleoli are destroyed. Centrioles diverge to the poles of the cell and begin to form a division spindle with the help of microtubules. At the end of prophase, the nuclear envelope completely disappears.

6. Metaphase

The second phase of mitosis is metaphase. In metaphase, chromosomes are attached by centromeres to the spindle threads extending from the centrioles (see video). Microtubules begin to align in length, as a result of which the chromosomes line up in the central part of the cell - at its equator. When the centromeres are located at an equal distance from the poles, their movement stops.

In a light microscope, you can see the metaphase plate, which is formed by chromosomes located at the equator of the cell. Metaphase and the following anaphase ensure uniform distribution of hereditary information of sister chromatids between cells.

7. Anaphase

The next phase of mitosis is anaphase. She is the shortest. The centromeres of the chromosomes divide, and each of the released sister chromatids becomes an independent chromosome.

Spindle filaments move sister chromatids to the cell poles.

As a result of anaphase, the same number of chromosomes is assembled at the poles as was in the original cell. The amount of DNA at the cell poles becomes 2C, and the number of chromosomes (sister chromatids) becomes 2n.

8. Telophase

The final stage of mitosis is telophase. The nuclear envelope begins to form around the chromosomes (sister chromatids) collected at the poles of the cell. In a cell, two nuclei appear at the poles.

Processes opposite to prophase occur: DNA and proteins of chromosomes begin to decondense, and chromosomes cease to be visible in a light microscope, nuclear membranes are formed, nucleoli are formed in which transcription begins, and spindle threads disappear.

The end of telophase predominantly coincides with the division of the mother cell body - cytokinesis.

9. Cytokinesis

Cytokinesis

The distribution of cytoplasm in plant and animal cells occurs differently. In plant cells, at the site of the metaphase plate, a cell wall is formed, which divides the cell into two daughter cells. The fission spindle is involved in this with the formation of a special structure - the phragmoplast. Animal cells divide to form a constriction.

Mitosis produces two cells that are genetically identical to the original one, although each contains only one copy of the parent cell's genetic information. Copying of hereditary information occurs during the synthetic period of interphase.

Sometimes cytoplasmic division does not occur, and bi- or multinucleated cells are formed.

The entire process of mitotic division takes from several minutes to several hours, depending on the species characteristics of living organisms.

10. Biological significance of mitosis

The biological significance of mitosis is to maintain a constant number of chromosomes and genetic stability of organisms.

In addition to mitosis, there are other types of division.

In almost all eukaryotic cells, so-called direct division occurs - amitosis.

During amitosis, the formation of a spindle and chromosomes does not occur. The distribution of genetic material occurs randomly.

As a rule, cells divide through amitosis and complete their life cycle. For example, epithelial cells of the skin or follicular cells of the ovaries. Amitosis also occurs in pathological processes, for example, inflammation or malignant tumors.

Mitosis disorder

The correct course of mitosis can be disrupted by external factors. For example, under the influence x-ray radiation chromosomes can break. They are then restored using special enzymes. However, errors may occur. Substances such as alcohols and ethers can disrupt the movement of chromosomes to the poles of the cell, which leads to uneven distribution of chromosomes. In these cases, the cell usually dies.

There are substances that affect the spindle, but do not affect the distribution of chromosomes. As a result, the nucleus does not divide, and the nuclear envelope will unite together all the chromosomes that should have been distributed among the new cells. Cells with a double set of chromosomes are formed. Such organisms with a double or triple set of chromosomes are called polyploids. The method of obtaining polyploids is widely used in breeding to create resistant varieties plants.

11. Lesson summary

The lesson discussed cell division through mitosis. As a result of mitosis, as a rule, two cells are formed, identical in quantity and quality of genetic material to the mother cell.

Homework

1. What is the cell cycle? What phases make it up?

2. What process is called mitosis?

3. What happens to a cell during mitosis?

4. Describe each phase of mitosis. What biological meaning mitotic division?

5. Discuss with family and friends the importance of mitosis and its relationship to the growth and development of multicellular organisms, human health and lifespan.

