The biological meaning of mitosis is that. What is the biological significance of mitosis

The life cycle of a cell. Division of pro- and eukaryotic cells

The most important position of the cell theory says that new cells are formed by breeding(self-reproduction) previous, carried out thanks to division original cell. Thus, a huge number of cells that make up a multicellular organism arise as a result of successive divisions starting from a single zygote (fertilized egg).

When cells reproduce, hereditary information is preserved in a number of their generations (and organisms during asexual reproduction). This is ensured primarily by doubling the genetic material, since DNA molecules- going her replication. The duplicated DNA is then distributed equally (during cell division) between the two daughter cells.

Division of a prokaryotic cell. Self-reproduction of a prokaryotic cell is carried out by simple division(see fig. 1).

Rice. one. D division bacterial cell. DNA duplicates and diverges into two daughter cells

This process begins with DNA replication. Two daughter molecules of the latter are attached to the plasmalemma, and the membrane begins to grow rapidly between the points of their attachment. As a result, mechanical “pulling apart” of two daughter DNA molecules to the opposite poles of the cell occurs. After that, the mother cell is divided into two daughter cells with the same DNA molecules.

division of eukaryotic cells. Their genetic information is contained in the chromosomes of the nucleus, the number of which can reach a significant number, and therefore a uniform and accurate distribution of chromosomes that have doubled before this between daughter cells is provided by a special apparatus - spindle fission. It consists of filaments formed by microtubules. A special organelle is involved in the formation of the spindle of division - cell center.

The main mode of division of eukaryotic cells is mitosis. Sex cells are formed as a result meiosis.

Two periods can be distinguished in the life of a cell: its division is preceded by interphase, and then the actual mitosis. During the interphase period, the cell grows, the number of organelles increases in it, it reaches maturity and prepares for division - DNA replication (doubling) is carried out.

The set of stages through which a cell passes from the moment of its origin (in the process of division of the original, maternal) to its own division with the formation of two new cells is called life (cell) cycle. The life cycle of a cell reflects all the regular structural and functional changes that occur with the cell over time. Thus, the life cycle of a cell is the time of a cell's existence from the moment of its formation by dividing the mother cell to its own division or natural death.

Cells complex organism(for example, human) the life cycle of a cell can be different. highly specialized cells (erythrocytes, nerve cells, striated muscle cells) do not multiply. Their life cycle consists of birth, performance of intended functions, death.

Mitosis, its phases, biological significance

Main stages of mitosis.

1.Reduplication (self-doubling) of the genetic information of the mother cell and its uniform distribution between the 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 successive periods: presynthetic G1, synthetic S, postsynthetic G2, and mitosis proper. They constitute the autocatalytic interphase (preparatory period).

Phases cell cycle:

interphase:

Immediately after cell division, DNA synthesis does not yet occur. The cell actively grows in size, stores the substances necessary for division: proteins, RNA, ATP molecules. Mitochondria and chloroplasts divide. The features of the organization of the interphase cell are restored after the previous division;

Then the genetic material is duplicated by DNA replication when the double helix of the DNA molecule diverges into two strands and a complementary strand is synthesized on each of them.

As a result, two identical DNA double helixes are formed, each of which consists of one new and one old DNA strand. The amount of hereditary material is doubled. In addition, the synthesis of RNA and proteins continues.

mitosis

This is followed by mitosis itself, 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.

stages of mitosis.

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

prophase, metaphase, anaphase and telophase(Fig. 1–3). Since it is continuous, the phase change is carried out smoothly - one imperceptibly passes into another.

in prophase the volume of the nucleus increases, and due to the spiralization of chromatin, chromosomes are formed. By the end of prophase, each chromosome is seen to consist of two chromatids. Gradually, the nucleoli and nuclear membrane dissolve, and the chromosomes are randomly located in the cytoplasm of the cell. The centrioles move towards the poles of the cell. A spindle of division is formed, part of the threads of which goes from pole to pole, and part is attached to the centromeres of chromosomes. The content of genetic material in the cell remains unchanged (2n4c).

Rice. one.Diagram of mitosis in onion root cells

Rice. 2.Scheme of mitosis in onion root cells: 1 - interphase; 2,3 - prophase; 4 - metaphase; 5.6 - anaphase; 7.8 - telophase; 9 - formation of two cells

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

In metaphase chromosomes reach maximum spiralization and are arranged in an orderly manner at the equator of the cell, so their counting and study is carried out during this period. The content of genetic material does not change (2n4c).

in anaphase each chromosome "splits" into two chromatids, which from then on are called daughter chromosomes. The spindle fibers attached to the centromeres contract and pull the chromatids (daughter chromosomes) to 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 chromosomes located at the poles despiralize and become poorly visible. Around the chromosomes at each pole, a nuclear membrane forms from the membrane structures of the cytoplasm, and nucleoli form in the nuclei. The spindle of division 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).

Atypical forms of mitosis

To atypical forms mitosis include amitosis, endomitosis, polythenia.

1. Amitosis is a direct division of the nucleus. At the same time, the morphology of the nucleus is preserved, the nucleolus and the nuclear membrane are visible. Chromosomes are not visible, and their uniform distribution does not occur. The nucleus is divided into two relatively equal parts without the formation of a mitotic apparatus (a system of microtubules, centrioles, structured chromosomes). If division ends at the same time, a binuclear cell appears. But sometimes the cytoplasm is also laced.

This type of division exists in some differentiated tissues (in cells of skeletal muscles, skin, connective tissue), as well as in pathologically altered tissues. Amitosis never occurs in cells that need to preserve full genetic information - fertilized eggs, cells of a normally developing embryo. This method of division cannot be considered a full-fledged way of reproduction of eukaryotic cells.

2. Endomitosis. In this type of division, after DNA replication, chromosomes do not separate into two daughter chromatids. This leads to an increase in the number of chromosomes in a cell, sometimes by tens of times in comparison with the diploid set. This is how polyploid cells are formed. Normally, this process takes place in intensively functioning tissues, for example, in the liver, where polyploid cells are very common. However, from a genetic point of view, endomitosis is a genomic somatic mutation.

3. Polythenia. There is a multiple increase in the content of DNA (chromonemes) in the chromosomes without an increase in the content of the chromosomes themselves. At the same time, the number of chromonemes can reach 1000 or more, while the chromosomes become gigantic. During polythenia, all phases of the mitotic cycle fall out, except for the reproduction of primary DNA strands. This type of division is observed in some highly specialized tissues (liver cells, cells of the salivary glands of Diptera). The polylithic chromosomes of Drosophila are used to construct cytological maps of genes in chromosomes.

