The process of indirect cell division. Types of division of the cell nucleus. Eukaryotic cell division

Cell division is the central point of reproduction.

During the process of division, two cells arise from one cell. Based on the assimilation of organic and inorganic substances, a cell creates its own cell 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:

doubling of chromosome substance;
changes in the physical state and chemical organization of chromosomes;
divergence of daughter, or rather sister, chromosomes to the poles of the cell;
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, the dual nature of chromosomes can be observed. This suggests that in the nucleus it is in early or late interphase that the most important process of mitosis occurs - 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 appear in the general mass of cytoplasm and karyoplasm, which now form myxoplasm. This ends prophase; the cell enters metaphase.

Recently, between prophase and metaphase, researchers have begun 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 environmental factors 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 rest and sleep of animals, the mitotic activity of various tissues is much higher than during wakefulness. 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:

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;
chromosome doubling;
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 different types of 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 the nuclei in pollen grains divide, 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 occurs constantly 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 the functional characteristics of 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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The ability to divide is the most important property of cells. Without division, it is impossible to imagine an increase in the number of single-celled creatures, the development of a complex multicellular organism from a single fertilized egg, the renewal of cells, tissues and even organs lost during the life of the organism. Cell division occurs in stages. At each stage of division certain processes occur. They lead to the doubling of genetic material (DNA synthesis) and its distribution between daughter cells. The period of cell life from one division to the next is called the cell cycle.

Amitosis

Amitosis, or direct division, is the division of the interphase nucleus by constriction without the formation of a division spindle (chromosomes are generally indistinguishable in a light microscope). This division occurs in unicellular organisms (for example, polyploid large nuclei of ciliates are divided by amitosis), as well as in some highly specialized cells of plants and animals with weakened physiological activity, degenerating, doomed to death, or in various pathological processes, such as malignant growth, inflammation and etc. Amitosis can be observed in the tissues of a growing potato tuber, endosperm, walls of the pistil ovary and parenchyma of leaf petioles. This type of division is characteristic of liver cells, cartilage cells, and the cornea of the eye. Very often, during amitosis, only nuclear division is observed; in this case, bi- and multinucleated cells may appear. If nuclear division is followed by cytoplasmic division, then the distribution of cellular components, like DNA, is arbitrary. Amitosis, unlike mitosis, is the most economical method of division, since the energy costs are very insignificant. Cell division in prokaryotes is close to amitosis. A bacterial cell contains only one, most often circular, DNA molecule attached to the cell membrane. Before a cell divides, DNA is replicated to produce two identical DNA molecules, each also attached to the cell membrane. When a cell divides, the cell membrane grows between these two DNA molecules, so that each daughter cell ends up with one identical DNA molecule. This process was called direct binary fission.

Preparing for division. Eukaryotic organisms, consisting of cells with nuclei, begin preparation for division at a certain stage of the cell cycle, in interphase. It is during interphase that the process of protein biosynthesis occurs in the cell, and all the most important structures of the cell are doubled. Along the original chromosome, an exact copy of it is synthesized from the chemical compounds present in the cell, and the DNA molecule is doubled. A doubled chromosome consists of two halves of chromatids. Each chromatid contains one DNA molecule. Interphase in plant and animal cells lasts on average 10-20 hours. Then the process of cell division begins - mitosis.

Mitosis

Mitosis (from the Greek Mitos - thread) indirect division is the main method of division of eukaryotic cells. Mitosis is the division of the nucleus, which leads to the formation of two daughter nuclei, each of which has exactly the same set of chromosomes as the parent nucleus. Nuclear division is usually followed by division of the cell itself, so the term “mitosis” is often used to refer to division of the entire cell. Mitosis was first observed in the spores of ferns, horsetails and club mosses by G. E. Russov, a teacher at the University of Dorpat in 1872, and the Russian scientist I. D. Chistyakov in 1874. Detailed studies of the behavior of chromosomes in mitosis were carried out by the German botanist E. Strassburger in 1876- 1879 on plants and by the German histologist W. Flemming in 1882 on animals.

Rice. 1. Schematic representation of mitosis in animal cells

During interphase, DNA replication occurs as the cell prepares to divide. During prophase, the nuclear envelope is destroyed and a spindle is formed between the two centrioles. At the metaphase stage, chromosomes are located in the equatorial plane of the cell. When anaphase occurs, the duplicated chromosomes (called chromatids) separate. At the telophase stage, the chromosomes reach the spindle poles and the cell begins to divide into two daughter cells. In terms of the number and type of chromosomes, daughter cells are identical to the mother

Mitosis is a continuous process, but for ease of study, biologists divide it into four stages depending on how the chromosomes look under a light microscope at this time. Mitosis is divided into prophase, metaphase, anaphase and telophase. In prophase, chromosomes shorten and thicken due to their spiralization. At this time, double chromosomes consist of two sister chromatids connected to each other. Simultaneously with the spiralization of chromosomes, the nucleolus disappears and the nuclear membrane fragments (breaks up into separate tanks). After the collapse of the nuclear membrane, the chromosomes lie freely and randomly in the cytoplasm. In prophase, centrioles (in those cells where they exist) diverge to the cell poles. At the end of prophase, a fission spindle begins to form, which is formed from microtubules by polymerization of protein subunits.

In metaphase, the formation of the fission spindle is completed, which consists of two types of microtubules: chromosomal, which bind to the centromeres of the chromosomes, and centrosomal (polar), which stretch from pole to pole of the cell. Each double chromosome is attached to the microtubules of the spindle. The chromosomes seem to be pushed by microtubules to the equator of the cell, i.e., they are located at an equal distance from the poles. They lie in the same plane and form the so-called equatorial, or metaphase record. In metaphase, the double structure of chromosomes is clearly visible, connected only at the centromere. During this period, it is easy to count the number of chromosomes and study their morphological features. In anaphase, the daughter chromosomes are stretched toward the cell poles with the help of spindle microtubules. During movement, the daughter chromosomes bend somewhat like a hairpin, the ends of which are turned towards the equator of the cell. Thus, in anaphase, the chromatids doubled in interphase of chromosomes diverge to the poles of the cell. At this moment, the cell contains two diploid sets of chromosomes.

In telophase, processes occur that are the opposite of those observed in prophase: despiralization (unwinding) of chromosomes begins, they swell and become difficult to see under a microscope. Around the chromosomes at each pole, a nuclear envelope is formed from membrane structures of the cytoplasm, and nucleoli appear in the nuclei. The fission spindle is destroyed. At the telophase stage, the cytoplasm separates (cytotomy) to form two cells. In animal cells, the plasma membrane begins to invaginate into the region where the spindle equator was located. As a result of invagination, a continuous furrow is formed, encircling the cell along the equator and gradually dividing one cell into two.

In plant cells in the equator region, a barrel-shaped formation arises from the remnants of the spindle filaments - phragmoplast. Numerous vesicles of the Golgi complex rush into this area from the cell poles, which merge with each other. The contents of the vesicles form the cell plate, which divides the cell into two daughter cells, and the membrane of the Golgi vesicles forms the missing cytoplasmic membranes of these cells. Subsequently, elements of cell membranes are deposited on the cell plate from the side of each of the daughter cells. As a result of mitosis, two daughter cells arise from one cell with the same set of chromosomes as in the mother cell.

The biological significance of mitosis therefore lies in the strictly identical distribution between the daughter cells of the material carriers of heredity - the DNA molecules that make up the chromosomes. Thanks to the uniform distribution of replicated chromosomes, organs and tissues are restored after damage. Mitotic cell division is also the cytological reproduction of organisms.