1. Biology. rf.

2. GlavRef.

3. Scientific and Educational portal "All Biology".

5. Trifonov E.V. Human pneumaticpsychosomatology. Russian-English-Russian encyclopedia.

6. Website of a chemistry and biology teacher.

7. Wikipedia.

Bibliography

1. Mamontov S. G., Zakharov V. B., Agafonova I. B., Sonin N. I. Biology. General patterns. - M.: Bustard, 2009.

2. Pasechnik V.V., Kamensky A.A., Kriksunov E.A. Biology. Introduction to general biology and ecology. Textbook for 9th grade. 3rd ed., stereotype. - M.: Bustard, 2002.

3. Ponomareva I. N., Kornilova O. A., Chernova N. M. Fundamentals of general biology. 9th grade: Textbook for 9th grade students of general education institutions / Ed. prof. I. N. Ponomareva. - 2nd ed. reworked - M.: Ventana-Graf, 2005.

Interphase is the period between two cell divisions. In interphase, the nucleus is compact, does not have a pronounced structure, and the nucleoli are clearly visible. The collection of interphase chromosomes is chromatin. The composition of chromatin includes: DNA, proteins and RNA in a ratio of 1: 1.3: 0.2, as well as inorganic ions. The structure of chromatin is variable and depends on the state of the cell.

Chromosomes are not visible in interphase, so they are studied by electron microscopy and biochemical methods. Interphase includes three stages: presynthetic (G1), synthetic (S) and postsynthetic (G2). The symbol G is an abbreviation for English. gap – interval; the symbol S is an abbreviation for English. synthesis - synthesis. Let's look at these stages in more detail.

Presynthetic stage (G1). Each chromosome is based on one double-stranded DNA molecule. The amount of DNA in a cell at the presynthetic stage is indicated by the symbol 2c (from the English content). The cell is actively growing and functioning normally.

Synthetic stage (S). Self-duplication, or DNA replication, occurs. In this case, some chromosome regions double earlier, while others later, that is, DNA replication proceeds asynchronously. In parallel, doubling of the centrioles (if any) occurs.

Postsynthetic stage (G2). DNA replication completes. Each chromosome contains two double DNA molecules, which are an exact copy of the original DNA molecule. The amount of DNA in a cell at the postsynthetic stage is indicated by the symbol 4c. Substances necessary for cell division are synthesized. At the end of interphase, synthesis processes stop.

Mitosis process

Prophase– first phase of mitosis. Chromosomes spiral and become visible in a light microscope in the form of thin threads. Centrioles (if present) diverge to the poles of the cell. At the end of prophase, the nucleoli disappear, the nuclear membrane is destroyed, and the chromosomes are released into the cytoplasm.

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. Centrioles diverge towards the poles of the cell. An achromatin fission spindle is formed, some of the threads of which go from pole to pole, and some are attached to the centromeres of the chromosomes. The content of genetic material in the cell remains unchanged (2n2хр).

Rice. 1. Scheme of mitosis in onion root cells

Rice. 2. Scheme of mitosis in onion root cells: 1- interphase; 2.3 - prophase; 4 - metaphase; 5.6 - anaphase; 7,8 - telophase; 9 - formation of two cells

Rice. 3. Mitosis in the cells of the tip of the onion root: a - interphase; b - prophase; c - metaphase; g - anaphase; l, e - early and late telophases

Metaphase. The beginning of this phase is called prometaphase. In prometaphase, chromosomes are located in the cytoplasm rather randomly. A mitotic apparatus is formed, which includes a spindle and centrioles or other microtubule organizing centers. In the presence of centrioles, the mitotic apparatus is called astral (in multicellular animals), and in their absence - anastal (in higher plants). The spindle (achromatin spindle) is a system of tubulin microtubules in a dividing cell that ensures the divergence of chromosomes. The spindle consists of two types of filaments: polar (supporting) and chromosomal (pulling).

After the formation of the mitotic apparatus, chromosomes begin to move to the equatorial plane of the cell; this movement of chromosomes is called metakinesis.

