Mitosis–mitos (Greek - threads) – indirect division cells, a universal method of dividing eukaryotic cells.

Main events of the mitotic cycle consist in reduplication (self-duplication) hereditary material of the mother cell and in uniform distribution of this material between daughter cells. These events are accompanied by natural changes in the chemical and morphological organization chromosomes- nuclear structures in which more than 90% of the genetic material of a eukaryotic cell is concentrated (the main part of extranuclear DNA animal cell located in mitochondria).

Chromosomes, in interaction with extrachromosomal mechanisms, provide: a) storage of genetic information; b) using this information to create and maintain cellular organization; c) regulation of reading hereditary information; d) doubling of genetic material; d) transfer it from the mother cell to the daughter cells.

Mitosis is a continuous process that is divided into phases.

In mitosis we can distinguish four phases. The main events for individual phases are presented below.

Biological significance of mitosis: the formation of cells with hereditary information that is qualitatively and quantitatively identical to the information of the mother cell. Ensuring the constancy of the karyotype over a number of cell generations. Mitosis serves as a cellular mechanism for the processes of growth and development of the body, its regeneration and asexual reproduction. Thus, mitosis is a universal mechanism for reproducing the cellular organization of the eukaryotic type in individual development.

Pathology of mitosis

Disturbances in one or another phase of mitosis lead to pathological changes in cells. Deviation from normal course The process of spiralization can lead to swelling and sticking together of chromosomes. Sometimes a fragment of a chromosome section is observed, which, if it is deprived of a centromere, does not participate in anaphase movement to the poles and is lost. Individual chromatids may lag behind during movement, which leads to the formation of daughter nuclei with unbalanced chromosome sets. Damage to the spindle leads to a delay in mitosis in metaphase and chromosome scattering. When the number of centrioles changes, multipolar or asymmetric mitoses occur. Violation of cytotomy leads to the appearance of bi- and multinucleated cells.

Based on the mitotic cycle, a number of mechanisms have emerged by which in a particular organ the amount of genetic material and, consequently, the intensity of metabolism can be increased while maintaining a constant number of cells.

Endomitosis. The doubling of a cell's DNA is not always accompanied by its division into two. Since the mechanism of such doubling coincides with premitotic DNA reduplication and it is accompanied by a multiple increase in the number of chromosomes, this phenomenon is called endomitosis. When cells are exposed to substances that destroy spindle microtubules, division stops, and chromosomes will continue the cycle of their transformations: replicate, which will lead to the gradual formation of polyploid cells - 4n, 8n, etc. This transformation process is otherwise called endoreproduction. From a genetic point of view, endomitosis is a genomic somatic mutation. The ability of cells to undergo endomitosis is used in plant breeding to obtain cells with a multiple set of chromosomes. For this purpose, colchicine and vinblastine are used, which destroy the filaments of the achromatin spindle. Polyploid cells (and later adult plants) differ large sizes, vegetative organs of such cells are large, with a large supply of nutrients. In humans, endoreproduction occurs in some hepatocytes and cardiomyocytes.

Polythenia. During polyteny in the S-period, as a result of replication and non-disjunction of chromosomal strands, a multi-stranded, polytene structure is formed. They differ from mitotic chromosomes in their larger size (200 times longer). Such cells are found in salivary glands dipteran insects, in the macronuclei of ciliates. On polytene chromosomes, swellings and puffs (transcription sites) are visible - an expression of gene activity. These chromosomes are the most important object of genetic research. Endomitosis and polyteny lead to the formation polyploid cells, characterized by a multiple increase in the volume of hereditary material. In such cells, unlike diploid cells, genes are repeated more than twice. In proportion to the increase in the number of genes, the cell mass increases, which increases its functionality. In the mammalian body, polyploidization with age is characteristic of liver cells.

Mitotic cycle abnormalities. The mitotic rhythm, usually adequate to the need for restoration of aging, dead cells, can be changed under pathological conditions. A slowdown of the rhythm is observed in aging or poorly vascularized tissues, an increase in the rhythm is observed in tissues under various types of inflammation, hormonal influences, in tumors, etc.

Anomalies in the development of mitoses. Some aggressive agents, acting on the S phase, slow down DNA synthesis and duplication. These include ionizing radiation, various antimetabolites (metatrexate, mercapto-6-purine, fluoro-5-uracil, procarbozine, etc.). They are used for antitumor chemotherapy. Other aggressive agents act on the phases of mitosis and interfere with the formation of the achromatic spindle. They change the viscosity of the plasma without splitting the chromosome strands. Such a cytophysiological change may entail a blockade of mitosis in metaphase, and then - acute death cells, or mitonecrosis. Mitonecrosis is often observed, in particular, in tumor tissue, in the foci of certain inflammations with necrosis. They can be caused with the help of podophyllin, which is used in the treatment of malignant neoplasms.

Abnormalities in mitotic morphology. With inflammation, exposure to ionizing radiation, chemical agents, and especially in malignant tumors morphological abnormalities of mitoses are detected. They are associated with severe metabolic changes in cells and can be referred to as “abortive mitoses.” An example of such an abnormality is mitosis with an abnormal number and shape of chromosomes; three-, four- and multipolar mitoses.