Meiosis or reduction division

Meiosis is a special method of cell division, which results in a reduction (decrease) in the number of chromosomes by half. It was first described by W. Flemming in 1882 in animals and by E. Strassburger in 1888 in plants. Meiosis produces gametes. As a result of the reduction of spores and germ cells of the chromosome set, each haploid spore and gamete contains one chromosome from each pair of chromosomes present in a given diploid cell. During the further process of fertilization (fusion of gametes), the organism of the new generation will again receive a diploid set of chromosomes, i.e., the karyotype of organisms of a given species remains constant over a number of generations. Thus, the most important significance of meiosis is to ensure the constancy of the karyotype in a number of generations of organisms of a given species during sexual reproduction.

Fig.2. Final scheme of meiosis

DNA and its associated proteins are replicated during interphase. During prophase, the nuclear envelope is destroyed and homologous chromosomes (each of which consists of two chromatids connected by a centromere) are arranged in pairs. At this time, an exchange of regions can occur between four homologous chromatids. After metaphase I, the two initially homologous chromosomes separate into different cells. During the second division, the centromere splits, resulting in each new cell containing one copy of each chromosome. We.

Reduction division is, in fact, a mechanism that prevents a continuous increase in the number of chromosomes during the fusion of gametes; without it, during sexual reproduction, the number of chromosomes would double in each new generation. In other words, thanks to meiosis, it maintains a certain and constant number of chromosomes in all generations of any species of plants, animals and fungi. Another important significance of meiosis is to ensure extreme diversity in the genetic composition of gametes, both as a result of crossing over and as a result of different combinations of paternal and maternal chromosomes during their independent divergence in anaphase I of meiosis, which ensures the appearance of diverse and different-quality offspring during sexual reproduction of organisms.

Education

The spindle is... Description, structure and functions

The spindle is a temporary structure formed during the processes of mitosis and meiosis, and ensures chromosome segregation and cell division.

Cell division

It is bipolar: the system of microtubules formed in the space between the poles is shaped like a spindle. In the centromere region, spindle microtubules are attached to the kinetochores of the chromosome. Along them, chromosomes move to the poles.

Structure

The spindle consists of three main structural elements: microtubules, division poles and chromosomes. Division poles in animals are organized by centrosomes, which contain centrioles. In the absence of centrosomes (in plants, and in oocytes in some animal species), the spindle has wide poles and is called acentrosomal. Another structure is involved in the formation of the spindle—motor proteins. They belong to the dyneins and kinesins.

The spindle is a bipolar structure. At both poles there are centrosomes - organelles that are centers for the organization of microtubules. In the structure of the centrosome, two centrioles are distinguished, surrounded by many different proteins. Condensed chromosomes, which look like two chromatids joined at the centromere, are located between the poles. In the centromere region there are kinetochores to which microtubules attach.

Formation

Since the spindle is the structure responsible for cell division, its assembly begins in prophase. In plants and oocytes, in the absence of centrosomes, the nuclear membrane serves as the center of microtubule organization. Microtubules approach the nuclear envelope and at the end of prophase their orientation ends, and a “prophase spindle” is formed - the axis of the future fission spindle.

Due to the fact that in animal cells it is the centrosome that serves as the center of organization, the beginning of the formation of the division spindle is the divergence of two centrosomes during prophase. This is possible thanks to the motor proteins dyneins: they are attached to the outer surface of the nucleus, as well as to the inner side of the cell membrane. A group of dyneins attached to the membrane connects to astral microtubules and they begin to move towards the minus end, due to which centrosomes spread along opposite sections of the cell membrane.

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Completion of assembly

The final formation of the division spindle occurs at the prometaphase stage; after the disappearance of the nuclear membrane, it becomes complete, because it is after this that centrosomes and microtubules can gain access to the components of the spindle.
However, there is one exception: in budding yeast, spindle formation occurs inside the nucleus.

The formation of spindle threads and their orientation is impossible without two processes: the organization of microtubules around chromosomes and their attachment to each other at opposite poles of division. Many of the elements necessary for the final formation of the spindle, including chromosomes and motor proteins, are located inside the cell nucleus, while microtubules and, in an animal cell, centrosomes are contained in the cytoplasm, that is, the components are isolated from each other. That is why the formation of the spindle ends only after the disappearance of the nuclear envelope.

Attachment of chromosomes

Protein, as well as many other structures, are involved in the formation of the spindle, and this process has been well studied in animal cells. During prophase, microtubules form a star-shaped structure around the centrosomes, which diverges in the radial direction. After the nuclear membrane is destroyed, dynamically unstable microtubules begin to actively probe this area and chromosome kinetochores can attach to them. Some of the chromosomes immediately appear at opposite poles, while the rest first bind to the microtubules of one of the poles, and only then begin to move towards the desired pole. When the process is completed, the chromosomes already associated with any pole begin to attach kinetochores to microtubules from the opposite pole, thus, during the metaphase process, from ten to forty tubes are attached to the kinetochores. This formation is called the kinetochore bundle. Gradually, each of the chromosomes becomes associated with the opposite pole, and they form a metaphase plate in the central part of the spindle.

Second option

There is another scenario in which a fission spindle can form. This is possible both for cells that have centrosomes and for cells that lack them. The process involves the gamma-tubulin ring complex, due to which the nucleation of short microtubules around chromosomes occurs. The tubules are attached to the kinetochores with the plus end, after which microtubule polymerization begins, that is, regulated growth. The minus ends “fuse” and remain at the division poles thanks to motor proteins. If a pair of centrosomes is involved in the formation of the spindle, this facilitates the connection of microtubules, but the process is possible without them.

Equally

A clear separation of chromosomes between two cells formed during division can only occur if paired chromatids are attached to different poles with their kinetochores. Bipolar chromatid segregation is called amphitepic, but there are other options that occur while the spindle is being assembled. These are monothepic (one kinetochore is connected to one pole) and synthepic (both kinetochores of the chromosome are connected to one pole). In merotepic, one kinetochore is captured by two poles at once. Only the usual, bipolar bonding is stable, which occurs due to tension forces from the poles; other methods of bonding are unstable and reversible, but are possible due to the location of the kinetochores.

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Search Lectures

Study the types of cell division. Enter into the protocol the table “Types of cell division”

2. Examine karyokinesis in onion root cells on microscopic slides and sketch it.

3. Using the educational table, study the scheme of meiotic cell division. Draw it in an album.

4. Solve situational problems.

WORK IN THE LABORATORY

8.Literature:

Main:

Types and types of cell division.

Biology: In 2 books. Book 1: Textbook. for medical specialists universities /ed. V.N. Yarygina. 6th ed. -M.: Higher School, 2004.- P.55-61

2. Biology/A.A.Slyusarev, S.V.Zhukova.- K.: Vishcha school. Head publishing house, 1992.- P.41-45

3. Biology. Guide to practical exercises for students of dental faculties, ed. acad. RANS prof. V.V. Markina. Ed. M. "GEOTAR-Media" 2010

Additional:

10. Medical biology: Pidruchnik /edited by V.P.Pishak, Yu.I.Bazhori.-Vinnytsia: New book, 2004.- P.26-28, 104-107, 118-125

11. Alberts G., Gray D., Lewis J. et al. Molecular biology of the cell. M.: Mir, 1986. – In 3 volumes, 2nd ed. T.1.- pp. 176-177

12. Logical structure graph.

13. Lecture notes.

OBJECTIVE (general): It is necessary to pay attention to general issues of cytology and molecular biology.

The lesson is conducted with the aim of consolidating previously learned material.

Students who have not missed lectures or practical classes and have completed and signed protocols by the teacher are allowed to attend the colloquium.

The final assessment consists of:

1. 40 test tasks (0 - 1 points) – max 40 points.