In metaphase, chromosomes are maximally spiralized. The centromeres of chromosomes are located in the equatorial plane of the cell independently of each other. The polar filaments of the spindle stretch from the cell poles to the chromosomes, and the chromosomal filaments stretch from the centromeres (kinetochores) to the poles. The collection of chromosomes in the equatorial plane of the cell forms the metaphase plate.

Anaphase. Chromosomes are divided into chromatids. From this moment, each chromatid becomes an independent single-chromatid chromosome, which is based on one DNA molecule. Single-chromatid chromosomes in anaphase groups disperse to the poles of the cell. When chromosomes diverge, chromosomal microtubules are shortened, and polar microtubules are lengthened. In this case, the polar and chromosomal threads slide along each other.

Telophase. The fission spindle is destroyed. Chromosomes at the cell poles despiral, and nuclear membranes form around them. Two nuclei are formed in the cell, genetically identical to the original nucleus. The DNA content in daughter nuclei becomes equal to 2c.

Cytokinesis. In cytokinesis, the cytoplasm is divided and the membranes of daughter cells are formed. In animals, cytokinesis occurs by cell ligation. In plants, cytokinesis occurs differently: vesicles are formed in the equatorial plane, which merge to form two parallel membranes.

At this point, mitosis ends and the next interphase begins.

Cell division is the central point of reproduction.

During the process of division, two cells arise from one cell. Cell based on the assimilation of organic and inorganic substances creates a similar one with a characteristic structure and functions.

In cell division, two main moments can be observed: nuclear division - mitosis and cytoplasmic division - cytokinesis, or cytotomy. The main attention of geneticists is still focused on mitosis, since, from the point of view of chromosome theory, the nucleus is considered an “organ” of heredity.

During the process of mitosis occurs:

1. doubling of chromosome substance;
2. changes in the physical state and chemical organization of chromosomes;
3. divergence of daughter, or rather sister, chromosomes to the poles of the cell;
4. subsequent division of the cytoplasm and complete restoration of two new nuclei in sister cells.

Thus, the entire life cycle of nuclear genes is laid down in mitosis: duplication, distribution and functioning; As a result of the completion of the mitotic cycle, sister cells end up with equal “inheritance”.

During division, the cell nucleus goes through five successive stages: interphase, prophase, metaphase, anaphase and telophase; some cytologists distinguish another sixth stage - prometaphase.

Between two successive cell divisions, the nucleus is in the interphase stage. During this period, the nucleus, during fixation and staining, has a mesh structure formed by dyeing thin threads, which in the next phase are formed into chromosomes. Although interphase is called differently phase of a resting nucleus, on the body itself, metabolic processes in the nucleus during this period occur with the greatest activity.

Prophase is the first stage of preparation of the nucleus for division. In prophase, the reticulate structure of the nucleus gradually turns into chromosomal strands. From the earliest prophase, even in a light microscope, one can observe dual nature chromosomes. This suggests that in the nucleus it is in the early or late interphase that the most important process of mitosis occurs - the doubling, or reduplication, of chromosomes, in which each of the maternal chromosomes builds a similar one - a daughter one. As a result, each chromosome appears longitudinally doubled. However, these halves of chromosomes, which are called sister chromatids, do not diverge in prophase, since they are held together by one common area - the centromere; the centromeric region divides later. In prophase, chromosomes undergo a process of twisting along their axis, which leads to their shortening and thickening. It must be emphasized that in prophase, each chromosome in the karyolymph is located randomly.

In animal cells, even in late telophase or very early interphase, the duplication of the centriole occurs, after which in prophase the daughter centrioles begin to converge to the poles and the formations of the astrosphere and spindle, called the new apparatus. At the same time, the nucleoli dissolve. An essential sign of the end of prophase is the dissolution of the nuclear membrane, as a result of which the chromosomes end up in the general mass of cytoplasm and karyoplasm, which now form myxoplasm. This ends prophase; the cell enters metaphase.

IN Lately Between prophase and metaphase, researchers began to distinguish an intermediate stage called prometaphase. Prometaphase is characterized by the dissolution and disappearance of the nuclear membrane and the movement of chromosomes towards the equatorial plane of the cell. But by this moment the formation of the achromatin spindle has not yet been completed.