Multinucleated cells. Cells containing many nuclei are also found in in good condition eg: osteoclasts, megakaryocytes, syncytiotrophoblasts. But they are often prescribed in pathological conditions - for example: Langhans cells in tuberculosis, giant cells foreign bodies, a bunch of tumor cells. The cytoplasm of such cells contains granules or vacuoles; the number of nuclei can vary from a few to several hundred, and the volume is reflected in the name - giant cells. Their origin is variable: epithelial, mesenchymal, histiocytic. The mechanism of formation of giant multinucleated cells is different. In some cases, their formation is due to the fusion of mononuclear cells, in others it is carried out due to the division of nuclei without division of the cytoplasm. It is also believed that their formation may be a consequence of certain mitotic abnormalities after irradiation or the administration of cytostatics, as well as during malignant growth.

Amitosis

Direct fission or amitosis- This is the division of a cell in which the nucleus is in an interphase state. In this case, chromosome condensation and spindle formation do not occur. Formally, amitosis should lead to the appearance of two cells, but most often it leads to the division of the nucleus and the appearance of bi- or multinucleated cells.

Amitotic division begins with fragmentation of the nucleoli, followed by division of the nucleus by constriction (or invagination). There may be multiple nuclear fission, usually of unequal magnitude (with pathological processes). Numerous observations have shown that amitosis almost always occurs in cells that are obsolete, degenerating and unable to produce full-fledged elements in the future. Normally, amitotic division occurs in the embryonic membranes of animals, in the follicular cells of the ovary, and in giant trophoblast cells. Positive value amitosis occurs in the process of tissue or organ regeneration (regenerative amitosis). Amitosis in aging cells is accompanied by disturbances in biosynthetic processes, including replication, DNA repair, as well as transcription and translation. The physicochemical properties of chromatin proteins of cell nuclei, the composition of the cytoplasm, the structure and functions of organelles change, which entails functional disorders at all subsequent levels - cellular, tissue, organ and organismal. As destruction increases and restoration fades, natural cell death occurs. Amitosis often occurs when inflammatory processes And malignant neoplasms(induced amitosis).

Mitosis - indirect cell division, the most common method of reproduction in eukaryotic cells. The most important component cell cycle is mitotic (proliferative) cycle. It is a complex of interrelated and coordinated phenomena during cell division, as well as before and after it. Mitotic cycle- this is a set of processes occurring in a cell from one division to the next and ending with the formation of two cells of the next generation. In addition, the concept of the life cycle also includes the period during which the cell performs its functions and periods of rest. At this time, the further cell fate is uncertain: the cell may begin to divide (enters mitosis) or begin to prepare to perform specific functions.

Main stages of mitosis:

Reduplication(self-duplication) of the genetic information of the mother cell and its uniform distribution between daughter cells. This is accompanied by changes in the structure and morphology of chromosomes, in which more than 90% of the information of a eukaryotic cell is concentrated.

Mitotic cycle consists of four consecutive periods (phases):

• presynthetic (or postmitotic) G1,
• synthetic S,
• postsynthetic (or premitotic) G2,
• mitosis itself.

They make up autocatalytic interphase(preparation period).

Presynthetic (G1). Occurs immediately after cell division. DNA synthesis has not yet occurred. The cell is actively growing in size, storing substances necessary for division: proteins (histones, structural proteins, enzymes), RNA, ATP molecules. Division of mitochondria and chloroplasts (i.e., structures capable of self-reproduction) occurs. The organizational features of the interphase cell are restored after the previous division.

Synthetic (S). Genetic material is duplicated through DNA replication. It occurs in a semi-conservative manner, when the double helix of the DNA molecule diverges into two chains and a complementary chain is synthesized on each of them. The result is two identical DNA double helices, each consisting of one new and one old DNA strand. The amount of hereditary material doubles. In addition, the synthesis of RNA and proteins continues. Also, a small part of mitochondrial DNA undergoes replication (the main part of it is replicated in the G2 period).

Postsynthetic (G2). DNA is no longer synthesized, but the defects made during its synthesis in the S period are corrected (repair). Energy and nutrients are also accumulated, and the synthesis of RNA and proteins (mainly nuclear) continues.

S and G2 are directly related to mitosis, so they are sometimes separated into a separate period - preprophase.

After this comes mitosis proper, which consists of four phases. The division process includes several successive phases and is a cycle. Its duration varies and ranges from 10 to 50 hours in most cells. In human body cells, the duration of mitosis itself is 1-1.5 hours, the G2 period of interphase is 2-3 hours, the S period of interphase is 6-10 hours .

The process of mitosis is usually divided into four main phases:

• prophase,
• metaphase,
• anaphase,
• telophase.

Since it is continuous phase changes are carried out smoothly- one imperceptibly passes into the other.

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 (2n4c).

In metaphase chromosomes reach maximum spiralization and are arranged in an orderly manner at the equator of the cell, so they are counted and studied during this period. The content of genetic material does not change (2n4c).