The biological significance of mitosis.

It consists in the fact that mitosis ensures the hereditary transmission of traits and properties in a number of generations of cells during the development of a multicellular organism. Due to the exact and uniform distribution of chromosomes during mitosis, all cells of a single organism are genetically the same.

Mitotic division cells underlie all forms asexual reproduction in both unicellular and multicellular organisms. Mitosis causes the most important phenomena of vital activity: growth, development and restoration of tissues and organs and asexual reproduction of organisms.

Cell cycle. Mitosis

One of the most important properties of life is the self-reproduction of biological systems, which is based on cell division: “Not only the phenomena of heredity, but also the very continuity of life depend on cell division” (E. Wilson). The universal way of dividing eukaryotic cells is indirect division, or mitosis (from the ancient Greek "mitos" - a thread). The biological significance of mitosis lies in the preservation of the volume and quality of hereditary information.

A Brief History of the Discovery of Mitosis

For the first time, cell division (crushing of frog eggs) was observed by the French scientists Prevost and Dumas (1824). This process was described in more detail by the Italian embryologist M. Rusconi (1826). The process of nuclear fission during the crushing of eggs in sea urchins was described by K. Baer (1845). The first description of cell division in algae was made by B. Dumortier (1832). Separate phases of mitosis were observed by the German botanist W. Hofmeister (1849; cells of the filament of tradescantia), Russian botanists E. Russov (1872; mother cells of spores of ferns, horsetails, lilies) and I.D. Chistyakov (1874; spores of horsetail and club moss), German zoologist A. Schneider (1873; crushing eggs of flatworms), Polish botanist E. Strasburger (1875; spirogyra, club moss, onion).

To indicate the processes of movement constituent parts nucleus, the German histologist W. Schleichner proposed the term karyokinesis (1879), and the German histologist W. Flemming introduced the term mitosis (1878). In the 1880s The general morphology of chromosomes was described in the works of Hofmeister, but only in 1888 did the German histologist W. Waldeyer introduce the term chromosome. The leading role of chromosomes in the storage, reproduction and transmission of hereditary information was proved only in the twentieth century.

biological significance

The process of mitosis ensures a strictly uniform distribution of chromosomes between two daughter nuclei, so that in a multicellular organism all cells have exactly the same (in number and character) sets of chromosomes. Chromosomes contain genetic information encoded in DNA, and therefore a regular, ordered mitotic process also ensures the complete transfer of all information to each of the daughter nuclei; as a result, each cell has all the genetic information necessary for the development of all the characteristics of the organism. In this regard, it becomes clear why one cell taken from a fully differentiated adult plant can suitable conditions develop into a whole plant. We have described mitosis in a diploid cell, but this process proceeds in a similar way in haploid cells, for example, in cells of the gametophyte generation of plants.

Those. The biological significance of mitosis lies in the fact that mitosis ensures the hereditary transmission of traits and properties in a number of cell generations during the development of a multicellular organism. Due to the exact and uniform distribution of chromosomes during mitosis, all cells of a single organism are genetically the same.

Mitotic cell division underlies all forms of asexual reproduction in both unicellular and multicellular organisms. Mitosis causes the most important phenomena of vital activity: growth, development and restoration of tissues and organs and asexual reproduction of organisms.

It consists in the fact that mitosis ensures the hereditary transmission of traits and properties in a number of generations of cells during the development of a multicellular organism. Due to the exact and uniform distribution of chromosomes during mitosis, all cells of a single organism are genetically the same.

Mitotic cell division underlies all forms of asexual reproduction in both unicellular and multicellular organisms. Mitosis causes the most important phenomena of vital activity: growth, development and restoration of tissues and organs and asexual reproduction of organisms.

Certain structural transformations take place in each phase.

Prophase characterized by morphological changes in the nucleus and cytoplasm. In the nucleus, condensation of chromatin and the formation of chromosomes consisting of two chromatids, the disappearance of the nucleolus, the disintegration of the karyolemma into separate vesicles. In the cytoplasm there is reduplication(doubling) of centrioles and their divergence to opposite poles of the cell, the formation of a fission spindle from microtubules, reproduction of the granular endoplasmic reticulum, as well as a decrease in the number of free and attached ribosomes.

occurs in metaphase the formation of a metaphase plate, or parent star, incomplete separation of sister chromatids from each other.

Anaphase is characterized complete isolation (divergence) of chromatids and the formation of two equivalent diploid sets of chromosomes, the divergence of chromosome sets to the poles of the mitotic spindle and the divergence of the poles themselves.

Telophase is characterized decondensation of the chromosomes of each chromosome set, the formation of a nuclear envelope from vesicles, cytotomy by constriction of a binuclear cell into two daughter independent cells, the appearance of a nucleolus in the nuclei of daughter cells.

In prophase, centrioles are clearly visible - formations located in the cell center and playing a role in the division of the daughter chromosomes of animals. (Recall that higher plants do not have centrioles in the cell center that organizes the division of chromosomes.) We will consider mitosis using the example of an animal cell, since the presence of centrioles makes the process of cell division more obvious. Centrioles divide and diverge to different poles of the cell. Microtubules extend from the centrioles, forming the fission spindle, which regulates the divergence of chromosomes to the poles of the dividing cell.

The morphology of mitotic chromosomes is best studied at the moment of their greatest condensation, in metaphase and at the beginning of anaphase. Chromosomes in this state are rod-shaped structures of varying length with a fairly constant thickness. Most chromosomes can easily find the zone primary constriction(centromere), which divides the chromosome into two arms (Fig. 22). Chromosomes with equal or almost equal arms are called metacentric, those with arms of unequal length are called submetacentric. Rod-shaped chromosomes with a very short, almost imperceptible second arm are called acrocentric.

In the region of the primary constriction is located kinetochore. Microtubules of the cell spindle depart from this zone during mitosis, associated with the movement of chromosomes during cell division. Some chromosomes also have secondary constrictions, located near one of the ends of the chromosome and separating a small area - a satellite of the chromosome. Secondary constrictions are also called nucleolar organizers(see the previous lecture), since it is on these parts of the chromosomes that the nucleolus is formed in interphase. In these places, the DNA responsible for the synthesis of ribosomal RNA is localized.