Metaphase called the stage of completion of the arrangement of chromosomes at the equator of the spindle. The characteristic arrangement of chromosomes in the equatorial plane is called the equatorial, or metaphase, plate. The arrangement of chromosomes in relation to each other is random. In metaphase, the number and shape of chromosomes are clearly revealed, especially when examining the equatorial plate from the poles of cell division. The achromatin spindle is fully formed: the spindle filaments acquire a denser consistency than the rest of the cytoplasm and are attached to the centromeric region of the chromosome. The cytoplasm of the cell during this period has the lowest viscosity.

Anaphase called the next phase of mitosis, in which the chromatids divide, which can now be called sister or daughter chromosomes, and diverge to the poles. In this case, first of all, the centromeric regions repel each other, and then the chromosomes themselves diverge to the poles. It must be said that the divergence of chromosomes in anaphase begins simultaneously - “as if on command” - and ends very quickly.

During telophase, the daughter chromosomes despiral and lose their apparent individuality. The core shell and the core itself are formed. The nucleus is reconstructed in the reverse order compared to the changes it underwent in prophase. In the end, the nucleoli (or nucleolus) are also restored, and in the same quantity as they were present in the parent nuclei. The number of nucleoli is characteristic of each cell type.

At the same time, the symmetrical division of the cell body begins. The nuclei of the daughter cells enter the interphase state.

The figure above shows a diagram of cytokinesis in animal and plant cells. In an animal cell, division occurs by lacing the cytoplasm of the mother cell. In a plant cell, the formation of a cell septum occurs with areas of spindle plaques, forming a partition called a phragmoplast in the equatorial plane. This ends the mitotic cycle. Its duration apparently depends on the type of tissue, the physiological state of the body, external factors (temperature, light conditions) and lasts from 30 minutes to 3 hours. According to various authors, the speed of passage of individual phases is variable.

Both internal and external factors environments acting on the growth of the organism and its functional state affect the duration of cell division and its individual phases. Since the nucleus plays a huge role in the metabolic processes of the cell, it is natural to believe that the duration of the mitotic phases can vary in accordance with the functional state of the organ tissue. For example, it has been established that during the rest and sleep of animals, the mitotic activity of various tissues is much higher than during the waking period. In a number of animals, the frequency of cell divisions decreases in the light and increases in the dark. It is also assumed that hormones influence the mitotic activity of the cell.

The reasons that determine the readiness of a cell to divide still remain unclear. There are reasons to suggest several reasons:

1. doubling the mass of cellular protoplasm, chromosomes and other organelles, due to which nuclear-plasma relations are disrupted; To divide, a cell must reach a certain weight and volume characteristic of the cells of a given tissue;
2. chromosome doubling;
3. secretion of special substances by chromosomes and other cell organelles that stimulate cell division.

The mechanism of chromosome divergence to the poles in anaphase of mitosis also remains unclear. An active role in this process appears to be played by spindle filaments, representing protein filaments organized and oriented by centrioles and centromeres.

The nature of mitosis, as we have already said, varies depending on the type and functional state of the tissue. Cells of different tissues are characterized by Various types mitoses. In the described type of mitosis, cell division occurs in an equal and symmetrical manner. As a result of symmetrical mitosis, sister cells are hereditarily equivalent in terms of both nuclear genes and cytoplasm. However, in addition to symmetrical, there are other types of mitosis, namely: asymmetrical mitosis, mitosis with delayed cytokinesis, division of multinucleated cells (division of syncytia), amitosis, endomitosis, endoreproduction and polyteny.

In the case of asymmetric mitosis, sister cells are unequal in size, amount of cytoplasm, and also in relation to their future fate. An example of this is the unequal size of the sister (daughter) cells of the grasshopper neuroblast, animal eggs during maturation and during spiral fragmentation; when nuclear fission occurs pollen grains one of the daughter cells can further divide, the other cannot, etc.

