Biological significance 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 the majority of biological processes occurring on our planet. 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 the central partition, the same number of formed components remains in each new cell.

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 environmental conditions, for example, ambient temperature, light phases, 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.

Not direct division Eukaryotic cells - containing a nucleus - are called mitosis. In this article you will learn what the biological significance of mitosis is and the history of the study of this process.

Stages of mitosis

The individual development of any living organism is impossible without the process of cell division. The uniqueness of mitosis is that during the division of a diploid somatic cell, two daughter cells are formed that have the same genetic information and have an equal number of chromosomes. In other words, continuity is maintained between generations of eukaryotic cells.

The whole process consists of four stages:

• Prophase;
• Metaphase;
• Anaphase;
• Telophase.

Rice. 1. Stages of mitosis

In some sources you can find a detailed list of the phases of mitosis. For example, prophase is preceded by preprophase, the so-called preparation for division. And also between prophase and metaphase the stage of prometaphase is considered. However, most scientists combine preprophase, prophase and prometaphase into one single stage - prophase.

History of process research

The first mention of the process of cell division was found in scientific literature in 1870. But these descriptions were incomplete and concerned only changes in the behavior of nuclei inside the cell.

The first attempts to study this process belonged to Russian scientists Russov, Chistyakov, as well as the German scientist Schneider.

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In 1879, Schleicher, a German scientist, proposed the process of cell division to be called karyokinesis. The concept of “mitosis” was first introduced by the German histologist W. Flemming in the early 1880s. It is this term that has become generally accepted to name the process that completes the division of chromosomes between daughter cells.

Rice. 2. Walter Flemming

Biological significance of mitosis

The key role of mitosis is to copy the genetic code and pass it on to subsequent generations. Thanks to this process, a constant number of chromosomes is maintained in the nucleus, which is strictly equally distributed between daughter cells. With the help of mitotic division, plant tissue cells grow. In animal organisms, mitosis underlies the fragmentation of a fertilized egg and tissue growth.

In addition, the biological meaning of mitosis is:

• Development and growth of a living organism;

Thanks to this process, a multicellular organism develops and grows from a single-celled zygote. Mitosis is the basis of embryonic development.

• Cell replacement;

Some areas of the body require constant replacement during life, for example, skin cells, intestinal epithelium, red blood cells.

• Regeneration and restoration;

Through mitosis, some organisms can regenerate themselves from one part of the body. For example, a starfish can recover from just one of its rays. A lizard can grow a new tail, and a person's skin can be restored.

Rice. 3. Starfish recovery

• Asexual reproduction;

This process underlies the vegetative propagation of plants. In animals, hydra reproduces through mitosis. A new individual is formed by budding, which is impossible without division and an increase in the number of cells.

What have we learned?

The process of indirect division of eukaryotic cells, during which genetic information is copied and stored, is called mitosis. This process takes place in 4 stages: prophase, metaphase, anaphase and telophase. Scientists first described the process of cell division in the 70-80s of the 19th century. The term “mitosis” was introduced by the German scientist Walter Flemming. The biological significance of mitosis is to ensure the formation of daughter cells with identical genetic information. Indirect division underlies the development and growth of all living organisms, restoration and regeneration of body parts, as well as asexual reproduction.

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Amitosis, or direct division- this is the division of the interphase nucleus by constriction without the formation of a fission spindle. This division occurs in unicellular organisms, as well as in some highly specialized cells of plants and animals with weakened physiological activity, degenerating, doomed to death, or during various pathological processes, such as malignant growth, inflammation, etc. Amitosis can be observed in the tissues of a growing potato tuber, endosperm, walls of the pistil ovary and parenchyma of leaf petioles. This type of division is characteristic of liver cells, cartilage cells, and the cornea of ​​the eye. Very often, during amitosis, only nuclear division is observed; in this case, bi- and multinucleated cells may appear. Cell division in prokaryotes is close to amitosis. A bacterial cell contains only one, most often circular, DNA molecule attached to the cell membrane. Before a cell divides, DNA is replicated to produce two identical DNA molecules, each also attached to the cell membrane. When a cell divides, the cell membrane grows between these two DNA molecules, so that each daughter cell ends up with one identical DNA molecule. This process is called direct binary fission.