In anaphase each chromosome “splits” into two chromatids, which from this point on are called daughter chromosomes. The spindle strands attached to the centromeres contract and pull the chromatids (daughter chromosomes) toward opposite poles of the cell. The content of genetic material in the cell at each pole is represented by a diploid set of chromosomes, but each chromosome contains one chromatid (4n4c).

In telophase The chromosomes located at the poles despiral and become poorly visible. Around the chromosomes at each pole, a nuclear membrane is formed from membrane structures of the cytoplasm, and nucleoli are formed in the nuclei. The fission spindle is destroyed. At the same time, the cytoplasm is dividing. Daughter cells have a diploid set of chromosomes, each of which consists of one chromatid (2n2c).

All processes occurring during the cell cycle are controlled certain genes. Mutations of these genes lead to disruption of the cell cycle at its different stages. Mitosis is common to all eukaryotes. His biological significance is that as a result, all daughter cells have the same number of chromosomes as the parent. The individuality of chromosomes is completely preserved. In this and is the genetic significance of mitosis, because each of the cells arising as a result of division carries a complete set of genes characteristic of the initial cell. The latter is very important with the increasingly widespread introduction into practice of biotechnological methods, thanks to which normal fertile plants develop from individual somatic cells

Mitosis(from gr. mitos- thread), or 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 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.

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. In mitosis there are prophase and metaphase; anaphase and telophase.

IN prophase shortening and thickening of chromosomes occurs due to their spiralization. At this time, double chromosomes consist of two sister chromatids connected to each other. Chromosome duplication occurred in the S-period of interphase. 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 it begins to form spindle, which is formed from microtubules by polymerization of protein subunits.

Microtubules begin to form from the centrioles.

IN metaphase the formation of the fission spindle is completed, which consists of two types of chromosomal microtubules, which bind to the centromeres of chromosomes, and centrosomal (polar) microtubules, which stretch from pole to pole of the cell.

Each double chromosome is attached to the spindle microtubules. 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 plate. 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 Daughter chromosomes, with the help of spindle microtubules, are stretched to the poles of the cell. 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 of chromosomes duplicated in interphase 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 area 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, the phragmoplast, arises from the remnants of the filament spindle filaments. 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 with the same set of chromosomes as in the mother cell arise from one cell.

Biological significance Mitosis thus consists in a 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 basis for asexual reproduction of organisms.

The biological significance of mitosis is very high. It is difficult for the uninitiated to even imagine what role the process of simple cell division in the body plays in life. The ability of cells to divide is their most important and fundamental function. Without this, it is impossible to continue life on Earth, to increase the populations of unicellular organisms, it is impossible to develop and continue the existence of a large multicellular organism, and it is also impossible to develop new life from a fertilized egg.

The biological significance of mitosis would be much less if it were not the essence of most events occurring on our planet biological processes. This process occurs in several stages. Each of them involves several actions within the cell. The result of this is the obligatory multiplication of the genetic basis of one cell in two by duplicating DNA, so that subsequently the mother cell gives life to two daughter cells.

The entire life of a cell can be concluded in the period from the formation of a daughter cell to its subsequent division in two. This period is called the “cell cycle” in biology.

The very first phase of mitosis is the actual preparation for the The period in which cells endowed with nuclei make direct preparations for division is called interphase. All the most important things happen in it, namely the doubling of the DNA chain and other structures, as well as the synthesis of large amounts of protein. Thus, the chromosomes of the cell become doubled, and each half of such a double chromosome is called a “chromatid”.

After interphase, the division process itself begins - mitosis. It also takes place in several stages. As a result, all doubled parts are stretched symmetrically across the cell, so that after the formation of a central partition in each new cage the same number of formed components remained.

And meiosis is similar, but in the latter (during division there are two divisions, and as a result, not two, but four “daughter” cells are obtained. Also, before the second division, there is no doubling of chromosomes, so their set in the daughter cells remains half.

1. Prophase. In this phase, the centrioles of the cell are very clearly visible. They are present only in animal and human cells. Plants do not have centrioles.
2. Prometaphase. At this moment, prophase ends and metaphase begins.
3. Metaphase. At this moment, the chromosomes lie at the “equator” of the cell.
4. Anaphase. Chromosomes move to different poles.
5. Telophase. One “mother” cell divides by forming a central partition into two “daughter” cells. This is how cell division or mitosis ends.

The most important biological significance of mitosis is the absolutely identical division of the doubled chromosomes into 2 identical parts and their placement in two “daughter” cells. Different types cells and cells of different organisms have varying duration of division - mitosis, but on average it lasts about one and a half hours. There are many factors influencing this very fragile process. Any changing conditions external environment, for example, ambient temperature, light phase regime, pressure in the environment and inside the body and cell, as well as many other factors, can significantly affect both the duration and quality of the cell division process. Also, the duration of the entire mitosis and its individual stages can directly depend on the type of tissue in whose cells it occurs.

The biological significance of mitosis becomes more valuable with each new discovery in the field of cytology, because without this process life on the planet is impossible.