Arms of chromosomes end telomeres- end sections. Telomeric regions of chromosomes are characterized by a lack of ability to connect with other chromosomes or their fragments and perform a protective function. With each cycle of cell division, the cell's telomere shortens, due to the inability of DNA polymerase to synthesize a copy of the DNA from the very end. This phenomenon is called terminal underreplication and is one of the most important factors of biological aging. Special Enzyme telomerase using its own RNA template, it completes telomeric repeats and lengthens telomeres. In most differentiated cells, telomerase is blocked, but is active in stem and germ cells.

Meiosis

The central event of gametogenesis is a special form of cell division - meiosis. Unlike 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, gametes form an organism of a new generation with a diploid karyotype ( ps + ps == 2n 2c). This is the most important biological significance of meiosis, which arose and became fixed in the process of evolution in all species that reproduce sexually (see Section 3.6.2.2).

Meiosis consists of two rapidly following one after the other divisions that occur during the maturation period. DNA doubling for these divisions is carried out once during the growth period. The second division of meiosis follows the first almost immediately so that the hereditary material is not synthesized in the interval between them (Fig. 5.5).

The first meiotic division is called reductional. since it leads to the formation of diploid cells (2 P 2With) haploid cells P 2With. This result is ensured due to the features of the prophase of the first division of meiosis. In prophase I of meiosis, as in ordinary mitosis, a compact packing 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. closely related areas.

As a result of conjugation, chromosome pairs are formed, or bivalents, number P. Since each chromosome entering meiosis consists of two chromatids, the bivalent contains four chromatids. Formula of genetic material in prophase I remains 2 n 4c. By the end of the prophase, the chromosomes in bivalents, strongly spiraling, are shortened. Just as in mitosis, in prophase I of meiosis, the formation of a division spindle begins, with the help of which the chromosomal material will be distributed between daughter cells (Fig. 5.5).

Rice. 5.5. meiosis stages

Paternal chromosomes are shown in black, maternal chromosomes are unstained. The figure does not show metaphase I, in which the bivalents are located in the plane of the fission spindle equator, and telophase I, which quickly passes into prophase II

The processes occurring in the prophase I of meiosis and determining its results cause a longer course of this phase of division compared to mitosis and make it possible to distinguish several stages within it (Fig. 5.5).

Leptotena - the earliest stage of prophase I of meiosis, in which the spiralization of chromosomes begins, and they become visible under a microscope as long and thin threads. Zygoten characterized by the beginning of the conjugation of homologous chromosomes, which are combined by the synaptonemal complex into a bivalent (Fig. 5.6). Pachytene - the stage in which, against the background of the ongoing spiralization of chromosomes and their shortening, between homologous chromosomes, crossing over - cross with the exchange of the corresponding sections. Diploten characterized by the emergence of repulsive forces between homologous chromosomes, which begin to move away from each other primarily in the region of centromeres, but remain connected in the regions of the past crossing over - chiasmus(Fig. 5.7).

Diakinesis - the final stage of prophase I of meiosis, in which homologous chromosomes are held together only at separate points in the chiasm. 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, absent in spermatogenesis. At this stage, which is reached in humans even in embryogenesis, the chromosomes, having adopted a special morphological form"lampbrushes", stop any further structural changes for many years. Upon reaching the female body reproductive age Under the influence of the pituitary luteinizing hormone, as a rule, one oocyte renews meiosis every month.

AT metaphase I meiosis completes the formation of the fission spindle. Its filaments are attached to the centromeres of chromosomes united in bivalents in such a way that only one filament goes from each centromere to one of the spindle poles. As a result, the threads associated with the centromeres of homologous chromosomes, heading to different poles, establish bivalent in the plane of the equator of the division spindle.

AT anaphase I Meiosis weakens the bonds between homologous chromosomes in bivalents and they move away from each other, heading to different poles of the division spindle. In this case, a haploid set of chromosomes consisting of two chromatids departs to each pole (see Fig. 5.5).

AT telophase In meiosis I, a single, haploid set of chromosomes is assembled at the poles of the spindle, each of them contains twice the amount of DNA.

The formula of the genetic material of the resulting daughter cells corresponds to P 2With.

Second meiotic (equational) division leads to the formation of cells in which the content of genetic material in the chromosomes will correspond to their single-stranded structure ps(see figure 5.5). This division proceeds like mitosis, only the cells entering into it carry a haploid set of chromosomes. In the process of such division, the maternal double-stranded chromosomes, splitting, form single-stranded daughter ones.

One of the main tasks of meiosis is creation of cells with a haploid set of single-stranded chromosomes - is achieved due to a single DNA replication for two consecutive meiotic divisions, as well as due to the formation of pairs of homologous chromosomes at the beginning of the first meiotic division and their further divergence into daughter cells.

The processes occurring in the reduction division also provide an equally important consequence - genetic diversity of gametes, formed by the body. Such processes include crossing over, segregation of homologous chromosomes into different gametes and independent behavior of bivalents in the first meiotic division(See section 3.6.2.3).

Crossing over provides a recombination of paternal and maternal alleles in linkage groups (see Fig. 3.72). Due to the fact that chromosome crossing can occur in different areas, crossing over in each individual case leads to the exchange of genetic material of different amounts. It is also necessary to note the possibility of the occurrence of several crossings between two chromatids (Fig. 5.9) and the participation of more than two bivalent chromatids in the exchange (Fig. 5.10). The noted features of crossing over make this process an effective mechanism for recombination of alleles.

Segregation of homologous chromosomes into different gametes in the case of heterozygosity, it leads to the formation of gametes that differ in the alleles of individual genes (see Fig. 3.74).

Random arrangement of bivalents in the plane of the fission spindle equator and their subsequent divergence in anaphase I meiosis provide recombination of parental linkage groups in the haploid set of gametes (see Fig. 3.75).

Answer 25! Tissues as systems of cells and their derivatives are one of the hierarchical levels of organization of the living. Cells as the leading elements of tissue. Non-cellular structures - symplasts and intercellular substance as derivatives of cells. Syncytia. The concept of cell populations. Differentons. Patterns of the origin and evolution of tissues, the theory of parallelism by A.A. Zavarzin and divergent evolution by N.G. Khlopin, their synthesis at the present level of development of science.

Tissue is a historically (phylogenetically) established system of cells and non-cellular structures that has a common structure, and sometimes origin, and is specialized in performing certain functions. Tissue is a new (after cells) level of organization of living matter.

Structural components of tissue: cells, cell derivatives, intercellular substance.