2. 2 problems (0-5-15 points for each problem) - max 30 points.

3. Theoretical question (0-5-10 points) - max 10 points.

__________________________________max 80 points.

ASSESSMENT CRITERIA:

SCORE - EXCELLENT

BALLA - GOOD

POINTS - SATISFACTORY

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Core. Nuclear and cell division

The core usually has a spherical or oval shape. The nucleus includes: nuclear envelope, karyoplasm, nucleoli and chromatin (chromosomes).

Nuclear envelope formed by two membranes (external and internal). The holes in the nuclear envelope are called pores. Through them, the exchange of substances occurs between the nucleus and the cytoplasm.

Karyoplasm (nucleoplasm, nuclear juice)– jelly-like internal contents of the nucleus.

Nucleolus– a spherical structure whose function is rRNA synthesis.

Chromatin- an unspiraled DNA molecule bound to proteins.

Types of cell division

In this form, DNA is present in non-dividing cells.

In this case, DNA doubling (replication) and the implementation of information contained in DNA are possible.

Chromosome- a spiralized DNA molecule bound to proteins. DNA is coiled before cell division to more accurately distribute the genetic material. At the metaphase stage, each chromosome consists of two chromatids, formed as a result of DNA duplication. The chromatids are connected to each other in the region of the primary constriction, or centromere. The centromere divides the chromosome into two arms.

The set of chromosomes contained in the nucleus is called chromosome set. The number of chromosomes in a cell and their shape are constant for each type of living organism.

Kernel functions: storage of genetic information, transferring it to daughter cells during division; control of cell activity.

Each cell begins its life when it separates from the mother cell, and ends its existence, giving the opportunity to its daughter cells to appear. Nature provides more than one way to divide their nucleus, depending on their structure.

Methods of cell division

Nuclear division depends on the cell type:

— Binary fission (found in prokaryotes).

— Amitosis (direct method of division).

— Mitosis (found in eukaryotes).

— Meiosis (intended for the division of germ cells).

The types of nuclear division are determined by nature and correspond to the structure of the cell and the function that it performs in the macroorganism or on its own.

Binary fission

This type is most often found in prokaryotic cells. It consists in doubling the circular DNA molecule. Binary nuclear fission is so called because from the mother cell two daughter cells of equal size appear.

After the genetic material (DNA or RNA molecule) is prepared accordingly, that is, doubled, a transverse partition begins to form from the cell wall, which gradually narrows and divides the cell cytoplasm into two approximately equal parts.

The second division process is called budding, or uneven binary fission. In this case, a protrusion appears in a section of the cell wall, which gradually grows. After the sizes of the “bud” and the mother cell are equal, they will separate. And a section of the cell wall is synthesized again.

Amitosis

This nuclear division is similar to that described above, with the difference that there is no doubling of the genetic material. This method was first described by the biologist Remak. This phenomenon occurs in pathologically altered cells (tumor degeneration), and is also a physiological norm for liver tissue, cartilage and cornea.

The process of nuclear division is called amitosis because the cell retains its functions rather than losing them, as during mitosis. This explains the pathological properties inherent in cells with this method of division. In addition, direct nuclear division occurs without a spindle, so the chromatin in daughter cells is unevenly distributed. Subsequently, such cells cannot use the mitotic cycle. Sometimes, as a result of amitosis, multinucleated cells are formed.

Mitosis

This is an indirect fission of the nucleus. Most often found in eukaryotic cells. The main difference between this process is that the daughter cells and the mother cell contain the same number of chromosomes. Thanks to this, the required number of cells is maintained in the body, and regeneration and growth processes are also possible. Flemming was the first to describe mitosis in an animal cell.

The process of nuclear division in this case is divided into interphase and mitosis itself. Interphase is a state of rest of the cell in the interval between divisions. There are several phases in it:

1. Presynthetic period - the cell grows, proteins and carbohydrates accumulate in it, ATP (adenosine triphosphate) is actively synthesized.

2. Synthetic period - genetic material doubles.

3. Postsynthetic period - cellular elements double, proteins appear that make up the spindle.

Phases of mitosis

The division of the nucleus of a eukaryotic cell is a process that requires the formation of an additional organelle - a centrosome. It is located next to the nucleus, and its main function is the formation of a new organelle - the spindle. This structure helps distribute chromosomes evenly between daughter cells.

There are four phases of mitosis:

1. Prophase: chromatin in the nucleus condenses into chromatids, which assemble near the centromere, forming chromosomes in pairs. The nucleoli disintegrate, and the centrioles move toward the cell poles. A fission spindle is formed.

2. Metaphase: The chromosomes are arranged in a line passing through the center of the cell, forming the metaphase plate.

3. Anaphase: chromatids from the center of the cell diverge to the poles, and then the centromere splits in two. This movement is possible thanks to the spindle, the threads of which contract and stretch the chromosomes in different directions.

4. Telophase: daughter nuclei are formed. The chromatids turn back into chromatin, a nucleus is formed, and in it - nucleoli. It all ends with the division of the cytoplasm and the formation of the cell wall.

Meaning of Mitosis

Mitotic nuclear division is a way to maintain a constant set of chromosomes. Daughter cells have the same set of genes as the mother, and all the characteristics inherent in it. Mitosis is necessary for:

— growth and development of a multicellular organism (from the fusion of germ cells);

- movement of cells from the lower layers to the upper ones, as well as replacement of blood cells (erythrocytes, leukocytes, platelets);

- restoration of damaged tissues (in some animals, regeneration abilities are a necessary condition for survival, for example, in starfish or lizards);

- asexual reproduction of plants and some animals (invertebrates).

Meiosis

The mechanism of division of the nuclei of germ cells is somewhat different from somatic ones. The result is cells that have half as much genetic information as their predecessors. This is necessary in order to maintain a constant number of chromosomes in each cell of the body.

Meiosis occurs in two stages:

— reduction stage;

— equational stage.

The correct course of this process is possible only in cells with an even set of chromosomes (diploid, tetraploid, hexaproid, etc.). Of course, it remains possible for meiosis to occur in cells with an odd set of chromosomes, but then the offspring may not be viable.

It is this mechanism that ensures sterility in interspecific marriages. Since germ cells contain different sets of chromosomes, this makes it difficult for them to fuse and produce viable or fertile offspring.

Amitosis

Mitosis

Rice. 1. Schematic representation of mitosis in animal cells

The biological significance of mitosis therefore lies in the strictly identical distribution between daughter cells of the material carriers of heredity - the DNA molecules that make up the chromosomes. Thanks to the uniform distribution of replicated chromosomes, organs and tissues are restored after damage. Mitotic cell division is also the cytological reproduction of organisms.

Meiosis or reduction division

Fig.2. Final scheme of meiosis

Reduction division is, in fact, a mechanism that prevents a continuous increase in the number of chromosomes during the fusion of gametes; without it, during sexual reproduction, the number of chromosomes would double in each new generation. In other words, thanks to meiosis, it maintains a certain and constant number of chromosomes in all generations of any species of plants, animals and fungi. Another important significance of meiosis is ensuring extreme diversity in the genetic composition of gametes, both as a result of crossing over and as a result of different combinations of paternal and maternal chromosomes with their independent divergence in anaphase I of meiosis, which ensures the appearance of diverse and different-quality offspring during sexual reproduction of organisms.