28. Mitosis, its biological significance.

The most important component of the cell cycle is the mitotic (proliferative) cycle. It is a complex of interrelated and coordinated phenomena during cell division, as well as before and after it. Mitotic cycle- this is a set of processes occurring in a cell from one division to the next and ending with the formation of two cells of the next generation. In addition, the concept of the life cycle also includes the period during which the cell performs its functions and periods of rest. At this time, the further cell fate is uncertain: the cell may begin to divide (enters mitosis) or begin to prepare to perform specific functions.

Main stages of mitosis.

1. Reduplication (self-duplication) of the genetic information of the mother cell and its uniform distribution between daughter cells. This is accompanied by changes in the structure and morphology of chromosomes, in which more than 90% of the information of a eukaryotic cell is concentrated.

2. The mitotic cycle consists of four consecutive periods: presynthetic (or postmitotic) G1, synthetic S, postsynthetic (or premitotic) G2 and mitosis itself. They constitute the autocatalytic interphase (preparatory period).

Cell cycle phases:

1) presynthetic (G1). Occurs immediately after cell division. DNA synthesis has not yet occurred. The cell is actively growing in size, storing substances necessary for division: proteins (histones, structural proteins, enzymes), RNA, ATP molecules. Division of mitochondria and chloroplasts (i.e., structures capable of self-reproduction) occurs. The organizational features of the interphase cell are restored after the previous division;

2) synthetic (S). Genetic material is duplicated through DNA replication. It occurs in a semi-conservative manner, when the double helix of the DNA molecule diverges into two chains and a complementary chain is synthesized on each of them.

The result is two identical DNA double helices, each consisting of one new and one old DNA strand. The amount of hereditary material doubles. In addition, the synthesis of RNA and proteins continues. Also, a small part of mitochondrial DNA undergoes replication (the main part of it is replicated in the G2 period);

3) postsynthetic (G2). DNA is no longer synthesized, but the defects made during its synthesis in the S period are corrected (repair). Energy and nutrients are also accumulated, and the synthesis of RNA and proteins (mainly nuclear) continues.

S and G2 are directly related to mitosis, so they are sometimes separated into a separate period - preprophase.

After this, mitosis proper occurs, which consists of four phases. The division process includes several successive phases and is a cycle. Its duration varies and ranges from 10 to 50 hours in most cells. In human body cells, the duration of mitosis itself is 1-1.5 hours, the G2 period of interphase is 2-3 hours, the S period of interphase is 6-10 hours .

Biological significance of mitosis

∙ Mitosis underlies the growth and vegetative reproduction of all organisms that have a nucleus - eukaryotes.

∙ Thanks to mitosis, the constancy of the number of chromosomes is maintained in cell generations, i.e. daughter cells receive the same genetic information that was contained in the nucleus of the mother cell.

∙ Mitosis determines the most important phenomena of life: growth, development and restoration of tissues and organs and asexual reproduction of organisms.

∙ Asexual reproduction, regeneration of lost parts, cell replacement in multicellular organisms

Genetic stability - ensures the stability of the karyotype of somatic cells throughout the life of one generation (i.e., throughout the entire life of the organism.

29. Meiotic division, its features, characteristics of the stages of prophase 1.

The central event of gametogenesis is a special form of cell division - meiosis. Unlike the widespread mitosis, which maintains a constant diploid number of chromosomes in cells, meiosis leads to the formation of haploid gametes from diploid cells. During subsequent fertilization, the gametes form a new generation organism with a diploid karyotype (ps + ps == 2n2c). This is the most important biological significance of meiosis, which arose and became established in the process of evolution in all species that reproduce sexually.

Meiosis consists of two divisions that quickly follow one another, occurring during the period of maturation. DNA doubling for these divisions occurs once during the growth period. The second meiotic division follows the first almost immediately so that the hereditary material is not synthesized in the interval between them (Fig. 5.5).

First meiotic division is called reduction, since it leads to the formation of haploid n2c cells from diploid cells (2n2c). This result is ensured due to the peculiarities of the prophase of the first division of meiosis. In prophase I of meiosis, as well as in ordinary mitosis, compact packaging of genetic material (chromosome spiralization) is observed. At the same time, an event occurs that is absent in mitosis: homologous chromosomes conjugate with each other, i.e. are closely approximated by the corresponding areas.

As a result of conjugation, chromosome pairs, or bivalents, number n are formed. Since each chromosome entering meiosis consists of two chromatids, the bivalent contains four chromatids. The formula of the genetic material in prophase I remains 2n4c. Towards the end of prophase, the chromosomes in bivalents, strongly spiraling, shorten. As in mitosis, in prophase I of meiosis, the formation of the spindle begins, with the help of which chromosomal material will be distributed between daughter cells (Fig. 5.5).

The processes occurring in prophase I of meiosis and determining its results determine the longer duration of this division phase compared to mitosis and make it possible to distinguish several stages within it.

Leptotene is the earliest stage of prophase I of meiosis, in which the spiralization of chromosomes begins, and they become visible under the microscope as long and thin threads.