Date added: 2015-05-19 | Views: 690 | Copyright infringement

Each somatic cell goes through a specific life cycle, including division into two somatic cells. This division - mitosis - occurs according to a certain order, the biological meaning of which is that each of the daughter cells receives exactly the same double set of chromosomes as the parent cell. Mitosis does not introduce any changes in hereditary information, and both daughter cells are identical to the parent cell. Before mitosis begins, the cell's DNA doubles. Each chromosome now consists of two identical chromatids, which will become the chromosomes of the daughter cells. Mitosis consists of four consecutive phases - prophase, metaphase, telophase. In prophase, the chromosomes become clearly visible (when stained). The nuclear membrane disintegrates, the chromosomes are freely located in the cytoplasm of the cell. In the cells of animals and lower plants, the centrioles of the cell. In the cells of animals and lower plants, centrioles (organelles that control cell division) diverge towards the poles of the cell. Spindle threads extend from the centrioles between the poles, ensuring the divergence of the chromosomes to the poles. In metaphase, chromosomes are located along the equator of the cell. In anaphase, cretin filaments begin to pull the chromatids of each chromosome to opposite poles of the cell. The separated chromatids (now called chromosomes) gather at the poles. In the telophase, which completes mitosis, the chromosomes that have diverged to the poles again become poorly visible. The threads of the spindle are destroyed. Two new nuclei form around the chromosomes. A constriction appears in the middle of the cell, which divides the cell in half, into two new cells. Mitosis continues for a relatively short time - usually from half an hour to three hours.

2. Due to what is the diploid set of chromosomes preserved in generations during asexual reproduction?

During asexual reproduction, a diploid set of chromosomes is preserved, asexual reproduction occurs without the formation of gametes (sex cells), and only one individual participates in it, which divides, buds or forms spores.

3. Histo- and orthogonesis

Organogenesis (from organ and ... genesis) in animals is the formation and development of organs. There are ontogenetic organogenesis, studied by embryology and developmental biology, and phylogenetic organogenesis, studied by comparative anatomy. In addition to describing and analyzing the course of organogenesis processes, the task of these disciplines includes the disclosure and causal explanation of these processes in phylogenesis and ontogenesis. Comparative anatomy considers the emergence of new organs, their transformation, division, progressive development and reduction, rudimentation processes, etc. The study of the development of the form of organs in connection with their function has led to the discovery of the main patterns of phylogenetic organogenesis. These are the principles of differentiation and integration, as well as the change of functions as a guiding principle in the phylogenetic transformation of organs. Ontogenetic organogenesis to a certain extent repeats phylogenetic organogenesis. During the first, a consistent differentiation and integration of organs is carried out, as well as uneven growth and active movement of cellular material. The causal study of ontogenetic organogenesis is available for accurate study, especially due to the possibility of using an experimental method.

In plants, the term "organogenesis" usually refers to the formation and development of the main organs (root, stem, leaves, flowers) in the process of ontogenesis from an area of ​​\u200b\u200bundifferentiated tissue - the meristem.

Histogenesis is a set of naturally occurring processes in animal organisms that ensure the emergence, existence and restoration of tissues with their specific properties in different organs.

Histogenesis (from the Greek histos - tissue and ... genesis), tissue development, a set of naturally occurring processes that ensure the emergence, existence and restoration of tissues of animal organisms with their specific properties in different organs. G.'s studying of different fabrics and its patterns — one of the most important tasks of histology. The term "histogenesis" is used to denote the development of tissues in ontogenesis. However, the patterns of histogenesis cannot be considered in isolation from the evolutionary development of tissues (phylogistogenesis). Histogenesis is based on starting from the most early stages embryogenesis cellular differentiation - the development of increasing morpho-functional differences between specialized cells. This is a complex molecular genetic process of regular activation of the activity of genes that determine the specifics of protein synthesis in the cell. Reproduction of cells, their mutual movements, and other processes lead to the formation of embryonic rudiments, which are groups of cells regularly located in the body of the embryo. As a result of tissue differentiation of embryonic rudiments, the whole variety of tissues arises. various organs body. In the post-embryonic period, G.'s processes are divided into 3 main types: in tissues whose cells do not multiply (for example, nervous tissue); in tissues, the reproduction of cells of which is mainly associated with the growth of the organ (for example, the parenchyma of the digestive glands, kidneys); in tissues characterized by constant cell renewal (for example, hematopoietic tissue, many integumentary epithelium). The totality of cells that perform a certain histogenesis is divided into a number of successive groups (funds): a fund of ancestral cells capable of both differentiation and replenishment of the loss of their own kind; fund of progenitor cells, differentiating and capable of reproduction; a fund of mature cells that have completed differentiation. Restoration of damaged or partially lost tissues after injuries is carried out due to the so-called reparative histogenesis. Under pathological conditions, the processes of histogenesis can undergo profound qualitative changes and lead to the development of tumor tissues.

Embryonic histogenesis is the process of the emergence of specialized tissues from poorly differentiated cellular material of embryonic primordia, occurring during the embryonic development of the organism. Embryonic rudiments are the sources of development of tissues and organs in ontogenesis, represented by groups of more or less numerous poorly differentiated (non-specialized) cells; intercellular substance rudiments do not have.

Histogenesis is accompanied by reproduction and growth of cells, their movement - migration, differentiation of cells and their derivatives, intercellular and intertissue interactions - correlations, cell death. At different stages individual development one or the other of the listed components may take precedence.

In the process of histogenetic differentiation, specialization of tissue rudiments and the formation of various kinds fabrics. When cells differentiate from the original stem cell, diferons are formed - successive rows of cells (stem differons). The number of differentons in each type of tissue may be different.

The result of histogenetic processes is the formation of the main groups of tissues - epithelial, blood and lymph, connective, muscle and nerve. Their formation begins in the embryonic period and ends after birth. The sources of post-embryonic development of tissues are stem and semi-stem cells, which have high potentialities of development. The process of differentiation from stem cells has been studied in detail using blood cells as an example.

4. At what stage of spermatogenesis and oogenesis does a decrease in the number of chromosomes and the formation of haploid cells take place?

Significant cell growth is pronounced in oogenesis. The main content of the maturation period is meiosis, as a result of which 4 cells with a haploid set of chromosomes are formed from each diploid precursor cell. During spermatogenesis, these cells are identical in size and later become spermatozoa, and during oogenesis, meiosis provides an uneven division of the cytoplasm. As a result, only one of the four haploid cells becomes an egg capable of fertilization, while the other three are reduction bodies containing excess chromatin and eventually die. In addition to providing haploidy, meiosis also leads to the emergence of a qualitative diversity of germ cells. In the prophase of the first meiotic division, homologous chromosomes of paternal and maternal origin, spiraling, approach each other in pairs by corresponding sections (the so-called conjugation), forming bivalents. In this case, individual chromatids are intertwined with each other and can break in similar areas.