The NUCLEUS (nucleus) has a different shape, most often round, oval, less often rod-shaped or irregular. The shape of the nucleus sometimes depends on the shape of the cell. For example, in smooth myocytes, which have a spindle shape, the shape of the nucleus is rod-shaped. Typically, in round cells or cuboidal epithelial cells, the nuclei are round in shape. For example, blood lymphocytes are round in shape and their nuclei are usually round. But often the shape of the nucleus does not depend on the shape of the cells. For example, in blood granulocytes, which are round in shape, the nucleus may be segmented or rod-shaped. In neutrophil granulocytes in the blood of a woman, the nuclei may have a companion or satellite, which is sex chromatin, shaped like a drumstick. What is the CORE? This is a system of genetic determination and regulation of protein synthesis. What is determination? DETERMINATION is predestination or, more simply, it is the program according to which the cell develops. Thus, the nucleus performs 2 FUNCTIONS: 1) storage and transmission of hereditary information to daughter cells; 2) regulation of protein synthesis.

How is the 1st function carried out, i.e. storage of hereditary information? The storage of hereditary information is ensured by the fact that the DNA of chromosomes contains repair enzymes that restore nuclear chromosomes after they are damaged. How is information transmitted to daughter cells? During interphase, an exact copy of it is attached to each DNA molecule. These exactly identical copies of DNA are then distributed evenly between daughter cells when the mother cell divides. How does the nucleus participate in the regulation of protein synthesis? Protein synthesis is regulated due to the fact that all types of RNA are transcribed on the surface of chromosome DNA: informational, ribosomal and transport, which are involved in protein synthesis on the surface of the granular EPS of the cell cytoplasm. If the amount of all these RNAs and ribosomes increases, protein synthesis increases. If a small amount of RNA is produced in the nucleus, then protein synthesis decreases. So the nucleus is involved in the regulation of protein synthesis.

STRUCTURE OF THE CORE. The nucleus includes chromatin (chromatinum), nucleolus (nucleolus), nuclear envelope (nucleolemma) and nuclear sap (nucleoplasma). CHROMATIN of the interphase nucleus is so called because it is capable of perceiving (staining) basic dyes. What is chromatin? Chromatin is despiralized chromosomes, i.e. chromosomes that have lost their normal shape. In the event that a section of the DNA of a chromosome is most dispersed, then loose chromatin, called EUHROMATIN, which has high activity, is formed in this place. If a section of chromosome DNA is not dispersed, then it has a compacted structure. Such chromatin is called HETEROCHROMATIN (heterochromatinum). Heterochromatin is not active.

Why is euchromatin active and heterochromatin inactive? The ACTIVITY of euchromatin is explained by the fact that the DNA fibrils of chromosomes are despiralized, i.e. genes on the surface of which RNA transcription occurs have been discovered. This creates the conditions for RNA transcription. If the DNA of the chromosomes is not despiraled, then the genes here are closed, which makes it difficult to transcribe RNA from their surface. Consequently, the amount of RNA decreases and protein synthesis decreases. This is why heterochromatin is not active.

DNA FIBRILLS. Both the mitotic chromosomes and the chromatin of the interphase nucleus include filaments - primitive or elementary fibrils, which consist of DNA in the amount of 1 unit, histone and non-histone proteins, amounting to 1.3 units, and RNA, the amount of which is 0.2 units. The length of the fibrils can range from several hundred microns to 7 cm. The total length of the fibrils of all chromosomes of the human nucleus is 170 cm. The fibrils contain regions of independent chromosome replication, called REPLICONS, their length is 30 microns, the total number in the human genome is up to 50,000 replicons.

HISTONE proteins form blocks, each of which consists of 8 molecules. These blocks are called NUCLEOSOMES. A DNA fibril 5 nm thick is wrapped around the nucleosomes; the thickness of the nucleosome together with the fibril is 10 nm. With further spiralization of this already spiralized fibril, its thickness reaches 20 nm. Among chromatin proteins, histone proteins account for up to 80 percent. Their FUNCTION is 1) a special arrangement of chromosome DNA and 2) regulation of protein synthesis. Regulation of protein synthesis is carried out through the laying of DNA fibrils on chromosomes. If during the laying of DNA fibrils there is a sharp condensation, then dense chromatin (heterochromatin) is formed, which, as is already known, is inactive; if during the laying of fibrils they weakly spiral, then active euchromatin is formed. The function of NON-HIST proteins is that they form the nuclear matrix.

The amount of RNA in chromatin is ???, if in several places there are several nucleoli. In the place where the nucleolar organizers of chromosomes are located, there are several hundred genes on the surface of which ribosomal RNAs are transcribed, from which ribosomal subunits are then formed. The nucleoli consist of two components: 1) fibrillar, located in the center, and 2) granular, localized on the surface. The fibrillar component is RNA fibrils transcribed from the surface of nucleolar organizer genes. The granular component is the subunits of ribosomes. Ribosomal subunits are formed as a result of complexation (combination) of ribosomal proteins with ribosomal RNA fibrils. Ribosomal proteins are synthesized on the surface of the granular ER of the cytoplasm and enter the nucleus through nuclear pores, where they combine with r-RNA. The resulting ribosomal subunits are transported through nuclear pores into the cytoplasm of the cell, where they combine into ribosomes, which settle on the surface of the granular ER or form clusters in the cytoplasm. Such associations of ribosomes in the cytoplasm are called polysomes. Thus, the regulation of protein synthesis in the cell is carried out by the nucleolus, since protein synthesis occurs on ribosomes formed in the nucleoli.

Nucleoli can disappear both normally and in pathology. When do nucleoli disappear normally? Normally, nucleoli disappear when the period of cell division comes and spiralization of DNA fibrils begins, including in the region of nucleolar organizers, then the genes of nucleolar organizers on which r-RNA is transcribed are closed, r-RNA transcription stops and the nucleolus disappears. This can also happen if the cell is exposed to some toxic substances. Before disappearing, the nucleolus is dissected, i.e. the internal fibrillar part is separated from the external granular part. Then the granular component of the nucleolus disappears, i.e. ribosomal subunits and the fibrillar component disappears, i.e. rRNA molecules. Thus, the larger the size of the nucleoli or the greater their number, the more intense the formation of ribosomal subunits and the increase in protein synthesis in the cell

The nuclear membrane (nucleolemma) consists of two membranes: the outer membrane (membrana nuclearis externa) and the inner membrane (membrana nuclearis interna). There is a space between the membranes (cysterna nucleolemmae). The outer nuclear membrane is covered with ribosomes and is closely associated with the ER. You can often see how the outer membrane continues into the tubules of the granular ER. The inner nuclear membrane is associated with chromatin and fibrillar nuclear components. The nucleolemma has nuclear pores (pori nuclearis). Nuclear pores include pore complexes (complexus pori). Which include: a pore opening (annulus pori) with a diameter of about 90 microns, pore granules (granula pori) and a pore membrane (membrana pori).

The pore opening is formed by the fusion of the outer and inner membranes. The second component of the pore complex is granules. The granules are arranged in 3 rows, 8 granules in each row. The size of the granules is about 25 nm. The granules of each row are located along the periphery of the pore opening. The outer layer of granules faces the cytoplasm, the inner layer faces the karyoplasm, and the third layer is located between the outer and inner. Fibrils extend from the granules. These fibrils connect to the central granule, forming a pore membrane (membrana pori).

THE FUNCTION of nuclear pores is that through them the exchange of substances occurs between the karyoplasm and the cytoplasm of the cell. The more pores in the nucleolemma, the more active the nucleus. If the activity of the nucleus is reduced, then the number of pores decreases; if the synthetic activity of the nucleus is close to zero, then there are no pores in the nucleus. For example, there are no pores in the karyolemma of the sperm nucleus.

Under various adverse influences, the core may observe

pathological changes may occur: pyknosis - coagulation of nuclear chromatin, karyorrhexis - disintegration of the nucleus into parts, there may be swelling of the perinuclear space.