Zygotene is characterized by the beginning of conjugation of homologous chromosomes, which are united by the synaptonemal complex into a bivalent (Fig. 5.6).

Pachytene is a stage in which, against the background of ongoing spiralization of chromosomes and their shortening, crossing over occurs between homologous chromosomes - crossover with the exchange of corresponding sections.

Diplotene is characterized by the emergence of repulsive forces between homologous chromosomes, which begin to move away from each other primarily in the centromere region, but remain connected in the areas of past crossing over - chiasmachs (Fig. 5.7).

Diakinesis is the final stage of prophase I of meiosis, in which homologous chromosomes are held together only at individual points of the chiasmata. Bivalents take on the bizarre shape of rings, crosses, eights, etc. (Fig. 5.8).

Thus, despite the repulsive forces that arise between homologous chromosomes, the final destruction of bivalents does not occur in prophase I. A feature of meiosis in oogenesis is the presence of a special stage - dictyoten, which is absent in spermatogenesis. At this stage, reached in humans even in embryogenesis, the chromosomes, having taken on a special morphological form of “lamp brushes”, stop any further structural changes for many years. Upon reaching female body reproductive age under the influence of luteinizing hormone of the pituitary gland, as a rule, one oocyte monthly resumes meiosis.

PECULIARITIES

Sexual reproduction of organisms is carried out with the help of specialized cells, the so-called. gametes - oocytes (eggs) and sperm (sperm). Gametes fuse to form one cell - a zygote. Each gamete is haploid, i.e. has one set of chromosomes. Within the set, all the chromosomes are different, but each chromosome of the egg corresponds to one of the chromosomes of the sperm. The zygote, therefore, already contains a pair of chromosomes corresponding to each other, which are called homologous. Homologous chromosomes are similar because they have the same genes or their variants (alleles) that determine specific traits. For example, one of the paired chromosomes may have a gene encoding blood type A, and the other may have a variant encoding blood type B.

The chromosomes of the zygote originating from the egg are maternal, and those originating from the sperm are paternal.

As a result of repeated mitotic divisions, either a multicellular organism or numerous free-living cells arise from the resulting zygote, as occurs in protozoa that have sexual reproduction and in unicellular algae.

During the formation of gametes, the diploid set of chromosomes present in the zygote must be reduced by half. If this did not happen, then in each generation the fusion of gametes would lead to a doubling of the set of chromosomes. Reduction to the haploid number of chromosomes occurs as a result of reduction division - the so-called. meiosis, which is a variant of mitosis.

Cleavage and recombination. The peculiarity of meiosis is that during cell division the equatorial plate is formed by pairs of homologous chromosomes, and not by duplicated individual chromosomes, as in mitosis. Paired chromosomes, each of which remains single, diverge to opposite poles of the cell, the cell divides, and as a result, the daughter cells receive half the set of chromosomes compared to the zygote.

For example, assume that the haploid set consists of two chromosomes. In the zygote (and accordingly in all cells of the organism that produces gametes) maternal chromosomes A and B and paternal chromosomes A" and B" are present. During meiosis they can divide as follows:

The most important thing in this example is the fact that when chromosomes diverge, the original maternal and paternal set is not necessarily formed, but recombination of genes is possible,

Now suppose that the pair of chromosomes AA" contains two alleles - a and b - of the gene that determines blood groups A and B. Similarly, the pair of chromosomes BB" contains alleles m and n of another gene that determines blood groups M and N. The separation of these alleles can proceed as follows : Obviously, the resulting gametes can contain any of the following combinations of alleles of the two genes: am , bn , bm or an .

If there are more chromosomes, then pairs of alleles will segregate independently according to the same principle. This means that the same zygotes can produce gametes with different combinations of gene alleles and give rise to different genotypes in the offspring.

Meiotic division. Both examples illustrate the principle of meiosis. In fact, meiosis is much more difficult process, since it includes two consecutive divisions. The main thing in meiosis is that chromosomes are doubled only once, while the cell divides twice, as a result of which the number of chromosomes is reduced and the diploid set turns into a haploid one.

During the prophase of the first division, homologous chromosomes conjugate, that is, they come together in pairs. As a result of this, very precise process each gene appears opposite its homologue on another chromosome. Both chromosomes then double, but the chromatids remain connected to each other by a common centromere. In metaphase, the four connected chromatids line up to form the equatorial plate, as if they were one duplicated chromosome. Contrary to what happens in mitosis, centromeres do not divide. As a result, each daughter cell receives a pair of chromatids still connected by the centromere. During the second division, the chromosomes, already individual, line up again, forming, as in mitosis, an equatorial plate, but their doubling does not occur during this division. The centromeres then divide and each daughter cell receives one chromatid.

Cytoplasmic division. As a result of two meiotic divisions of a diploid cell, four cells are formed. When male reproductive cells are formed, four sperm of approximately the same size are obtained. When eggs are formed, the division of the cytoplasm occurs very unevenly: one cell remains large, while the other three are so small that they are almost entirely occupied by the nucleus. These small cells, the so-called. polar bodies serve only to accommodate excess chromosomes formed as a result of meiosis. The bulk of the cytoplasm necessary for the zygote remains in one cell - the egg.