Reproductive phase: diploid cells divide repeatedly by mitosis. The number of cells in the gonads grows, they are called oogonia and spermatogonia. Set of chromosomes 2n. In the growth phase, their growth occurs, the resulting cells are called oocytes of the 1st order and spermatocytes of the 1st order. In the maturation phase, meiosis occurs, as a result of the first meiotic division, gametocytes of the 2nd order are formed (a set of chromosomes n2c), which enter the second meiotic division, and cells with a haploid set of chromosomes (nc) are formed. Ovogenesis at this stage practically ends, and spermatogenesis includes another formation phase, during which spermatozoa are formed.

In the process of restoring the integrity of chromatids, homologous chromosomes are able to exchange the corresponding sections. This process is called crossing over. In the anaphase of the first meiotic division, an independent divergence of maternal and paternal chromosomes occurs to the poles of the cell,
as a result, in the haploid set of future gametes, different combinations maternal and paternal chromosomes. The last period of gametogenesis (formation period) is observed only during spermatogenesis, during which haploid cells - spermatids - acquire structural features characteristic of mature spermatozoa.

5. How big can eggs be?

The size of the eggs varies widely - from several tens of micrometers to several centimeters (a human egg is about 100 microns, an ostrich egg, which has a length of about 155 mm with a shell, is also an egg). The egg cell has a number of membranes located on top of the plasma membrane and spare nutrients. In mammals, the eggs have a shiny shell, on top of which there is a radiant crown - a layer of follicular cells.

6. Self-fertilization and parthenogenesis

Self-fertilization is the fusion of heterosexual or sister nuclei formed in one individual.

Autogamy, self-fertilization - autogamy or self-fertilization - - a type of reproduction in which a zygote is formed due to the fusion of two haploid nuclei inside the cell of the same organism (ciliate) or by the fusion of gametes formed in the same flower

Self-fertilization, the fusion of male and female germ cells belonging to the same bisexual individual. In nature, self-fertilization is rare: in the process of evolution, organisms usually developed adaptations that eliminate the possibility of self-fertilization and provide cross-fertilization, as a result of which the genetic diversity of offspring increases, which contributes both to the development of new adaptations and the development of more viable offspring. Among animals, self-fertilization is sometimes observed in hydras, flatworms, some annelids, mollusks, fish; among plants - in many algae, fungi, flowering plants (in the latter as a result of self-pollination)

Self-fertilization is the closest form of inbreeding.

Parthenogenesis - A type of sexual reproduction in which an animal develops from an unfertilized egg; characteristic of wasps, bees and some other arthropods.

Parthenogenesis (from the Greek parthénos - virgin and ... genesis), virgin reproduction, one of the forms of sexual reproduction of organisms, in which female germ cells (eggs) develop without fertilization. Parthenogenesis - sexual, but same-sex reproduction - arose in the process of evolution of organisms in dioecious forms. In cases where parthenogenetic species are represented (always or periodically) only by females, one of the main biological advantages of parthenogenesis is to accelerate the rate of reproduction of the species, since all individuals of such species are able to leave offspring. In those cases where females develop from fertilized eggs, and males develop from unfertilized eggs, parthenogenesis contributes to the regulation of the numerical ratios of the sexes (for example, in bees). Often parthenogenetic species and races are polyploid and arise as a result of distant hybridization, revealing in connection with this heterosis and high viability. Parthenogenesis should be distinguished from asexual reproduction, which is always carried out with the help of somatic organs and cells (reproduction by division, budding, etc.). Distinguish between parthenogenesis. natural - normal way reproduction of some organisms in nature and artificial, caused experimentally by the action of various stimuli on an unfertilized egg, which normally needs to be fertilized.

There are obligate parthenogenesis, in which eggs are only capable of parthenogenetic development, and facultative parthenogenesis, in which eggs can develop both through parthenogenesis and as a result of fertilization [in many Hymenoptera insects, for example, bees, males (drones) develop from unfertilized eggs, from fertilized - females (wombs and worker bees)]. Often reproduction through parthenogenesis alternates with bisexual - the so-called cyclic parthenogenesis. Parthenogenetic and sexual generations in cyclic parthenogenesis. outwardly different. Thus, successive generations in aphids of the genus Chermes differ sharply in morphology (winged and wingless forms) and ecology (associated with different fodder plants); in some gall wasps, individuals of parthenogenetic and bisexual generations are so different that they were taken for different types and even childbirth. Usually (in many aphids, daphnia, rotifers, etc.), summer parthenogenetic generations consist of only females, and in autumn generations of males and females appear, which leave fertilized eggs for the winter. Many animal species that do not have males are capable of long-term reproduction through parthenogenesis - the so-called constant parthenogenesis. In some species, along with the parthenogenetic female race, there is a bisexual race (the original species), which sometimes occupies a different area - the so-called geographical parthenogenesis (sheath-bearing butterflies, many beetles, centipedes, mollusks, rotifers, daphnia, lizards among vertebrates, etc.).

According to the ability to give through parthenogenesis, males or females are distinguished: arrhenotoky, in which only males develop from unfertilized eggs (bees and other hymenoptera, mealybugs, mites, and from vertebrates - parthenogenetic lines of turkeys); thelytoky, in which only females develop (the most common case); deuterotokia, in which individuals of both sexes develop (for example, with random parthenogenesis in butterflies; in the bisexual generation with cyclic parthenogenesis in daphnia, rotifers, aphids).

Highly great importance has a cytogenetic mechanism for the maturation of an unfertilized egg. It is precisely because of whether the egg undergoes meiosis and a halving of the number of chromosomes - reduction (meiotic parthenogenesis) or does not pass (ameiotic parthenogenesis), whether the number of chromosomes characteristic of the species is preserved due to the loss of meiosis (zygotic parthenogenesis) or this number is restored after reduction by fusion of the nucleus of the egg with the nucleus of the directional body or in some other way (automictic parthenogenesis), ultimately the hereditary structure (genotype) of the parthenogenetic embryo and all its most important hereditary features- gender, preservation or loss of heterosis, acquisition of homozygosity, etc.