CELL CYCLE (cyclus cellularis) is the period from one cell division to another, or the period from cell division to its death. The cell cycle is divided into 4 periods. The first period is the period of mitosis, the 2nd period is postmitotic or presynthetic, it is designated by the letter G-1, the 3rd period is synthetic, it is designated by the letter S and the 4th period is postsynthetic or premitotic, it is designated by the letter G-2 , the mitotic period is designated by the letter M. After mitosis, the next period G-1 begins. During this period, the daughter cell's mass is 2 times less than the mother cell. This cell has 2 times less protein, DNA and chromosomes, i.e. Normally there should be chromosomes 2n and DNA 2c. What happens in the G-1 period? At this time, transcription of RNA occurs on the surface of the DNA, which takes part in the synthesis of proteins. Due to proteins, the mass of the daughter cell increases. At this time, DNA precursors and enzymes involved in the synthesis of DNA and DNA precursors are synthesized. The main processes in the G-1 period are the synthesis of proteins and cell receptors. Then comes the S-period. During this period, DNA replication of chromosomes occurs. As a result, by the end of the S-period the DNA content is 4c. But there will be 2n chromosomes, although in fact there will also be 4n chromosomes, but the DNA of the chromosomes during this period is so mutually intertwined with each other that each sister chromosome in the mother chromosome is not yet visible. As a result of DNA synthesis, its quantity increases and the transcription of ribosomal, messenger and transport RNAs increases, and protein synthesis naturally increases. At this time, doubling of centrioles in cells can occur. Thus, a cell from the S period enters the G-2 period. At the beginning of the G-2 period, the active process of transcription of various RNAs and the process of protein synthesis, mainly tubulin proteins, which are necessary for the division spindle, continue. Centriole duplication may occur. Mitochondria intensively synthesize ATP, which is a source of energy, and energy is necessary for mitotic cell division. After the G-2 period, the cell enters the mitotic period.

Some cells may exit the cell cycle. The exit of a cell from the cell cycle is indicated by the letter G-o. A cell entering this period loses its ability to undergo mitosis. Moreover, some cells lose their ability to mitosis temporarily, while other cells permanently.

If a cell temporarily loses the ability to undergo mitotic division, it undergoes initial differentiation. In this case, a differentiated cell specializes in performing a specific function. After initial differentiation, this cell is able to return to the cell cycle and enter the G-1 period and, after passing through the S period and the G-2 period, undergo mitotic division. Where in the body are cells located in the G-o period? Such cells are found in the liver. But if the liver is damaged or part of the liver is surgically removed by a surgeon, then all the cells that have undergone initial differentiation return to the cell cycle and, due to their division, rapid restoration of liver parenchyma cells occurs.

Stem cells are also in the G-o period, but when a stem cell begins to divide, it goes through all the interphase periods: G-1, S, G-2.

Those cells that finally lose the ability to mitotic division undergo first initial differentiation and perform certain functions, and then final differentiation. At terminal differentiation, the cell is unable to return to the cell cycle and eventually dies. Where in the body are these cells located? Firstly, these are blood cells. Blood granulocytes that have undergone differentiation function for 8 days and then die. Red blood cells function for 120 days, then they also die in the spleen. Secondly, the epidermal cells of the skin. Epidermal cells undergo first initial and then final differentiation. During mitosis, chromosomal material is evenly distributed between daughter cells. Mitosis is divided into 4 phases. The 1st phase is called prophase, the 2nd is metaphase, the 3rd is anaphase, the 4th is telophase.

If a cell has a half (happloid) set of chromosomes, constituting 23 chromosomes (sex cells), then such a set is designated by the symbol 1n chromosomes and 1c DNA, if diploid - 2n chromosomes and 2c DNA (somatic cells immediately after mitotic division), an aneuploid set of chromosomes - in abnormal cells.

PROPHASE OF MITOSIS is divided into early and late. During early prophase, spiralization of chromosomes occurs and they become visible in the form of thin threads and form a dense ball, i.e., the figure of a dense ball is formed. With the onset of late prophase, the chromosomes spiral even more, as a result of which the genes for the nucleolar chromosome organizers are closed. Therefore, r-RNA transcription stops, the formation of chromosome subunits stops, and the nucleolus disappears. At the same time, fragmentation of the nuclear membrane occurs. Fragments of the nuclear membrane fold into small vacuoles. The amount of granular EPS in the cytoplasm decreases. The granular EPS tanks are fragmented into smaller structures. The number of ribosomes on the surface of the ER membranes decreases sharply. This leads to a decrease in protein synthesis by 75%. At this point, the cell center doubles. The resulting 2 cell centers begin to diverge towards the poles. Each of the newly formed cell centers consists of two centrioles: a mother and a daughter. With the participation of cell centers, a fission spindle begins to form, which consists of microtubules. Chromosomes continue to spiralize, resulting in the formation of a loose ball of chromosomes located in the cytoplasm. Thus, late prophase is characterized by a loose ball of chromosomes.

METAPHASE. During metaphase, the chromatids of the maternal chromosomes become visible. Maternal chromosomes line up in the equatorial plane. If you look at these chromosomes from the equator of the cell, they are perceived as the equatorial plate (lamina equatorialis). In that case, if you look at the same plate, but from the side of the pole, then it is perceived as the mother star (monastr). During metaphase, spindle formation is completed. Two types of microtubules are visible in the spindle. Some microtubules are formed from the cell center, i.e. from the centriole and are called centriolar microtubules (microtubuli cenriolaris). Other microtubules begin to form from the kinetochores of the chromosomes. What are kinetochores? In the area of primary chromosome constrictions there are so-called kinetochores. These kinetochores have the ability to induce self-assembly of microtubules. This is where the microtubules begin, which grow towards the cell centers. Thus, the ends of the kinetochore microtubules extend between the ends of the centriolar microtubules.

ANAPHASE. During anaphase, the simultaneous separation of daughter chromosomes (chromatids) occurs, which begin to move, some to one, and others to the other pole. In this case, a double star appears, i.e. 2 daughter stars (diastr). The movement of stars is carried out thanks to the spindle and due to the fact that the poles of the cell themselves move somewhat away from each other.

MECHANISM OF MOTION OF DAUGHTER STARS. This movement is ensured by the fact that the ends of the kinetochore microtubules slide along the ends of the centriolar microtubules and pull the chromatids of the daughter stars towards the poles.

TELOPHASE. During telophase, the motion of daughter stars stops and cores begin to form. Chromosomes undergo despirilization, and a nuclear envelope (nucleolemma) begins to form around the chromosomes. Since the DNA fibrils of chromosomes undergo despiralization, RNA transcription begins on the opened genes. Since despiralization of chromosome DNA fibrils occurs in the region of nucleolar organizers, rRNA begins to be transcribed in the form of thin threads, i.e., the fibrillar apparatus of the nucleolus is formed. Then ribosomal proteins are transported to the r-RNA fibrils, which are complexed with r-RNA, resulting in the formation of ribosomal subunits, i.e. a granular component of the nucleolus is formed. This occurs already in late telophase. CYTOTOMY, i.e. constriction formation. When a constriction forms along the equator, the cytolemma invaginates. The mechanism of invagination is as follows. Tonofilaments, consisting of contractile proteins, are located along the equator. These tonofilaments retract the cytolemma. Then the cytolemma of one daughter cell separates from another similar daughter cell. Thus, as a result of mitosis, new daughter cells are formed. Daughter cells are 2 times smaller in mass compared to the mother. The amount of DNA is also reduced here. It corresponds to 2c and the number of chromosomes is reduced by 2 times. It corresponds to 2n. This is how the cell cycle ends with mitotic division.