Conjugation and crossing over. During conjugation, the chromatids of homologous chromosomes can break and then join in a new order, exchanging sections as follows:

This exchange of sections of homologous chromosomes is called crossing over. As shown above, crossing over leads to the emergence of new combinations of alleles of linked genes. So, if the original chromosomes had the combinations AB and ab, then after crossing over they will contain Ab and aB. This mechanism for the emergence of new gene combinations complements the effect of independent chromosome sorting that occurs during meiosis.

The difference is that crossing over separates genes on the same chromosome, whereas independent sorting separates only genes on different chromosomes.

30. Mutations of the hereditary apparatus. Their classification. Factors causing mutations of the hereditary apparatus

Factors causing mutations can be a wide variety of environmental influences: temperature, ultraviolet radiation, radiation (both natural and artificial), the effects of various chemical compounds- mutagens.

Mutagens are agents of the external environment that cause certain changes in the genotype - mutation, and the process of formation of mutations is called mutagenesis.

Radiation mutagenesis started practicing in the 20s of the last century. In 1925, Soviet scientists G.S. Filippov and G.A. Nadson for the first time in the history of genetics used X-rays to produce mutations in yeast. A year later, the American researcher G. Meller (later twice a laureate Nobel Prize), long time working in Moscow, at the institute headed by N.K. Koltsov, used the same mutagen on Drosophila. It was found that a radiation dose of 10 rad doubles the frequency of mutations in humans. Radiation can induce mutations leading to hereditary diseases and cancer.

Chemical mutagenesis For the first time, N.K. Koltsov’s collaborator V.V. Sakharov began to purposefully study it in 1931 on Drosophila when its eggs were exposed to iodine, and later M.E. Lobashov.

Chemical mutagens include a wide variety of substances (hydrogen peroxide, aldehydes, ketones, nitric acid and its analogues, salts heavy metals, aromatic substances, insecticides, herbicides, drugs, alcohol, nicotine, some medicinal substances and many others. From 5 to 10% of these compounds have mutagenic activity (capable of disrupting the structure or functioning of the hereditary material).

Genetically active factors can be divided into 3 categories: physical, chemical and biological.

Physical factors. These include various types of ionizing radiation and ultraviolet radiation. A study of the effect of radiation on the mutation process showed that in this case there is no threshold dose, and even the smallest doses increase the likelihood of mutations occurring in the population. An increase in the frequency of mutations is dangerous not so much in an individual sense, but from the point of view of increasing the genetic load of the population.

For example, irradiation of one of the spouses with a dose within the range of doubling the frequency of mutations (1.0 - 1.5 Gy) slightly increases the risk of having a sick child (from a level of 4 - 5% to a level of 5 - 6%). If the population of an entire region receives the same dose, then the number hereditary diseases the population will double in a generation.

Chemical factors. Chemicalization Agriculture and other areas human activity, the development of the chemical industry led to the synthesis of a huge flow of substances, including those that had never existed in the biosphere for millions of years of previous evolution. This means, first of all, indecomposability and long-term preservation. foreign substances entering the environment. What was initially taken as an achievement in the fight against pests later turned into a complex problem. The widespread use of the insecticide DDT in the 40s - 60s of the last century led to its spread throughout the globe, right up to the ice of Antarctica.

Most pesticides are highly resistant to chemical and biological degradation and have high level toxicity.

Biological factors. Along with physical and chemical mutagens, some factors of biological nature also have genetic activity. The mechanisms of the mutagenic effect of these factors have been studied in the least detail. At the end of the 30s, S. and M. Gershenzon began research on mutagenesis in Drosophila under the influence of exogenous DNA and viruses. Since then, the mutagenic effect of many viral infections and for humans.

Chromosome aberrations in somatic cells are caused by smallpox, measles, chickenpox, mumps, influenza, hepatitis, etc.

Classification of mutations

The classification of mutations was proposed in 1932 by G. Meller. Highlight:

- hypomorphic mutations - the manifestation of a trait controlled by a pathological gene is weakened compared to a trait controlled by a normal gene (synthesis of pigments).

- amorphous mutations- a trait controlled by a pathological gene does not appear, since the pathological gene is not active compared to the normal gene (albinism gene).

Hypomorphic and amorphous mutations underlie diseases inherited in a recessive manner.

Antimorphic mutations- the value of a trait controlled by a pathological gene is opposite to the value of a trait controlled by a normal gene (dominantly inherited traits and diseases).

- neomorphic mutations- the value of the trait controlled by the pathological gene is opposite to the value of the gene controlled by the normal gene (synthesis in the body of new antibodies to the penetration of the antigen).

- hypermorphic mutations- a trait controlled by a pathological gene is more pronounced than a trait controlled by a normal gene (Fanconi anemia).