Parthenogenesis is also divided into generative, or haploid, and somatic (it can be diploid and polyploid). During generative parthenogenesis, a haploid number of chromosomes (n) is observed in the dividing cells of the body; this case is relatively rare and is combined with arrhenotoky (haploid males are drones of bees). During somatic parthenogenesis in dividing cells of the body, the initial diploid (2n) or polyploid (3n, 4n, 5n, rarely even 6n and 8n) number of chromosomes is observed. Often within one species there are several races characterized by multiple numbers of chromosomes - the so-called polyploid series. In terms of the very high frequency of polyploidy, parthenogenetic animal species present a sharp contrast with bisexual animals, in which polyploidy, on the contrary, is very rare. Polyploid, dioecious animal species appear to have evolved through parthenogenesis and distant hybridization.

A peculiar form of parthenogenesis - pedogenesis - parthenogenetic reproduction in the larval state.

Artificial parthenogenesis in animals was first obtained by the Russian zoologist A. A. Tikhomirov. He showed (1886) that unfertilized silkworm eggs can be stimulated to develop by solutions of strong acids, friction, and other physical and chemical stimuli. Later, artificial parthenogenesis was obtained by J. Loeb and other scientists in many animals, mainly in marine invertebrates ( sea ​​urchins and stars, worms, mollusks), as well as in some amphibians (frog) and even mammals (rabbit). At the end of the 19th - beginning of the 20th centuries. experiments on artificial parthenogenesis attracted special attention of biologists, giving hope to penetrate into the essence of fertilization processes with the help of this physicochemical model of egg activation. Artificial parthenogenesis is caused by the action of hypertonic or hypotonic solutions on the eggs (the so-called osmotic parthenogenesis), the prick of the egg with a needle moistened with hemolymph (the so-called traumatic P. of amphibians), sharp cooling and especially heating (the so-called temperature parthenogenesis), as well as the action of acids, alkalis etc. With the help of artificial parthenogenesis, it is usually possible to obtain only the initial stages of the development of the organism; complete parthenogenesis is rarely achieved, although cases of complete parthenogenesis are known even in vertebrate animals (frog, rabbit). The method of mass production of complete parthenogenesis, developed (1936) for the silkworm by B. L. Astaurov, is based on accurately dosed short-term heating (up to 46 ° C for 18 minutes) of unfertilized eggs extracted from the female. This method makes it possible to obtain only female individuals from the silkworm, hereditarily identical with the original female and with each other. The resulting di-, tri- and tetraploid clones can be propagated by parthenogenesis indefinitely. At the same time, they retain their original heterozygosity and "hybrid strength". Selection has produced clones that reproduce through parthenogenesis as easily as bisexual breeds through fertilization (more than 90% hatching of activated eggs and up to 98% viability). Parthenogenesis is of diverse interest for the practice of sericulture.

Parthenogenesis in plants. Parthenogenesis, common among seed and spore plants, is usually of the constant type; facultative parthenogenesis was found in isolated cases (in some species of hawkweed and in the cornflower Thalictrum purpurascens). As a rule, the sex of parthenogenetically reproducing plants is female: in dioecious plants, parthenogenesis is associated with the absence or extreme rarity male plants, in monoecious - with the degeneration of male flowers, the absence or abortivity of pollen. As with animal parthenogenesis, there are: generative, or haploid, parthenogenesis and somatic, which can be diploid or polyploid. Generative parthenogenesis occurs in algae (cutleria, spirogyra, ectocarpus) and fungi (saprolegnia, mucor, endomyces). In flowering plants, it is observed only under experimental conditions (tobacco, skerda, cotton, cereals, and many others). Somatic parthenogenesis occurs in algae (hara, cocconeis), in ferns (Drummond's marcelia) and in higher flowering plants (chondrilla, cuff, hawkweed, cat's foot, dandelion, etc.). Polyploid parthenogenesis in plants is very common; however, polyploidy is not a feature of parthenogenetic species here, since it is also widespread in bisexual plants. Other methods of reproduction are close to plant parthenogenesis—apogamy, in which the embryo develops not from an egg but from other cells of the gametophyte, and apomixis. Artificial parthenogenesis in plants has been obtained in some algae and fungi by the action of hypertonic solutions, as well as by short-term heating of female germ cells. The Austrian scientist E. Cermak obtained (1935–48) artificial parthenogenesis in flowering plants (cereals, legumes, and many others) by stimulating the stigma with killed or alien pollen or powdered substances (talc, flour, chalk, etc.). The Soviet scientist E. M. Vermel obtained (1972) diploid parthenogenesis in currants, tomatoes, and cucumbers by the action of dimethyl sulfoxide.

Parthenogenesis also includes peculiar ways of development of animals and plants - gynogenesis and androgenesis, in which the egg is activated to develop by penetrating sperm of its own or a close species, but the nuclei of the egg and sperm do not merge, fertilization turns out to be false, and the embryo develops only with female (gynogenesis) or only with the male (androgenesis) nucleus.

7. What is the significance of conjugation of homologous chromosomes and crossing over between them in evolution?

Chromosome conjugation is the convergence of homologous chromosomes during meiosis, as a result of which mutual exchange of individual sections is possible between them (crossing over).

Crossing over is an exchange of equal sections of homologous conjugating chromosomes that occurs in the prophase of the first meiosis and leads to the redistribution of genes in them. Chiases are an external manifestation of crossing over.

Crossing over is one of the mechanisms of hereditary variability.

In the prophase of the first division of meiosis, chromosomes spiralize. At the end of prophase, when spiralization ends, the chromosomes acquire their characteristic shape and size. The chromosomes of each pair, i.e. homologous, connected to each other along the entire length and twisted. This process of connecting homologous chromosomes is called conjugation. During conjugation between some homologous chromosomes, there is an exchange of sections - genes (crossing over), which means the exchange of hereditary information. After conjugation, homologous chromosomes separate from each other.

When the chromosomes are completely separated, a division spindle is formed, the metaphase of meiosis occurs and the chromosomes are located in the plane of the equator. Then comes the anaphase of meiosis, and not halves of each chromosome, including one chromatid, as in mitosis, go to the poles of the cell, but whole chromosomes, each of which consists of two chromatids. Consequently, only one of each pair of homologous chromosomes enters the daughter cell.

Following the first division, the second division of meiosis occurs, and this division is not preceded by DNA synthesis. The interphase before the second division is very short. Prophase 2 is short. In metaphase, 2 chromosomes line up in the equatorial plane of the cell. In anaphase 2, their centromeres separate and each chromatid becomes an independent chromosome. In telophase 2, the divergence of sister chromosomes to the poles is completed and cell division begins. As a result, four haploid daughter cells are formed from two haploid cells.