PATHOLOGY OF MITOSIS. ANEUPLOID CELLS DESTRUCTION OF THE SPINDLE is observed when the cell temperature decreases and when the cell is exposed to colchicine, as a result of which the spindle microtubules begin to disintegrate.

DISRUPTION OF CELL DIVISION WITH INCREASE IN THE NUMBER OF CENTROSOMES occurs when instead of 2 cytocenters 3 or 4 are formed. In this case, 2 or more division spindles are formed, as a result of which the mother cell is divided into 3 or more cells. The nucleus of each such cell will contain an incorrect, aneuploid set of chromosomes.

CHROMOSOMAL ABBERATION occurs when tissue is exposed to ultraviolet or radioactive rays. During anaphase of mitosis, part of such a damaged chromosome can separate from its arm and, after telophase, end up in one of the daughter cells. This piece of chromosome is surrounded by a nucleolemma and represents a “micronucleus”. Chromosomal aberration can manifest itself in the fact that chromosomes can stick together with each other, while the 2 primary constrictions of such a double chromosome are located in different places and stretch to opposite poles. When the daughter stars diverge, this pair of chromosomes will take a position along the spindle axis. In this case, the daughter stars will be connected by a “bridge”. In all cases of chromosomal aberration, the chromosome content in the nucleus will be aneuploid, i.e. wrong.

AMITOSIS (direct division) is characterized by the fact that first a nuclear constriction appears, which divides the nucleus not necessarily into absolutely equal parts, then the cytoplasm is divided by the constriction. During amitosis, the chromosomal material from the nucleus of the mother cell may be distributed unevenly between daughter cells. This is how amitosis differs fundamentally from mitosis.

Direct division separates cells that cannot be considered normal. This division is also considered abnormal.

POLYPLOIDY. ENDOREPRODUCTION POLYPLOIDY is the process of increasing the number of chromosomes in the cell nucleus. As a result, polyploid cells are formed.

In the process of polyploidy, 2 mechanisms take part: 1) blocking one of the phases of mitosis; 2) violation of cytotomy during telophase. Let's consider the 1st mechanism, i.e. blocking the G-2 period, prophase or metaphase. In this case, the undivided cell enters the G-1 period with a tetraploid set of chromosomes (4n), then into the S-period, after which it will have 8c DNA and 8n chromosomes. Then this cell enters prophase, then metaphase. In a metaphase star there will be 8n. Then, during anaphase, the diverging daughter stars will each have 4n chromosomes. After telophase, the daughter cells will have tetraploid nuclei. The 2nd mechanism of formation of polyploid cells is observed when cytotomy is violated. After anaphase has occurred, the cell

entered telophase, nuclei formed, but cytotomy of the mother cell did not occur. Each of the 2 nuclei of an undivided cell contains 2n and 2c. When this cell enters the G-1 period, then the S period, then at its end there will be 4n and 4c in each nucleus of the undivided cell. Then this cell enters the glandulocytes of the acini of the salivary glands, pancreas, and the pigment layer of the retina. In this case, the core can contain 4n, 8n, 16n, 32n. Severe polyploidy is especially characteristic of red bone marrow megakaryocytes.

ENDOREPRODUCTION is the sequential multiple doubling of DNA, resulting in an increase in the number of chromosomes, while the chromosomes are connected by thin threads. These structures are called polytenes, characteristic of placental cells.

MEIOSIS is a division in which the daughter cells end up with a half (haploid) set of chromosomes - 1n and 1c. This division occurs during the formation of germ cells.

Let's consider the formation of germ cells in the male body, called spermatogenesis. Spermatogenesis includes 4 periods: 1) reproduction; 2) growth period, or prophase; 3) ripening, which consists of two divisions: the 1st division of ripening and the 2nd division of ripening and 4) the formation period. But we will not consider the formation period.

BREEDING PERIOD. Reproducing (dividing) cells during the reproduction period are called spermatogonia. During division, spermatogonia undergo all phases characteristic of mitotic division, i.e. after the division of the maternal (stem) spermatogonia, 2 daughter spermatogonia are formed with a set of chromosomes 2n and a set of DNA 2c, then these spermatogonia go through the entire cell cycle and by the upcoming new division they will have 4n and 4c. These spermatogonia with 4n and 4c enter the 2nd period: the GROWTH period or the PROPHASE period of the 1st meiotic division. From this moment on, the cells are called 1st order SPERMATOCYTES. During the development of 1st order spermatocytes, 5 phases take place: leptotene, synaptene, pachytene, diplotene and diakinesis.

LEPTOTENE. During leptotene, spiralization of chromosomes occurs, which become visible, resembling thin threads. Then comes ZYGOTEN (synaptene). During zygotene, homologous chromosomes move closer to each other and join together, crossing over (crossing over). The united chromosomes exchange genes. A pair of united chromosomes is called a bivalent. How many bivalents are there in the nucleus of a 1st order spermatocyte in the zygotene phase? 23 bivalents. Then comes PACHITENA. During pachytene, each of the bivalent chromosomes undergoes further spiralization, but at the same time it shortens and thickens. Noticeable gaps appear between the chromatids of the bivalent chromosomes. After this, DIPLOTENA occurs, during which the chromatids of the bivalent chromosomes begin to diverge, but find themselves connected in the region of crossover. Then DIAKINESIS occurs, during which further spiralization of chromosomes occurs, resulting in the formation of tetrads at the end of prophase. Their number is 23. Each tetrad consists of 4 monads, or chromatids. Thus, in the nucleus of a first-order spermatocyte at the end of prophase there will be 23 tetrads and 92 monads. Then the cell enters the 1st division of MATURATION. Moreover, in metaphase there will be 23 tetrads in the mother star. The tetrads are lined up in the equatorial plane in such a way that one half of the tetrad faces one pole of the cell, the other half faces the other. During anaphase, the halves of the tetrads, called dyads, move toward the poles. Then, as a result of telophase, 2 new cells are formed from the 1st order spermatocyte, called 2nd order spermatocytes. Each 2nd order spermatocyte will have 23 dyads (2n) or 46 monads. Spermatocytes of the 2nd order without a preliminary S-period, G-2 period and prophase immediately enter the metaphase of the 2nd division of MATURATION. In the mother star of a 2nd order spermatocyte there will be 23 dyads, which are lined up in the equatorial plane in such a way that one half of the dyad (monad) faces one pole, and the other half faces the other pole. These halves are called monads. During anaphase, the daughter stars, consisting of monads, move towards the poles. During the telophase of the 2nd maturation division, 2 new cells called spermatids are formed. Spermatids will have a haploid set of chromosomes (1n).

STRUCTURE OF MITOTIC CHROMOSOMES. Mitotic chromosomes appear during mitosis. They are especially visible during metaphase and anaphase. During metaphase, each maternal chromosome is seen to consist of two sister chromosomes, or chromatids. Each chromosome consists of one DNP molecule, which is folded in a special way and takes on a characteristic shape. Each chromosome has a primary constriction, or centromere. The sections of chromosomes extending from the primary constriction are called chromosome arms. If the chromosome arms have the same or approximately the same length, then such chromosomes are called metacentric; if the chromosome arms are clearly unequal in length, then such a chromosome is called submetocentric; if one arm is clearly many times longer than the second, then such a chromosome is called acrocentric. The ends of the chromosome arms are called telomeres. In addition to the primary constriction, some chromosomes have secondary constrictions. The secondary constriction is the nucleolar organizer. The section of the chromosome arm between the secondary constriction and the telomere is called a satellite, or satellite. The set of chromosomes in the human nucleus constitutes a karyotype. What characterizes a KARYOTYPE? The karyotype is characterized by the number of chromosomes, their sizes and structural features.