Modern classification of mutations includes:

- gene or point mutations. This is a change in one gene (any point), leading to the appearance of new alleles. Point mutations are inherited as simple Mendelean traits, such as, for example, Huntington's chorea, hemophilia, etc. ( example s-m Martina - Bel, cystic fibrosis)

- chromosomal mutations- disrupt the structure of the chromosome (gene linkage group) and lead to the formation of new linkage groups. These are structural rearrangements of chromosomes as a result of deletion, duplication, translocation (movement), inversion or insertion of hereditary material (example with Down's, sm cat scream)

- genomic mutations lead to the emergence of new genomes or parts thereof through the addition or loss of entire chromosomes. Another name for them is numerical (numerical) mutations of chromosomes as a result of a violation of the amount of genetic material. (example from Shereshevsky - Turner, from Klinefelter).

31. Factors of mutagenesis of the hereditary apparatus.

Mutations are divided into spontaneous and induced. Spontaneous mutations are those that arose under the influence of unknown to us natural factors. Induced mutations are caused by special targeted effects.

Factors capable of inducing a mutation effect are called mutagenic. The main mutagenic factors are: 1) chemical compounds, 2) various types of radiation.

Chemical Mutagenesis

IN 1934 M.E. Lobashev noted that chemical mutagens must have 3 qualities:

1) high penetrating ability,

2) the ability to change the colloidal state of chromosomes, 3) a certain effect on changing a gene or chromosome.

Many chemical substances have a mutagenic effect. A number of chemical substances have even more powerful action than physical factors. They are called supermutagens.

Chemical mutagens are used to produce mutant forms of molds, actinomycetes, and bacteria that produce hundreds of times more penicillin, streptomycin and other antibiotics.

It was possible to increase the fermentative activity of fungi used for alcoholic fermentation. Soviet researchers have obtained dozens of promising mutations in various varieties of wheat, corn, sunflower and other plants.

In experiments, mutations are induced by a variety of chemical agents. This fact indicates that, apparently, even in natural conditions similar factors also cause the appearance of spontaneous mutations in various organisms, including in humans. The mutagenic role of various chemical substances and even some medicines. This indicates the need to study the mutagenic effect of new pharmacological substances, pesticides and other chemical compounds increasingly used in medicine and agriculture.

Radiation mutagenesis Induced mutations caused by radiation were first obtained by Soviet scientists

G.A. Nadson and G.S. Filippov, who in 1925 observed a mutation effect in yeast after exposure to radium rays. In 1927, the American geneticist G. Meller showed that X-rays can cause many mutations in Drosophila, and later the mutagenic effect of X-rays was confirmed on many objects. Later it was found that hereditary changes are also caused by all other types of penetrating radiation. To obtain artificial mutations, gamma rays are often used, the source of which in laboratories is usually radioactive cobalt Co60. IN Lately Neutrons, which have high penetrating power, are increasingly being used to induce mutations. In this case, both chromosome breaks and point mutations occur. The study of mutations associated with the action of neutrons and gamma rays is of particular interest for two reasons. Firstly, it has been established that the genetic consequences atomic explosions associated primarily with the mutagenic effect of ionizing radiation. Secondly, physical methods of mutagenesis are used to obtain economically valuable varieties of cultivated plants. Thus, Soviet researchers, using methods of exposure to physical factors, obtained varieties of wheat and barley that were resistant to a number of fungal diseases and more productive.

Irradiation indicates both gene mutations and structural chromosomal rearrangements of all types described above: deficiency, inversion, duplication and translocation, i.e. all structural changes associated with chromosome breakage. The reason for this is some features of the processes occurring in tissues under the influence of radiation. Radiation causes ionization in tissues, as a result of which some atoms lose electrons, while others gain them: positively or negatively charged ions are formed. A similar process of intramolecular rearrangement, if it occurred in chromosomes, can cause their fragmentation. Radiation energy can cause chemical changes in the environment surrounding a chromosome, which lead to the induction of gene mutations and structural rearrangements in chromosomes.

Mutations can also be induced by post-radiation chemical changes that have occurred in the environment. One of the most dangerous consequences irradiation is the formation of free radicals OH or HO2 from water in the tissues.

Other mutagenic factors The first researchers of the mutation process underestimated the role of environmental factors in

phenomena of variability. Some researchers at the beginning of the twentieth century even believed that external influences had no significance for the mutation process. But later these ideas were refuted thanks to the artificial production of mutations using various factors external environment. At present, it can be assumed that, apparently, there are no environmental factors that would not, to some extent, affect changes in hereditary properties. From physical factors The mutagenic effect of ultraviolet rays, photons of light and temperature has been established on a number of objects. Increasing temperatures increase the number of mutations. But temperature is one of those agents in relation to which organisms have defense mechanisms. Therefore, the disturbance of homeostasis turns out to be insignificant. As a result, temperature effects have a slight mutagenic effect compared to other agents.

32. Inclusions in eukaryotic cells, their types, purpose.

Inclusions are relatively unstable components of the cytoplasm that serve as reserve nutrients(fat, glycogen), cytoplasms, which serve as reserve nutrients (fat, glycogen), products to be removed from the cell (secretion granules), ballast substances (some pigments).