The crossover of chromosomes occurring in meiosis, the exchange of sites, as well as the independent divergence of each pair of homologous chromosomes, determines the patterns of hereditary transmission of a trait from parents to offspring. Of each pair of two homologous chromosomes (maternal and paternal), which were part of the chromosome set of diploid organisms, only one chromosome is contained in the haploid set of the egg or sperm. She may be:

1. paternal chromosome;

2. maternal chromosome;

3. paternal with maternal plot;

4. maternal with paternal plot.

These processes of origin a large number qualitatively different germ cells contribute to hereditary variability.
In some cases, due to a violation of the process of meiosis, if homologous chromosomes do not diverge, germ cells may not have a homologous chromosome or, conversely, have both homologous chromosomes. It leads to serious violations in the development of the organism or to its death.

8. Name the types of regeneration on the examples of animals and humans

9. What type of division is used to split a fertilized cell?

10. Why is radioactive radiation dangerous?

A person receives the main part of ionizing radiation from natural sources of radiation. Most of them are such that it is absolutely impossible to avoid radiation from them. Throughout the history of the Earth's existence, different types of radiation fall on the Earth's surface from space and come from radioactive substances located in the earth's crust.

A person is exposed to radiation in two ways. Radioactive substances can be outside the body and irradiate it from the outside; in this case they talk about external exposure
. Or they can be in the air that a person breathes, in food or in water and get inside the body. This method of irradiation is called internal.

Radiation, by its very nature, is harmful to life. Small doses of radiation can “start” a not yet fully understood chain of events leading to cancer or genetic damage. At high doses, radiation can destroy cells, damage organ tissues and cause the death of an organism.

Damage caused by high doses of radiation usually shows up within hours or days. Cancers, however, appear many years after exposure, usually not earlier than one to two decades. BUT birth defects development and others hereditary diseases caused by damage to the genetic apparatus, by definition, appear only in the next or subsequent generations: these are the children, grandchildren and more distant descendants of the individual exposed to radiation.

Under the influence of ionizing radiation, complex molecules and cellular structures are destroyed, which leads to radiation injury organism.

While it is not difficult to identify the rapidly manifesting ("acute") effects of high doses of radiation, it is easy to detect long-term effects from low doses of radiation is almost always very difficult. This is partly because they take a very long time to manifest. But even having discovered some effects, it is also necessary to prove that they are explained by the action of radiation, since both cancer and damage to the genetic apparatus can be caused not only by radiation, but also by many other reasons.

To cause acute damage to the body, radiation doses must exceed a certain level, but there is no reason to believe that this rule applies in the case of consequences such as cancer or damage to the genetic apparatus. By at least theoretically, the smallest dose is sufficient for this. However, at the same time, no radiation dose leads to these consequences in all cases. Even with relatively high doses of radiation, not all people are doomed to these diseases: the reparation mechanisms operating in the human body usually eliminate all damage. In the same way, any person exposed to radiation does not necessarily have to develop cancer or become a carrier of hereditary diseases; however, the likelihood or risk of such consequences is greater than that of a person who has not been exposed. And this risk is greater, the greater the dose of radiation.

Acute damage to the human body occurs at high doses of radiation. Generally speaking, radiation has similar action, only starting from a certain minimum, or “threshold”, radiation dose.

The response of human tissues and organs to irradiation is not the same, and the differences are very large. The magnitude of the dose, which determines the severity of the damage to the body, depends on whether the body receives it immediately or in several doses. Most organs have time to heal radiation damage to one degree or another and therefore tolerate a series of small doses better than the same total dose of radiation received at one time.

The impact of ionizing radiation on living cells

charged particles. The a- and b-particles penetrating into the tissues of the body lose energy due to electrical interactions with the electrons of the atoms near which they pass. (g-radiation and X-rays transfer their energy to matter in several ways, which eventually also lead to electrical interactions.)

Electrical Interactions. In the order of ten trillionth of a second after the penetrating radiation reaches the corresponding atom in the tissue of the body, an electron is detached from this atom. The latter is negatively charged, so the rest of the initially neutral atom becomes positively charged. This process is called ionization. The detached electron can further ionize other atoms.

Physico-chemical changes. Both a free electron and an ionized atom usually cannot remain in this state for long, and over the next ten billionths of a second, they participate in a complex chain of reactions that form new molecules, including extremely reactive ones such as "free radicals."

Chemical changes. Over the next millionths of a second, the resulting free radicals react both with each other and with other molecules and, through a chain of reactions not yet fully understood, can cause chemical modification of biologically important molecules necessary for normal functioning cells.

Biological effects. Biochemical changes can occur within seconds or decades after irradiation and cause immediate cell death or changes in them that can lead to cancer.

Of course, if the radiation dose is high enough, the exposed person will die. Anyway, very large doses exposures of the order of 100 Gy cause so much serious defeat central nervous system that death usually occurs within hours or days. At radiation doses of 10 to 50 Gy for whole-body exposure, the damage to the central nervous system may not be so severe as to be fatal, but the exposed person is likely to die anyway in one to two weeks from hemorrhages in the gastrointestinal tract . Even lower doses may not cause serious damage to the gastric tract, or the body can cope with them, and yet death can occur after one to two months, from the time of exposure mainly due to the destruction of red cells. bone marrow- the main component of the hematopoietic system of the body: from a dose of 3-5 Gy during whole-body irradiation, about half of all exposed people die. Thus, in this range of radiation doses, large doses differ from smaller ones only in that death occurs earlier in the first case, and later in the second.

In the human body, ionizing effects cause a chain of reversible and irreversible changes. The triggering mechanism of influence is the processes of ionization and excitation of atoms and molecules in tissues. An important role in the formation of biological effects is played by free radicals H and OH, which are formed as a result of water radiolysis (the human body contains up to 70% of water). Possessing high activity, they enter into chemical reactions with protein molecules, enzymes and other elements of biological tissue, which leads to disruption of biochemical processes in the body. Hundreds and thousands of molecules not affected by radiation are involved in the process. As a result, metabolic processes are disturbed, tissue growth slows down and stops, new chemical compounds not characteristic of the body. This leads to disruption of the vital activity of individual functions of organs and systems of the body. Under the influence of ionizing radiation in the body, there is a violation of the function hematopoietic organs, an increase in the permeability and fragility of blood vessels, an upset of the gastrointestinal tract, a decrease in the body's resistance, its depletion, the degeneration of normal cells into malignant ones, etc. Effects develop over different periods of time: from fractions of seconds to many hours, days, years.

Radiation effects are usually divided into somatic and genetic. Somatic effects are manifested in the form of acute and chronic radiation sickness, local radiation injuries, such as burns, as well as long-term reactions of the body, such as leukemia, malignant tumors, early aging organism. Genetic effects may show up in later generations.