All chromosomes of the human nucleus are divided into 7 groups. The groups are designated by letters of the Latin alphabet from A to G. In each group, the chromosomes are morphologically similar to each other, but the chromosomes of different groups are different. But to distinguish chromosomes from each other in the same group, the method of differential staining is used. With differential staining, light and dark stripes appear on the chromosome arms. Moreover, the pattern formed by these stripes is individual for each chromosome, like fingerprints. Therefore, thanks to differential staining, chromosomes can be distinguished from each other.

CELL RESPONSE TO EXTERNAL INFLUENCES

When a cell is exposed to unfavorable external chemical, physical and biological factors, structural and functional disturbances occur in it. Depending on the intensity, duration and nature of the impact, such a cell can adapt to new conditions and return to its original state or may die.

CHANGES IN THE CYTOPLASM OF THE DAMAGED CELL. The cytoplasm loses its ability to form granules. In a normal cell, paint particles entering its cytoplasm are enclosed in granules. The cytoplasm and karyoplasm remain light. When the ability to form granules is lost, granules are not formed, and the cytoplasm and karyoplasm are diffusely stained.

CHANGES IN THE CORE. In the nucleus, swelling of the perinuclear space begins, its expansion. Chromatin condenses into rough clumps and coagulates. This is called pyknosis. The regulation of protein synthesis is disrupted. Subsequently, the core breaks into fragments. This is called karyorrhexis. Ultimately, the nucleus undergoes lysis - karyolysis.

CHANGES IN MITOCHONDRIA. At the initial stage, mitochondria shrink, then they swell, become rounded, their cristae are shortened and reduced, and ATP synthesis decreases. Ultimately, the mitochondrial membranes rupture and the matrix mixes with the hyaloplasm.

CHANGES IN THE ENDOPLASMIC RETICULUM. The granular EPS cisterns fragment and disintegrate into vacuoles. The number of ribosomes on the membrane surface decreases, protein synthesis decreases.

CHANGES IN THE GOLGI COMPLEX. The Golgi complex may undergo disintegration as a result of fragmentation of its cisterns.

CHANGES IN LYSOSOMES. The number of primary lysosomes and autophagosomes increases. The membranes of primary lysosomes are ruptured. The enzymes released from them carry out self-digestion (lysis) of the cell.

As a result of disruption of the permeability of cell membranes, the structure and function of organelles, cell metabolism is disrupted, which can be accompanied by the accumulation of lipids (fatty degeneration), glycogen (carbohydrate degeneration) and proteins (protein degeneration) in the cytoplasm.

DURING CELL REBIRTH. In some cases, regulatory processes in the cell are disrupted. This can lead to disruption of its differentiation, which is based on changes in the DNA gene of the chromosomes. As a result, the cell acquires relative autonomy, the ability to uncontrollably divide, and metastasize. The newly formed daughter cells will inherit the above properties. The tumor begins to grow rapidly.

CELL NECROSIS AND APOPTOSIS

CELL NECROSIS occurs during its unprogrammed death and is observed after its damage. In this case, the permeability of cell membranes is disrupted, compartments expand, the structure is damaged and the function of the EPS, Golgi complex, and mitochondria is disrupted, the number of autophagosomes increases, and ultimately everything ends in cell lysis.

APOPTOSIS is programmed cell death. Such cell death is due to the fact that the DNA of the chromosomes contains genes in which the cell death program is encoded. This program is launched in two cases: 1) when the cell is exposed to certain proteins or hormones; 2) in the event that the cell does not receive regulatory signals.

When a cell is exposed to proteins or hormones, a signaling molecule (cAMP or calmodulin) is synthesized in its cytoplasm, which triggers the cell death program. Example: glucocorticoids of the adrenal cortex, with their increased content in the blood, are captured by the receptors of the outer membrane of the lymphocyte karyolemma and, through a signaling molecule, trigger the cell’s self-destruction program.

In the absence of signals regulating cell function, a signal molecule is also synthesized, which activates the gene containing the cell death program. Examples: 1) signals are produced in the testis that regulate the functions of prostate cells; if you castrate a male, the flow of regulatory signals stops, which is accompanied by self-destruction of prostate cells; 2) the pituitary gland produces hormones that regulate the development and function of the corpus luteum of the ovaries; when the release of these hormones from the pituitary gland stops, self-destruction of the cells of the corpus luteum begins, as a result of which it completely disappears.

THE NATURE OF CHANGES IN THE CELL DURING APOPTOSIS. After activation of the cell's self-destruction genes, the division of chromosome DNA into nucleosomal fragments begins. The chromatin of the nucleus condenses, and rough clumps of chromatin are formed adjacent to the nucleolemma. The nucleus disintegrates into micronuclei fragments. Each such nucleus is surrounded by a nucleolemma. At the same time, the cytoplasm is fragmented with the subsequent formation of microcells - apoptotic bodies, which include micronuclei. Apoptotic bodies are then phagocytosed by macrophages or undergo lysis.

There are 3 ways of cell division - mitosis, amitosis, meiosis.

Mitosis

Mitosis- indirect cell division. Mitosis consists of 4 phases: prophase, metaphase, anaphase, telophase.

First phase - prophase. During prophase, chromosomes spiral, shorten, thicken and become visible. Each chromosome consists of two chromatids. They are connected by a centromere. By the end of prophase, the nuclear envelope and nucleoli dissolve. Centrioles diverge towards the poles of the cell. A fission spindle is formed (Fig. 42, 2).

IN metaphase chromosomes are located at the equator. The number and shape of chromosomes are clearly visible. The filaments of the spindle stretch from the poles to the centromeres (42, 3).

IN anaphase Centromeres divide and chromatids (daughter chromosomes) move to different poles. The movement of chromosomes occurs

walks thanks to spindle threads, which, contracting, stretch the daughter chromosomes from the equator to the poles (Fig. 42, 4).

Mitosis ends telophase. Chromosomes, consisting of one chromatid, are located at the poles of the cell. They despiral and become invisible (Fig. 42, 5).

The nuclear envelope is formed. The nucleolus is formed in the nucleus. The cytoplasm divides. In animal cells, the cytoplasm is divided by constriction, invagination of the membrane from the edges to the center.

Fig.42.Mitosis. The nucleus of a non-dividing cell. The round nucleolus is visible (1). 2 - prophase, 3 - metaphase, 4 - anaphase, 5 - telophase.

In plant cells, a partition is formed in the center, which grows towards the cell walls. After the formation of a transverse cytoplasmic membrane, a cellular wall is formed in plant cells (Fig. 43).

As a result of mitosis, each daughter cell receives exactly the same chromosomes as the mother cell had. The number of chromosomes in both daughter cells is equal to the number of chromosomes in the mother cell.

Biological significance of mitosis

Mitosis ensures the precise transfer of hereditary information to each of the daughter nuclei.

Mitotic cycle

Mitotic cycle - the period between the end of one division and the beginning of the next. This period in the cell's mitotic cycle is called interphase.

Interphase has 3 periods:

. Presynthetic G 1. During this period, RNA and protein synthesis and cell growth occur. Cells have a diploid (2n) set of chromosomes and 2c DNA genetic material.

Rice. 43.Formation of the cytoplasmic membrane in animal (1, 2) and plant cells (3, 4).

Rice. 44.Mitotic cycle of a diploid cell.

G 1 - presynthetic (postmitotic) period: S - synthetic period, G 2 - postsynthetic (premitotic) period. Mitosis: P - prophase; M - metaphase, A - anaphase, T - telophase; n - haploid set of chromosomes; 2n - diploid set of chromosomes; 4n - tetraploid set of chromosomes; c is the amount of DNA corresponding to the haploid set of chromosomes. Outside the circle, changes in chromosomes during different periods of the cell life cycle are schematically shown.