Inclusions are waste products of cells. They can be dense particles-granules, liquid droplets-vacuoles, as well as crystals. Some vacuoles and granules are surrounded by membranes. Depending on the functions performed, inclusions are conventionally divided into three groups: trophic, secretory and special. Inclusions of trophic significance - droplets of fat, starch granules. glycogen, protein. They are present in small quantities in all cells and are used in the assimilation process. But in some special cells they accumulate in large quantities. Thus, there are many starch grains in the cells of potato tubers, and glycogen granules in the liver cells. The quantitative content of these inclusions varies depending on physiological state cells and the whole organism. In a hungry animal, liver cells contain significantly less glycogen than in a well-fed one. Inclusions of secretory significance are formed mainly in gland cells and are intended for release from the cell. The number of these inclusions in the cell also depends on the physiological state of the body. Thus, the cells of the pancreas of a hungry animal are rich in droplets of secretion. but if they are well-fed, they are poor in them. Inclusions of special significance are found in the cytoplasm of highly differentiated cells. performing a specialized function. An example of them is hemoglobin, diffusely scattered in erythrocytes.

33. Variability, its types in human populations Variability is a property that is the opposite of heredity, associated with the appearance of characteristics that differ from the typical ones. If during reproduction only the

continuity of previously existing properties and characteristics, then the evolution of the organic world would be impossible, but variability is characteristic of living nature. First of all, it is associated with “errors” during reproduction. Differently constructed molecules nucleic acid carry new hereditary information. This new, changed information in most cases is harmful to the body, but in some cases, as a result of variability, the body acquires new properties that are useful under given conditions. New characteristics are picked up and fixed by selection. This is how new forms, new species are created. Thus, hereditary variability creates the prerequisites for speciation and evolution, and thereby the existence of life.

A distinction is made between non-hereditary and hereditary variability. The first of them is associated with a change in phenotype, the second - genotype. Non-hereditary variability Darwin called it definite; it is usually called modification, or phenotypic, variability. Hereditary variation, as defined by Darwin, is indeterminate (“genotypic variation”).

PHENOTYPIC (MODIFICATION) AND GENOTYPIC VARIATION Phenotypic variability Modifications are phenotypic changes that occur under the influence of conditions

environment. The range of modification variability is limited by the reaction norm. The developed specific modification change in a trait is not inherited, but the range of modification variability is determined by heredity. Modification changes do not entail changes in the genotype and correspond to living conditions and are adaptive.

Genotypic, or non-hereditary, is divided into combinative and mutational.

Combinative variability

Combinative variability is associated with the production of new combinations of genes in the genotype. This is achieved as a result of 2 processes: 1) chromosome divergence during meiosis and their random combination during fertilization, 2) gene recombination due to crossing over; the hereditary factors (genes) themselves do not change, but new combinations of them lead to the appearance of organisms with a new phenotype.

Mutational variability

A mutation is a change caused by a reorganization of the reproductive structures of a cell, a change in its genetic apparatus. These mutations differ sharply from modifications that do not affect the genotype of the individual. Mutations occur suddenly, spasmodically and sometimes sharply distinguish the organism from its original form. Mutational variability is characteristic of all organisms; it supplies material for selection; evolution, the process of formation of new species, varieties and breeds, is associated with it. Based on the nature of changes in the genetic apparatus, mutations are distinguished due to:

1) change in the number of chromosomes (polyploidy, heteroploidy, haploidy);

2) changes in chromosome structure (chromosomal aberrations);

3) changes in the molecular structure of a gene.

Polyploidy and heteroploidy (aneuploidy).

Polyploidy is an increase in the diploid number of chromosomes by adding (gene or point mutations) entire chromosome sets. Sex lettuces have a haploid set of chromosomes (n), and zygotes and all somatic cells are characterized by a diploid set (2n). In polyploid forms, there is an increase in the number of chromosomes, a multiple of the haploid set: 3n - triploid, 4n - tetroploid, etc.

Heteroploidy is a change in the number of chromosomes that is not a multiple of the haploid set. A diploid set may have only 1 chromosome more than normal, i.e. 2n+1 chromosome. Such forms are called trisomics. The opposite of trisomy, i.e. the loss of one chromosome from a pair in a diploid set is called monosomy, the organism is called monosomic. Monosomics, as a rule, have reduced viability or are completely nonviable.

The phenomenon of aneuploidy shows that a violation of the normal number of chromosomes leads to changes in the structure and a decrease in the viability of the organism.

Darwin's doctrine of variability.

He saw the reason for variability in the influence environment. He distinguished between definite and indefinite variability. A certain variability appears in individuals that have been subjected to some specific, in some cases more or less easily detectable, influence. This form of variability is called modification. Uncertain variability (these are mutations) manifests itself in certain individuals and occurs in a variety of directions. While studying the manifestation of variability, Darwin discovered the relationship between changes in various organs and their systems in the body. This variability is called correlative, or correlative. It lies in the fact that a change in any organ always or almost always entails a change in other organs or their functions. Correlative variability is based on the pleiotropic action of genes.

Variation introduces diversity into organisms, and heredity transmits these changes to descendants.