Acute lesions develop with a single uniform gamma irradiation of the whole body and an absorbed dose of more than 0.25 Gy. At a dose of 0.25 ... 0.5 Gy, temporary changes in the blood can be observed, which quickly normalize. In the dose range of 0.5 ... 1.5 Gy, a feeling of fatigue occurs, less than 10% of those exposed may experience vomiting, moderate changes in blood. At a dose of 1.5 ... 2.0 Gy, a mild form of acute radiation sickness is observed, which is manifested by a prolonged decrease in the number of lymphocytes in the blood (lymphopenia), vomiting is possible on the first day after exposure. Deaths are not recorded.

Radiation sickness moderate occurs at a dose of 2.5 ... 4.0 Gy. Almost everyone has nausea, vomiting on the first day, the content of leukocytes in the blood decreases sharply, subcutaneous hemorrhages appear, in 20% of cases death is possible, death occurs 2-6 weeks after exposure.

At a dose of 4.0 ... 6.0 Gy, a severe form of radiation sickness develops, leading to death in 50% of cases during the first month. At doses exceeding 6.0 ... 9.0 Gy, in almost 100% of cases, an extremely severe form of radiation sickness ends in death due to hemorrhage or infectious diseases.

The given data refer to cases where there is no treatment. Currently, there are a number of anti-radiation agents, which, with complex treatment, make it possible to exclude a lethal outcome at doses of about 10 Gy.

Chronic radiation sickness can develop with continuous or repeated exposure to doses significantly lower than those that cause sharp shape. The most characteristic signs of the chronic form are changes in the blood, disorders of the nervous system, local skin lesions, damage to the lens, and a decrease in the body's immunity.

The degree of exposure to radiation depends on whether the exposure is external or internal (when a radioactive isotope enters the body). Internal exposure is possible by inhalation, ingestion of radioisotopes and their penetration into the human body through the skin. Some substances are absorbed and accumulated in specific organs, resulting in high local doses of radiation. For example, calcium, radium, strontium accumulate in the bones, iodine isotopes cause damage to the thyroid gland, rare earth elements - mainly liver tumors. Isotopes of cesium and rubidium are evenly distributed, causing oppression of hematopoiesis, damage to the testicles, and soft tissue tumors. With internal irradiation, the most dangerous alpha-emitting isotopes of polonium and plutonium.
Integrity and discreteness of living systems as the basis for ideas about the levels of organization of living matter. Levels of organization by complexity The role of ATP in microbial metabolism, the mechanisms of ATP biosynthesis

1. Leads to an increase in the number of cells and ensures the growth of a multicellular organism.

2. Provides replacement for worn or damaged fabrics.

3. Maintains a set of chromosomes in all somatic cells.

4. Serves as a mechanism for asexual reproduction, in which offspring are created that are genetically identical to the parents.

5. Allows you to study the karyotype of the organism (in metaphase).

AMITOSIS

Amitosis is the division of the interphase nucleus by constriction without the formation of a fission spindle.

During chromosome amitosis light microscope indistinguishable. Such a division occurs in unicellular organisms (amoeba, a large nucleus of ciliates), as well as in some highly specialized with weakened physiological activity, degenerating, doomed to death, plant and animal cells, or with various pathological processes(endosperm, potato tuber). In animals and humans, this type of division is characteristic of the cells of the liver, cartilage, and cornea of ​​the eye. With amitosis, only nuclear division is often observed: in this case, two- and multi-nuclear cells can appear. If the division of the nucleus is followed by the division of the cytoplasm, then the distribution of cellular components, like DNA, is carried out arbitrarily.

Amitosis value: in binuclear and multinucleated cells the total area of ​​contact between the nuclear material and the cytoplasm increases. This leads to an increase in nuclear-plasma metabolism, an increase in the functional activity of the cell and greater resistance to the effects of adverse factors. Cells that have gone through amitosis lose their ability to mitotic division and reproduction.

MEIOSIS

During the formation of gametes, i.e. sex cells - sperm and eggs - cell division occurs, called meiosis.

The original cell has a diploid set of chromosomes, which then double. But, if during mitosis in each chromosome the chromatids simply diverge, then during meiosis the chromosome (consisting of two chromatids) is closely intertwined with its parts with another chromosome homologous to it (also consisting of two chromatids), and occurs crossing over - exchange of homologous regions of chromosomes. Then, new chromosomes with mixed “mother's” and “dad's” genes diverge and cells with a diploid set of chromosomes are formed, but the composition of these chromosomes already differs from the original one; recombination . The first division of meiosis is completed, and the second division of meiosis occurs without DNA synthesis, therefore, during this division, the amount of DNA is halved. From the original cells with a diploid set of chromosomes, gametes with a haploid set arise. Four haploid cells are formed from one diploid cell. The phases of cell division that follow interphase are called prophase, metaphase, anaphase, telophase, and after division again interphase.

Meiosis is of three types: zygotic (in the zygote after fertilization, which leads to the formation of zoospores in algae and mycelium of fungi); gametic (in the genital organs, leads to the formation of gametes) and spore (in seed plants leads to the formation of a haploid gametophyte).

Meiosis consists of two successive divisions, meiosis I and meiosis II. DNA duplication occurs only before meiosis I, and there is no interphase between divisions. In the first division, homologous chromosomes diverge and their number is halved, and in the second division, chromatids are formed and mature gametes are formed. A feature of the first division is a complex and long-term prophase.

Prophase I- the prophase of the first division is very complex and consists of 5 stages:

Leptotena or leptonema - packing of chromosomes, condensation of DNA with the formation of chromosomes in the form of thin threads (chromosomes shorten). Zygoten or zygonema - conjugation occurs - the connection of homologous chromosomes with the formation of structures consisting of two connected chromosomes, called tetrads or bivalents and their further compaction. Pachytene or pachinema - (the longest stage) - in some places, homologous chromosomes are tightly connected, forming chiasmata. Crossing over occurs in them - the exchange of sites between homologous chromosomes. Diploten or diplonema - partial decondensation of chromosomes occurs, while part of the genome can work, transcription processes (RNA formation), translation (protein synthesis) occur; homologous chromosomes remain connected to each other. In some animals, chromosomes in oocytes at this stage of prophase of meiosis acquire the characteristic shape of lampbrush chromosomes. diakinesis - DNA again condenses as much as possible, synthetic processes stop, the nuclear envelope dissolves; centrioles diverge towards the poles; homologous chromosomes remain connected to each other.