Mitosis is a division of the nucleus that leads to the formation of two daughter nuclei, each of which has exactly the same set of chromosomes as in 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 is divided into prophase, metaphase, anaphase and telophase.

1) 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. Simultaneously with the spiralization of chromosomes, the nucleolus disappears and the nuclear membrane fragments (breaks up into separate tanks). After the collapse of the nuclear membrane, the chromosomes lie freely and randomly in the cytoplasm. In prophase, centrioles (in those cells where they exist) diverge to the cell poles. At the end of prophase, a fission spindle begins to form, which is formed from microtubules by polymerization of protein subunits.

2) In metaphase The formation of the fission spindle is completed, which consists of two types of microtubules: chromosomal, which bind to the centromeres of the chromosomes, and centrosomal (polar), which stretch from pole to pole of the cell. Each double chromosome is attached to the 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 them morphological features. In anaphase, daughter chromosomes are stretched toward the cell poles with the help of spindle microtubules. During movement, the daughter chromosomes bend somewhat like a hairpin, the ends of which are turned towards the equator of the cell.

3) In anaphase chromatids duplicated in the interphase of chromosomes diverge to the poles of the cell. At this moment, the cell contains two diploid sets of chromosomes.

4) 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 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 - phragmoplast - arises from the remains of the filament spindle filaments. As a result of mitosis, two daughter cells arise from one cell with the same set of chromosomes as in the mother cell.

Biological significance of mitosis consists in the fact that it ensures the hereditary transmission of characteristics and properties in a number of generations of cells during the development of a multicellular organism.

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Cell cycle. Mitosis

Formation of new knowledge. Lecture block.

Topic study plan:

1. Cell cycle. Mitosis

2. Short story opening of mitosis

3. Cell division - mitosis

4. Types of mitosis

5.Regulation of the cell cycle

One of the most important properties 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 method of dividing eukaryotic cells is indirect division, or mitosis (from the ancient Greek “mitos” - thread). The biological significance of mitosis is to preserve the volume and quality of hereditary information.

Cell division (fragmentation of frog eggs) was first observed by French scientists Prevost and Dumas (1824). This process was described in more detail by the Italian embryologist M. Rusconi (1826). The process of nuclear division during egg crushing sea ​​urchins 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 stamen 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 moss), German zoologist A. Schneider (1873; crushed eggs of flatworms), Polish botanist E. Strasburger (1875; spirogyra, moss, onion).

To indicate movement processes components 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 Hoffmeister, but only in 1888. German histologist W. Waldeyer introduced the term chromosome. The leading role of chromosomes in the storage, reproduction and transmission of hereditary information was proven only in the twentieth century.

Cell cycle

V. Flemming formulated the idea of ​​mitosis as a cyclic process, the culminating moment of which is the splitting of each chromosome into two daughter chromosomes and their distribution among two newly formed cells. In single-celled organisms, the lifespan of a cell coincides with the lifespan of the organism. In the body of multicellular animals and plants, two groups of cells are distinguished: constantly dividing (proliferating) and resting (static). The collection of proliferating cells forms a proliferative pool.

In groups of proliferating cells, the interval between the completion of mitosis in the parent cell and the completion of mitosis in its daughter cell is usually called the cell cycle. The cell cycle is controlled by certain genes. The complete cell cycle includes interphase and mitosis itself. In turn, mitosis itself includes karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm).

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

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

Presynthetic stage (G1). At the root of each chromosome lies one double-stranded DNA molecule. The amount of DNA in a cell at the presynthetic stage is indicated by the symbol 2c (from the English.