. Synthetic(S). DNA molecules are reduplicated and a second chromatid is formed in the chromosome. Each chromosome consists of two chromatids and contains 4c DNA. The number of chromosomes does not change (2n).

. In postsynthetic During the G 2 period, the synthesis of proteins necessary for the formation of the fission spindle occurs. The duplication of centrioles is completed. ATP molecules accumulate the energy necessary for cell division. The cell is ready to divide. Neither the DNA content (4c) nor the number of chromosomes (2n) changes.

Cells have a diploid set of chromosomes. Each chromosome consists of two chromatids (Fig. 44).

Questions for self-control

1. What cell division is called mitosis?

2. What cells divide by mitosis?

3. What phases does mitosis consist of?

4. What happens in prophase of mitosis?

5. Where are chromosomes located in metaphase of mitosis?

6. What happens in anaphase of mitosis?

7. What happens in the telophase of mitosis?

8. What set of chromosomes do daughter cells resulting from mitosis have?

9. What is the biological significance of mitosis? 10.What periods is interphase divided into?

11.What happens in the presynthetic period of interphase? 12.What happens in the synthetic period of interphase? 13.What happens in the postsynthetic period of interphase?

Key words for the topic “Mitosis”

anaphase

spindle

division

meaning

interphase

information

cell

edge

membrane

metaphase

mitosis

direction thread

ending

partition

constriction

period

pole

prophase

plant

reduplication

result

height

synthesis

stage

wall

body

telophase

form

chromatid

chromosome

center

centrioles

centromere

equator

nuclear envelope core

nucleoli

Amitosis

Amitosis- direct cell division, in which the nucleus is in an interphase state. Chromosomes are not detected. The fission spindle is not formed. Amitosis leads to the appearance of two cells, but very often as a result of amitosis, binucleate and multinucleate cells appear.

Amitotic division begins with a change in the shape and number of nucleoli. Large nucleoli are divided by a constriction. Following the division of the nucleoli, nuclear division occurs. The nucleus can be divided by a constriction, forming two nuclei, or multiple divisions of the nucleus, its fragmentation, take place. The nuclei may be of unequal size.

Amitosis occurs in aging, degenerating cells that are unable to produce new viable cells.

Normally, amitotic nuclear division occurs in the embryonic membranes of animals and in the follicular cells of the ovary.

Amitotically dividing cells are found in various pathological processes (inflammation, malignant growth, etc.).

Questions for self-control

1. What is amitosis?

2. How does amitotic division occur?

3. In which cells does amitosis occur?

Keywords of the topic “Amitosis”

Amitosis

Binucleate cells

Multinucleated cells Fragmentation

Meiosis

Meiosisoccurs during the formation of gametes in animals and the formation of spores in plants. Meiosis is a reduction division. As a result of meiosis, the number of chromosomes is reduced from diploid (2n) to haploid (n). Meiosis involves 2 successive divisions. Each meiotic division has 4 stages: prophase, metaphase, anaphase and telophase.

Prophase of the first meiotic division

ProphaseThe first meiotic division is the most complex. There are 5 stages in it: leptotene, zygotene, pachytene, diplotene, diakinesis.

IN leptotene (stage I) chromosome spiralization begins. Chromosomes become visible under a microscope as long, thin threads. Each chromosome consists of two chromatids. A diploid set of chromosomes is visible in the nucleus (Fig. 45).

In II stage of prophase first meiotic division - zygotene- spiralization of chromosomes continues and conjugation of homologous chromosomes occurs. Homologous are chromosomes that have the same shape and size: one of them is received from the mother, and the other from the father. Homologous chromosomes attract and adhere to each other along their entire length. The centromere of one of the paired chromosomes is exactly adjacent to the centromere of the other, and each chromomere is adjacent to the homologous chromomere of the other (Fig. 46).

Figure 45. Leptotene.

Rice. 46. Zygotene.

Stage III- pachytene- stage of thick filaments. Conjugating chromosomes are closely adjacent to each other. Such double chromosomes are called bivalents. Each bivalent consists of a quadruple (tetrad) of chromatids. The number of bivalents is equal to the haploid set of chromosomes. Further spiralization of chromosomes occurs. Close contact between chromatids makes it possible to exchange identical regions in homologous chromosomes. This phenomenon is called crossing over (Fig. 47).

IN diplotene (IV stage) repulsive forces arise between homologous chromosomes. The chromosomes that make up the bivalent begin to move away from each other, primarily in the centromere region. When chromatids diverge, the phenomenon of crossover and cohesion is detected in some places (Fig. 48).

Stage V- diakinesis- characterized by maximum spiralization, shortening and thickening of chromosomes (Fig. 49). The repulsion of chromosomes continues, but they remain connected at their ends into bivalents. The nucleolus and nuclear envelope dissolve. Centrioles diverge towards the poles.

In prophase of the first meiotic division, 3 main processes occur: conjugation of homologous chromosomes; formation of chromosome bivalents or chromatid tetrads; crossing over.

Rice. 47. Pachytena.

Rice. 48. Diplotena.

Rice. 49. Diakinesis.

Metaphase of the first meiotic division

In metaphaseDuring the first meiotic division, chromosome bivalents are located along the equator of the cell. The spindle threads are attached to them (Fig. 50).

Anaphase of the first meiotic division

In anaphaseDuring the first meiotic division, chromosomes, not chromatids, disperse to the poles of the cell. Only one of a pair of homologous chromosomes enters the daughter cells (Fig. 51).

Telophase of the first meiotic division

In telophaseDuring the first meiotic division, the number of chromosomes in each cell becomes haploid. For a short time, a nuclear envelope is formed (Fig. 52).

Rice. 50. Metaphase I.

Rice. 51. Anaphase I.

Rice. 52. Telophase I.

Between the first and second divisions of meiosis in an animal cell there may be a short interphase. During interphase there is no reduplication of DNA molecules.

The second meiotic division occurs in the same way as mitosis.

Prophase of the second meiotic division

IN prophase During the second meiotic division, chromosomes thicken and shorten. The nucleolus and nuclear membrane are destroyed. A fission spindle is formed (Fig. 53).

Metaphase of the second meiotic division

IN metaphase During the second meiotic division, the chromosomes line up along the equator. The filaments of the spindle are suitable for them (Fig. 54).

Anaphase of the second meiotic division

In anaphase of the second meiotic division, the centromeres divide and pull the chromatids that have separated from each other towards opposite poles. Chromatids are called chromosomes (Fig. 55).

Rice. 53. Prophase II.

Rice. 54. Metaphase II.

Rice. 55. Anaphase II.

Rice. 56. Telophase II.

Telophase of the second meiotic division

IN telophase During the second meiotic division, the chromosomes despiral and become invisible. The nuclear envelope is formed. Each nucleus contains a haploid number of chromosomes. The cytoplasm divides. From the initial diploid cell, 4 haploid cells are formed (Fig. 56).

Thus, during meiosis, conjugation and crossing over occur between regions of homologous chromosomes and a reduction in the number of chromosomes (Fig. 57).

Questions for self-control

1. What division is called meiosis?

2. What happens during meiosis?

3. How many divisions does meiosis have?

4. What happens in prophase of the first meiotic division?

5. What happens in metaphase of the first meiotic division?

6. What happens in anaphase of the first division of meiosis?

7. What set of chromosomes do cells have in the telophase of the first meiotic division?

8. What happens in prophase of the second division of meiosis?

9. What happens in metaphase of the second meiotic division? 10.What happens in anaphase of the second division of meiosis? 11.What happens in the telophase of the second division of meiosis? 12. How many cells were formed as a result of meiosis? 13. What set of chromosomes do they have?