Mitosis with delayed cytokinesis is characterized by the fact that the cell nucleus divides many times, and only then does the cell body divide. As a result of this division, multinucleated cells like syncytium are formed. An example of this is the formation of endosperm cells and the production of spores.

Amitosis called direct nuclear fission without the formation of fission figures. In this case, the division of the nucleus occurs by “lacing” it into two parts; sometimes several nuclei are formed from one nucleus at once (fragmentation). Amitosis constantly occurs in the cells of a number of specialized and pathological tissues, for example in cancerous tumors. It can be observed under the influence of various damaging agents (ionizing radiation and high temperature).

Endomitosis This is the name given to the process in which nuclear fission doubles. In this case, chromosomes, as usual, reproduce in interphase, but their subsequent divergence occurs inside the nucleus with preservation of the nuclear envelope and without the formation of an achromatin spindle. In some cases, although the nuclear membrane dissolves, chromosomes do not diverge to the poles, as a result of which the number of chromosomes in the cell multiplies even several tens of times. Endomitosis occurs in cells of various tissues of both plants and animals. For example, A. A. Prokofieva-Belgovskaya showed that through endomitosis in the cells of specialized tissues: in the hypodermis of the cyclops, the fat body, the peritoneal epithelium and other tissues of the filly (Stenobothrus) - the set of chromosomes can increase 10 times. This increase in the number of chromosomes is associated with functional features differentiated tissue.

During polyteny, the number of chromosomal strands multiplies: after reduplication along the entire length, they do not diverge and remain adjacent to each other. In this case, the number of chromosomal threads within one chromosome is multiplied, as a result the diameter of the chromosomes increases noticeably. The number of such thin threads in a polytene chromosome can reach 1000-2000. In this case, so-called giant chromosomes are formed. With polythenia, all phases of the mitotic cycle drop out, except for the main one - the reproduction of the primary strands of the chromosome. The phenomenon of polyteny is observed in the cells of a number of differentiated tissues, for example, in the tissue of the salivary glands of dipterans, in the cells of some plants and protozoa.

Sometimes there is a duplication of one or more chromosomes without any nuclear transformations - this phenomenon is called endoreproduction.

So, all phases of cell mitosis, components, are mandatory only for a typical process.

In some cases, mainly in differentiated tissues, the mitotic cycle undergoes changes. The cells of such tissues have lost the ability to reproduce the whole organism, and the metabolic activity of their nucleus is adapted to the function of the socialized tissue.

Embryonic and meristem cells, which have not lost the function of reproducing the whole organism and belong to undifferentiated tissues, retain the full cycle of mitosis, on which asexual and vegetative reproduction is based.

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It is a continuous process, each stage of which imperceptibly passes into the next after it. There are four stages of mitosis: prophase, metaphase, anaphase and telophase (Fig. 1). When studying mitosis, the main focus is on the behavior of chromosomes.

Prophase . At the beginning of the first stage of mitosis - prophase - the cells retain the same appearance as in interphase, only the nucleus noticeably increases in size, and chromosomes appear in it. In this phase, it is clear that each chromosome consists of two chromatids, spirally twisted relative to each other. Chromatids shorten and thicken as a result of the process of internal spiralization. A weakly colored and less condensed region of the chromosome begins to emerge - the centromere, which connects two chromatids and is located in a strictly defined place on each chromosome.

During prophase, the nucleoli gradually disintegrate: the nuclear membrane is also destroyed, and the chromosomes end up in the cytoplasm. In late prophase (prometaphase), the mitotic apparatus of the cell is intensively formed. At this time, the centriole divides, and the daughter centrioles disperse to opposite ends of the cell. Thin ray-shaped filaments extend from each centriole; spindle filaments are formed between the centrioles. There are two types of filaments: spindle pulling filaments, attached to the centromeres of chromosomes, and supporting filaments, connecting the poles of the cell.

When chromosome contraction reaches its maximum extent, they turn into short rod-shaped bodies and are directed to the equatorial plane of the cell.