Mitosis, its biological significance, pathology

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

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

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The constancy of the structure and correct functioning of the organs and tissues of a multicellular organism would be impossible without preserving the same set of genetic material in countless cell generations. Mitosis provides important manifestations of life: embryonic development, growth, restoration of organs and tissues after damage, replacement of dead and dead cells.

Non-cellular life forms - viruses

Virus structure

Simply organized viruses are nucleoproteins, i.e.

consist of nucleic acid and several proteins that form the shell - the capsid. Complex viruses have an additional protein shell (influenza and herpes viruses). Viruses can enter the cell together with a pinocytotic or phagocytotic vesicle. As a rule, the virus binds to receptor proteins on the cell surface, is immersed in the cytoplasm and can be delivered to any part of the cell.

The receptor mechanism for virus penetration into the cell ensures the specificity of the infectious process. Hepatitis A or B virus penetrates and multiplies in liver cells, influenza virus - in upper epithelial cells respiratory tract, the AIDS virus binds to blood leukocytes responsible for immune system. The infectious process begins with the penetration of the virus into the cell and its reproduction. The accumulation of viral particles leads to their exit from the cell and further infection.

Control questions

1. What are the characteristics of the tissues of a living organism?

2. What is the life cycle of a cell?

3. What is the mitotic cycle? What periods does it consist of?

4. List and characterize the phases of mitosis.

5. What is the biological meaning of mitosis?

6. Characterize non-cellular life forms.

7. The structure and role of the virus in human life.

Section 3 REPRODUCTION AND INDIVIDUAL DEVELOPMENT OF ORGANISMS

Topic 3.1 Forms of reproduction of organisms

Terminology

1. Ontogenesis– individual development organisms.

2. Somatic cells- the cells from which the body is built.

3. Gametes- specialized germ cells that transmit hereditary information.

4. Controversy- a section of a DNA molecule covered with a dense shell.

5. Vegetative propagation- propagation by plant parts.

6. Gametogenesis– development of gametes.

7. Zygote- fertilized egg.

8. Parthenogenesis– development of an egg without fertilization.

Reproduction or self-reproduction is a property inherent in all living organisms - from bacteria to mammals.

The existence of any species of animals, plants, bacteria and fungi, continuity between parent individuals and their offspring is maintained through reproduction. Closely related to self-reproduction is another property of living organisms – development. It is also inherent in all life on Earth: both unicellular and multicellular organisms. At any level of organization, living matter is represented by elementary structural units. For a cell, this is an organelle: the integrity of the cell is maintained by the constant reproduction of new organelles in place of the lost ones. Every organism is made up of cells.

Reproduction– one of the most complex processes of life. Natural selection favors the preservation of any characteristics and properties that increase the viability of the offspring at all stages of the life of the organism. In the struggle for existence, organisms win, which in turn leave more offspring, who survive to adulthood and, in turn, leave offspring. This direction of selection leads to the fact that many structural and behavioral features serve for the most successful reproduction. There are many known methods of reproduction, but all of them can be combined into two large groups: asexual and sexual.

Asexual reproduction

Asexual reproduction is characterized by the fact that a new individual develops from non-sexual (somatic cells). With asexual reproduction, a new organism can arise from one cell or several cells of the parent individual that are not specialized for reproduction. Many simple unicellular algae reproduce by normal mitotic cell division. Other unicellular organisms: lower fungi and algae are characterized by spore formation. Multicellular organisms are also capable of sporulation: their spores are often formed in special cells or organs - sporangia. Examples of organisms that reproduce in this way are some plants: mosses, higher fungi, ferns. In unicellular and multicellular organisms, budding is also a method of asexual reproduction. For example, in yeast fungi and some ciliates, budding consists of the initial formation of a small tubercle on the mother cell - a bud containing a nucleus. She grows and reaches a size close to the mother's and then separates. In multicellular organisms, the kidney consists of a group of cells from both layers of the body wall. The bud grows, lengthens, and a mouth opening appears at its anterior end, surrounded by tentacles. Budding ends with the formation of a small hydra, which can separate from the mother’s body and begin an independent existence. In multicellular animals asexual reproduction It is also carried out by dividing the body into two or more parts: flatworms, annelids, echinoderms. From such parts full-fledged individuals develop. In plants, vegetative propagation (by body parts) is widespread: cuttings, tendrils, tubers. Thus, in potatoes, modified underground parts of the stem - tubers - are used for reproduction. In jasmine or willow, cut shoots - cuttings - take root easily. Grapes and currants are propagated by cuttings. Long creeping stems - strawberry tendrils form buds, which take root and give rise to a new plant.

Cell division: amitosis, mitosis. Biological meaning of mitosis.

Few plants can propagate from leaf cuttings. On the lower part of the leaf, in places where large veins branch, roots appear, on the upper part - buds, and then shoots.

Asexual reproduction, which evolved earlier than sexual reproduction, is an effective process. Based on it in favorable conditions the number of a species can increase rapidly, however, with any form of asexual reproduction, all offspring have a genotype identical to the maternal one. Remember that in the interphase of mitosis, an absolutely precise doubling of the cell’s genetic material occurs, as a result of which, during division, each of the daughter cells receives hereditary information similar to that of the mother cell. Since all somatic cells of the body arose through mitosis, and it is from them that a new organism develops, it becomes clear why all individuals during asexual reproduction are genetically similar: it is not accompanied by an increase in genetic diversity. New traits that may prove useful when environmental conditions change appear only as a result of relatively rare mutations.

Sexual reproduction

Sexual reproduction refers to the change of generations and the development of organisms based on the fusion of specialized germ cells - gametes, formed in the gonads. Sexual reproduction provides enormous evolutionary advantages over asexual reproduction. This is due to the fact that the genotype of the offspring is formed due to a combination of genes belonging to both parents. The emergence of new gene combinations ensures more successful and rapid adaptation of the species to changing living conditions and to the development of new ecological niches. Thus, the essence of sexual reproduction lies in the combination in the hereditary material of a descendant of genetic information from two different sources– parents and in increasing the genetic diversity of offspring. However, this process is not always accompanied by an increase in the number of individuals. It often happens that two individuals exchange only part of the hereditary information. The main direction of the evolution of the sexual process is the path to the fusion of germ cells belonging to dioecious organisms. This type of reproduction best ensures the genetic diversity of the offspring. Bisexual animals and plants have adaptations that prevent self-fertilization. This may be the mating of different individuals. In plants, self-fertilization is excluded if they are unisexual. When plants are bisexual, the pistils and stamens ripen in different time, which makes only cross-pollination possible.

Gametogenesis

Sex cells (gametes): male - sperm and female - eggs develop in the gonads. In the first case, the path of their development is spermatogenesis, in the second – oogenesis. Some animals contain characteristics of both sexes, but most often the animals are dioecious. The separation of the sexes has an obvious evolutionary advantage; it creates the possibility of specialization of parents in structure and behavior, promotes the development various forms caring for the offspring.

In the process of formation of germ cells, a number of stages are distinguished.

First stage– the period of reproduction in which primary germ cells divide through mitosis, as a result their number increases. Spermatogenesis begins at puberty and continues throughout the reproductive period. Reproduction of female germ cells in lower vertebrates continues throughout life. In humans, these cells multiply with the greatest intensity only in the prenatal period. After the formation of the female gonads, the primary germ cells stop dividing, most of them die, and the rest remain dormant until puberty.

Second stage– period of growth. Immature male gametes grow slowly, eggs grow quickly. In some animals, eggs grow over several days or weeks; in others, months or years. The growth of the egg occurs due to substances produced by other cells. In fish, amphibians, and birds, the bulk of the egg is the yolk. It is synthesized in the liver and delivered to the oocyte. In addition to the yolk, numerous proteins and RNA of all types are synthesized: i-RNA, t-RNA, r-RNA.