Metaphase . In metaphase, the chromosomes are completely located in the equatorial plane of the cell, forming the so-called metaphase or equatorial plate. The centromere of each chromosome, which holds both chromatids together, is located strictly in the equator of the cell, and the arms of the chromosomes are extended more or less parallel to the spindle threads.

In metaphase, the shape and structure of each chromosome is clearly revealed, the formation of the mitotic apparatus ends and the attachment of the pulling threads to the centromeres occurs. At the end of metaphase, simultaneous division of all chromosomes of a given cell occurs (and the chromatids turn into two completely separate daughter chromosomes).

Anaphase. Immediately after centromere division, the chromatids repel each other and move toward opposite poles of the cell. All chromatids begin to move towards the poles simultaneously. Centromeres play an important role in the oriented movement of chromatids. In anaphase, the chromatids are called sister chromosomes.

The movement of sister chromosomes in anaphase occurs through the interaction of two processes: contraction of the pulling threads and elongation of the supporting threads of the mitotic spindle.

Telophase. At the beginning of telophase, the movement of sister chromosomes ends, and they are concentrated at the poles of the cell in the form of compact formations and clots. Chromosomes despiral and lose their apparent individuality. A nuclear envelope is formed around each daughter nucleus; nucleoli are restored in the same quantity as they were in the mother cell. This completes nuclear division (karyokinesis) and the formation of a cell membrane. Simultaneously with the formation of daughter nuclei in telophase, the division of the entire contents of the original mother cell or cytokinesis occurs.

When a cell divides, a constriction or groove appears on its surface near the equator. It gradually deepens and divides the cytoplasm into

two daughter cells, each of which has a nucleus.

During the process of mitosis, two daughter cells arise from one mother cell, containing the same set of chromosomes as the original cell.

Figure 1. Mitosis diagram

Biological significance of mitosis . The main biological significance of mitosis is the precise distribution of chromosomes between two daughter cells. The regular and orderly mitotic process ensures the transfer of genetic information to each of the daughter nuclei. As a result, each daughter cell contains genetic information about all the characteristics of the organism.

Meiosis is a special division of the nucleus, which ends with the formation of a tetrad, i.e. four cells with a haploid set of chromosomes. Sex cells divide by meiosis.

Meiosis consists of two cell divisions in which the number of chromosomes is halved, so that the gametes receive half as many chromosomes as the rest of the body cells. When two gametes unite during fertilization, the normal number of chromosomes is restored. The decrease in the number of chromosomes during meiosis does not occur randomly, but quite naturally: the members of each pair of chromosomes disperse into different daughter cells. As a result, each gamete contains one chromosome from each pair. This is accomplished by pairwise joining of similar or homologous chromosomes (they are identical in size and shape and contain similar genes) and the subsequent divergence of members of the pair, each of which goes to one of the poles. During the convergence of homologous chromosomes, crossing over can occur, i.e. mutual exchange of genes between homologous chromosomes, which increases the level of combinative variability.

In meiosis, a number of processes occur that are important in the inheritance of traits: 1) reduction - halving the number of chromosomes in cells; 2) conjugation of homologous chromosomes; 3) crossing over; 4) random divergence of chromosomes into cells.

Meiosis consists of two successive divisions: the first, which results in the formation of a nucleus with a haploid set of chromosomes, is called reduction; the second division is called equational and proceeds as mitosis. In each of them, prophase, metaphase, anaphase and telophase are distinguished (Fig. 2). The phases of the first division are usually designated by the number Ι, the second - P. Between the Ι and P divisions, the cell is in a state of interkinesis (Latin inter - between + gr. kinesis - movement). Unlike interphase, in interkinesis DNA is not replicated and chromosome material is not doubled.

Figure 2. Meiosis diagram

Reduction division

Prophase I

The phase of meiosis during which complex structural transformations of chromosomal material occur. It is longer and consists of a number of successive stages, each of which has its own distinctive properties:

– leptotene – stage of leptonema (connection of threads). Individual strands - chromosomes - are called monovalents. Chromosomes in meiosis are longer and thinner than chromosomes in the earliest stage of mitosis;

– zygotene – stage of zygonema (connection of threads). Conjugation, or synapsis (joining in pairs), of homologous chromosomes occurs, and this process is carried out not just between homologous chromosomes, but between exactly corresponding individual points of homologues. As a result of conjugation, bivalents are formed (complexes of homologous chromosomes connected in pairs), the number of which corresponds to the haploid set of chromosomes.