Third stage– period of maturation or meiosis. Cells entering the period of meiosis contain diploid set chromosomes and already double the amount of DNA. During the process of sexual reproduction, organisms of any species retain their characteristic number of chromosomes from generation to generation. This is achieved by the fact that before the fusion of germ cells - fertilization in the process of maturation, the number of chromosomes in them decreases (reduces), i.e. from a diploid set a haploid one is formed. The essence of meiosis is that each sex cell receives a single haploid set of chromosomes. During meiosis, new combinations of genes are created through the combination of different maternal and paternal chromosomes.

Control questions

1. Reproduction, its essence and meaning.

2. Methods of reproduction.

3. Asexual reproduction, its essence and significance.

4. Vegetative propagation.

5. Sexual reproduction, its essence and advantage over asexual reproduction.

6. Gametogenesis and its stages.

7. Meiosis, its essence and significance.

8. Name the cells capable of reproducing by mycosis and meiosis.

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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. The mitotic cycle 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) (2n2c, where n is the number of chromosomes, c is the number of molecules). 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) (2n4c). 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) (2n4c). 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.

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 change of phases is 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).

Rice. 1.

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; V- metaphase; G- anaphase; l, e- early and late telophases

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).

Atypical forms of mitosis

Atypical forms of mitosis include amitosis, endomitosis, and polyteny.

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

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

2. Endomitosis. With this type of division, after DNA replication, the chromosomes do not separate into two daughter chromatids. This leads to an increase in the number of chromosomes in a cell, sometimes tens of times compared to the diploid set. This is how polyploid cells arise.

The biological meaning of mitotic cell division is

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 DNA content (chromonemas) in the chromosomes without an increase in the content of the chromosomes themselves. In this case, the number of chromonemas can reach 1000 or more, and the chromosomes acquire gigantic sizes. With polythenia, all phases of the mitotic cycle are lost, except for the reproduction of the primary DNA strands. This type of division is observed in some highly specialized tissues (liver cells, salivary glands dipteran insects). Drosophila polytene chromosomes are used to construct cytological maps of genes in chromosomes.

Biological significance of mitosis.

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

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

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Mitosis- indirect cell division, the most common method of reproduction in eukaryotic cells. 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. The mitotic cycle 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.

The mitotic cycle consists of four successive periods (phases):

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

They constitute the autocatalytic interphase (preparatory 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 occurs (i.e.

Indirect cell division (mitosis, or karyokinesis)

structures capable of self-reproduction). 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, 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 .

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

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

Since it is continuous, the change of phases is 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 are then 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).

Scheme of mitosis in onion root cells

All processes occurring during the cell cycle are controlled by certain genes. Mutations of these genes lead to disruption of the cell cycle at its different stages. Mitosis is common to all eukaryotes. Its biological significance lies in the fact that as a result, all daughter cells have the same number of chromosomes as the parent. The individuality of chromosomes is completely preserved. This is the genetic significance of mitosis, because each of the cells resulting from division carries the full 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

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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 special shape 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 during embryogenesis, the chromosomes, having taken on a special morphological form"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 signs. 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 a much more complex process, since it involves two successive 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 ends up 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 an 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 obtain mutations in yeast. A year later, the American researcher G. Meller (later twice a laureate Nobel Prize), who worked for a long time 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 there is no threshold dose in this case, and even the most small doses increase the likelihood of mutations occurring in the population. An increase in the frequency of mutations is dangerous not only because individual plan, how much in terms 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, the number of hereditary diseases in the population will double in a generation.

Chemical factors. Chemicalization Agriculture and other areas of 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, the indegradability and long-term preservation of foreign substances entering the environment. What was initially taken as an achievement in the fight against pests later turned into a complex problem. Wide Application in the 40s - 60s of the last century, the insecticide DDT 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 in humans has been established.

Chromosome aberrations in somatic cells are caused by smallpox, measles, chickenpox, mumps, influenza, hepatitis viruses, 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 expressed stronger sign 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 arise under the influence of natural factors unknown to us. 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 an even more powerful effect 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, 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 medications. 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. Recently, neutrons with high penetrating power have been increasingly 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 of atomic explosions are associated primarily with the mutagenic effect of ionizing radiation. Secondly, physical methods Mutagenesis is 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 the chromosome that lead to the induction 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 exposure is education free radicals OH or HO2 from water in 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 have no significance for the mutation process. But later these ideas were refuted thanks to the artificial production of mutations using various environmental factors. At present, it can be assumed that, apparently, there are no environmental factors that would not, to some extent, affect changes in hereditary properties. Of the 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 against which organisms have protective 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 nucleic acid molecules 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. Darwin called non-hereditary variability 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 of the 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 a relationship between changes 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.

Mitosis is indirect cell division, karyokinesis, [~ 1] the most common method of reproduction of eukaryotic cells. The biological significance of mitosis is the strictly identical distribution of chromosomes between daughter nuclei, which ensures the formation of genetically identical daughter cells and maintains continuity in a number of cell generations.

Mitosis consists of four phases: prophase, metaphase, anaphase, telophase.

In prophase The volume of the nucleus increases, chromosomes become visible due to spiralization, and two centrioles diverge to the poles of the cell. As a result of the spiralization of chromosomes, it becomes impossible to read genetic information from DNA

and RNA synthesis stops. The threads of the achromatin spindle are stretched between the poles: an apparatus is formed that ensures the divergence of chromosomes to the poles of the cell. At the end of prophase, the nuclear envelope breaks up into separate fragments, the edges of which close together. Small vesicles similar to the endoplasmic reticulum are formed.

During prophase, the spiralization of chromosomes continues, which thicken and shorten. After the collapse of the nuclear membrane, the chromosomes lie freely and randomly in the cytoplasm.

In metaphase chromosome spiralization reaches a maximum, and shortened chromosomes rush to the equator of the cell, located at an equal distance from the poles. It can be seen that the chromosomes consist of two chromatids, connected only at the centromere. The centromeric regions of chromosomes are located in the same plane. The mitotic spindle is already fully formed by this time. Some of the spindle threads go from pole to pole - these are continuous threads. Other threads - chromosomal - connect the poles to the centromeres of the chromosomes.

In anaphase The centromeres are separated, and from this moment the sister chromatids become independent daughter chromosomes. The mechanism for the movement of daughter chromosomes to the poles of the cell is provided by the following processes. Firstly, by the sliding of the chromosomal strand of the spindle to which the chromosome is attached. Secondly, by the cleavage of fragments of the chromosomal thread by enzymes in the region of the cell center (or centromeric region), as a result of which the thread is shortened and brings the chromosome closer to the pole. Thus, in anaphase, the chromatids of the chromosomes doubled in interphase precisely diverge to the poles of the cell. At this moment, the cell contains two diploid sets of chromosomes Mitosis ends with telophase. Chromosomes gathered at the poles despiral and become barely visible. The nuclear envelope is formed from the membrane structures of the cytoplasm. In animal cells, the cytoplasm is divided due to constriction of the cell body into two smaller ones, each of which contains one diploid set of chromosomes. In plant cells, the cytoplasmic membrane arises in the middle of the cell and extends to the periphery, dividing the cell in half. After the formation of a transverse cytoplasmic membrane, a cellulose wall appears in plant cells. Starting from a fertilized egg - a zygote - all daughter cells formed as a result of mitosis contain the same set of chromosomes and the same genes, ensuring continuity of the genotype over a series of cell generations. Thus, the biological meaning of mitosis as a method of cell division lies in the precise distribution of genetic material between daughter cells. As a result of mitosis, both daughter cells receive a diploid set of chromosomes. Biological significance of mitosis. The constancy of the structure and correct functioning of the organs and tissues of a multicellular organism would be impossible without preserving the same set of genetic material in countless cell generations. Mitosis provides important manifestations of vital activity: embryonic development, growth, restoration of organs and tissues after damage, maintaining the structural integrity of tissues with the constant loss of cells in the process of their functioning (replacement of dead red blood cells, damaged skin cells, intestinal epithelium, etc.). In protozoa, mitosis ensures asexual reproduction.