Synapsis occurs from the ends of chromosomes, so the locations of homologous genes on one chromosome or the other coincide. Since the chromosomes are doubled, there are four chromatids in the bivalent, each of which ultimately turns out to be a chromosome.

– pachytene – stage of pachynema (thick filaments). The dimensions of the nucleus and nucleolus increase, the bivalents shorten and thicken. The connection of homologues becomes so close that it is difficult to distinguish two separate chromosomes. At this stage, crossing over, or crossover of chromosomes, occurs;

– diplotene – stage of diplonema (double strands), or stage of four chromatids. Each of the homologous chromosomes of the bivalent is split into two chromatids, so that the bivalent contains four chromatids. Although the tetrads of chromatids move away from each other in some places, they are in close contact in other places. In this case, the chromatids of different chromosomes form X-shaped figures called chiasmata. The presence of a chiasm holds the monovalents together.

Simultaneously with the continuing shortening and, accordingly, thickening of the bivalent chromosomes, their mutual repulsion - divergence - occurs. The connection is preserved only in the plane of decussation - in the chiasmata. The exchange of homologous regions of chromatids is completed;

– diakinesis is characterized by maximum shortening of diplotene chromosomes. Bivalents of homologous chromosomes extend to the periphery of the nucleus, so they are easy to count. The nuclear envelope fragments and the nucleoli disappear. This completes prophase 1.

Metaphase I

– begins from the moment the nuclear membrane disappears. The formation of the mitotic spindle is completed, the bivalents are located in the cytoplasm in the equatorial plane. Chromosome centromeres attach to the mitotic spindle, but do not divide.

Anaphase I

- characterized by complete dissolution of the relationship between homologous chromosomes, repulsion of them from one another and divergence to different poles.

Note that during mitosis, single-chromatid chromosomes diverged to the poles, each of which consists of two chromatids.

Thus, it is during anaphase that reduction occurs—preservation of the number of chromosomes.

Telophase I

– it is very short-lived and poorly separated from the previous phase. In telophase 1, two daughter nuclei are formed.

Interkinesis

This is a short resting state between 1 and 2 divisions. Chromosomes are weakly despiralized, DNA replication does not occur, since each chromosome already consists of two chromatids. After interkinesis, the second division begins.

Triple division occurs in both daughter cells in the same way as in mitosis.

Prophase P

In the nuclei of cells, chromosomes are clearly visible, each of which consists of two chromatids connected by a centromere. They look like rather thin threads located along the periphery of the core. At the end of prophase P, the nuclear envelope fragments.

Metaphase P

In each cell, the formation of the division spindle is completed. Chromosomes are located along the equator. The spindle strands are attached to the centromeres of the chromosomes.

Anaphase P

Centromeres divide and chromatids usually move rapidly to opposite poles of the cell.

Telophase P

Sister chromosomes concentrate at the cell poles and despiral. The nucleus and cell membrane are formed. Meiosis ends with the formation of four cells with a haploid set of chromosomes.

Biological significance of meiosis

Like mitosis, meiosis ensures the precise distribution of genetic material into daughter cells. But, unlike mitosis, meiosis is a means of increasing the level of combinative variability, which is explained by two reasons: 1) free, random combination of chromosomes occurs in cells; 2) crossing over, leading to the emergence of new combinations of genes within chromosomes.

In each subsequent generation of dividing cells, as a result of the above reasons, new combinations of genes are formed in gametes, and when animals reproduce, new combinations of genes of parents are formed in their offspring. This each time opens up new possibilities for the action of selection and the creation of genetically different forms, which allows a group of animals to exist in variable environmental conditions.

Thus, meiosis turns out to be a means of genetic adaptation, increasing the reliability of the existence of individuals over generations.