Meiosis and its stages.

MEIOSIS is a cell division in which the number of chromosomes is reduced and their recombination occurs in daughter cells compared to the mother cell. Meiosis is the basis of sexual reproduction, in which the offspring are not identical to the parents. Its most important evolutionary role is as a barrier to non-viable combinations of chromosomes and genes. Meiosis occurs in two stages, the first of which is called reduction (during this particular stage the number of chromosomes in daughter cells is halved), and the second is equational (as a result of which chromosomes are evenly distributed among daughter cells, it is similar to mitosis). With a decrease in the number of chromosomes as a result of meiosis in life cycle there is a transition from the diploid phase to the haploid phase.

Due to the fact that in the prophase of the first, reduction stage, pairwise fusion (conjugation) of homologous chromosomes occurs, the correct course of meiosis is possible only in diploid cells or in even polyploids (tetra-, hexaploid, etc. cells). Meiosis can also occur in odd polyploids (tri-, pentaploid, etc. cells), but in them, due to the inability to ensure pairwise fusion of chromosomes in prophase I, chromosome divergence occurs with disturbances that jeopardize the viability of the cell or developing from it a multicellular haploid organism.

Phases of meiosis

Meiosis consists of 2 consecutive divisions with a short interphase between them.

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

o Leptotene or leptonema - packaging of chromosomes, condensation of DNA to form chromosomes in the form of thin threads (chromosomes are shortened).

o Zygotene or zygonema - conjugation occurs - the joining of homologous chromosomes with the formation of structures consisting of two connected chromosomes, called tetrads or bivalents and their further compaction.

o Pachytene or pachynema - (the longest stage) crossing over (crossover), exchange of sections between homologous chromosomes; homologous chromosomes remain connected to each other.

o Diplotene or diplonema - partial decondensation of chromosomes occurs, while part of the genome can work, the processes of transcription (RNA formation), translation (protein synthesis) occur; homologous chromosomes remain connected to each other. In some animals, the chromosomes in oocytes at this stage of meiotic prophase acquire the characteristic lampbrush chromosome shape.

o Diakinesis - DNA condenses to the maximum again, synthetic processes stop, the nuclear membrane dissolves; Centrioles diverge towards the poles; homologous chromosomes remain connected to each other.

By the end of Prophase I, centrioles migrate to the cell poles, spindle filaments are formed, and the nuclear membrane and nucleoli are destroyed.

Metaphase I - bivalent chromosomes line up along the equator of the cell.

Anaphase I - microtubules contract, bivalents divide and chromosomes move towards the poles. It is important to note that, due to the conjugation of chromosomes in zygotene, entire chromosomes, consisting of two chromatids each, diverge to the poles, and not individual chromatids, as in mitosis.

Telophase I - chromosomes despiral and a nuclear envelope appears.

The second division of meiosis follows immediately after the first, without a pronounced interphase: there is no S period, since DNA replication does not occur before the second division.

Prophase II - chromosome condensation occurs, the cell center divides and the products of its division diverge to the poles of the nucleus, the nuclear membrane is destroyed, and a fission spindle is formed.

Metaphase II - univalent chromosomes (consisting of two chromatids each) are located at the “equator” (at an equal distance from the “poles” of the nucleus) in the same plane, forming the so-called metaphase plate.

Anaphase II - univalents divide and chromatids move to the poles.

Telophase II - chromosomes despiral and a nuclear envelope appears.

As a result, four haploid cells are formed from one diploid cell. In cases where meiosis is associated with gametogenesis (for example, in multicellular animals), during the development of eggs, the first and second divisions of meiosis are sharply uneven. As a result, one haploid egg and two so-called reduction bodies are formed.