The term chromosomes was first proposed by V. It is very difficult to identify chromosome bodies in the nuclei of interphase cells using morphological methods. The chromosomes themselves, as clear, dense, well-visible bodies in a light microscope, are revealed only shortly before cell division.

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Lecture #6

CHROMOSOMES

Chromosomes are the main functional auto-reproducing structure of the nucleus, in which DNA is concentrated and with which the functions of the nucleus are associated. The term "chromosomes" was first proposed by W. Waldeyer in 1888.

It is very difficult to identify chromosome bodies in the nuclei of interphase cells using morphological methods. Chromosomes proper, as clear, dense, well-visible bodies under a light microscope, are revealed only shortly before cell division. In the interphase itself, chromosomes are not seen as dense bodies, since they are in a loosened, decondensed state.

Number and morphology of chromosomes

The number of chromosomes is constant for all cells of a given animal or plant species, but varies significantly in different objects. It is not related to the level of organization of living organisms. Primitive organisms may have many chromosomes, while highly organized organisms may have much less. For example, in some radiolarians, the number of chromosomes reaches 1000-1600. The record holder among plants in terms of the number of chromosomes (about 500) is the grass fern, 308 chromosomes in the mulberry tree. Let us give examples of the quantitative content of chromosomes in some organisms: crayfish - 196, humans - 46, chimpanzees - 48, soft wheat - 42, potatoes - 18, Drosophila - 8, houseflies - 12. The smallest number of chromosomes (2) is observed in one of roundworm races, the haplopapus composite plant has only 4 chromosomes.

The size of chromosomes in different organisms varies widely. So, the length of chromosomes can vary from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa, fungi, algae, very small chromosomes in flax and sea reeds; they are so small that they are hardly visible in a light microscope. The longest chromosomes are found in some orthopteran insects, in amphibians and in lilies. The length of human chromosomes is in the range of 1.5-10 microns. The thickness of the chromosomes ranges from 0.2 to 2 microns.

The morphology of chromosomes is best studied at the time of their greatest condensation, in metaphase and at the beginning of anaphase. The chromosomes of animals and plants in this state are rod-shaped structures of different lengths with a fairly constant thickness, most of the chromosomes can easily find a zoneprimary constrictionthat divides a chromosome into two shoulder . In the region of the primary constriction is located centromere or kinetochore . It is a plate-like structure shaped like a disk. It is connected by thin fibrils with the body of the chromosome in the region of the constriction. The kinetochore is poorly understood structurally and functionally; Thus, it is known that it is one of the centers of tubulin polymerization; bundles of microtubules of the mitotic spindle grow from it, going towards the centrioles. These bundles of microtubules are involved in the movement of chromosomes to the poles of the cell during mitosis. Some chromosomes havesecondary constriction. The latter is usually located near the distal end of the chromosome and separates a small area - satellite . The dimensions and shape of the satellite are constant for each chromosome. The size and length of the secondary constrictions are also quite constant. Some secondary constrictions are specialized sections of chromosomes associated with the formation of the nucleolus (nucleolar organizers), the rest are not associated with the formation of the nucleolus and their functional role is not fully understood. Arms of chromosomes end in end segments - telomeres. The telomeric ends of chromosomes are not able to connect with other chromosomes or their fragments, in contrast to the ends of chromosomes that lack telomeric regions (as a result of breaks), which can join the same broken ends of other chromosomes.

According to the location of the primary constriction (centromere), the following are distinguished types of chromosomes:

1. metacentric- the centromere is located in the middle, the arms are equal or almost equal in length, in metaphase it acquires V-shaped;

2. submetacentric- the primary constriction is slightly shifted to one of the poles, one shoulder is slightly longer than the other, in metaphase it has L-shaped;

3. acrocentric- the centromere is strongly shifted to one of the poles, one arm is much longer than the other, does not bend in metaphase and has a rod-shaped shape;

4. telocentric- the centromere is located at the end of the chromosome, but such chromosomes are not found in nature.

Usually each chromosome has only one centromere (monocentric chromosomes), but chromosomes may occur dicentric (with 2 centromeres) andpolycentric(possessing multiple centromeres).

There are species (for example, sedges) in which the chromosomes do not contain visible centromeric regions (chromosomes with diffusely located centromeres). They're called acentric and are not able to perform an ordered movement during cell division.

Chemical composition of chromosomes

The main components of chromosomes are DNA and basic proteins (histones). DNA complex with histonesdeoxyribonucleoprotein(DNP) - makes up about 90% of the mass of both chromosomes isolated from interphase nuclei and chromosomes of dividing cells. The content of DNP is constant for each chromosome of a given type of organism.

From mineral components Calcium and magnesium ions, which give plasticity to chromosomes, are of the greatest importance, and their removal makes the chromosomes very fragile.

Ultrastructure

Each mitotic chromosome is covered on top pellicle . Inside is matrix , in which a spirally curled thread of DNP is located, 4-10 nm thick.

Elementary fibrils of DNP are the main component, which is part of the structure of mitotic and meiotic chromosomes. Therefore, in order to understand the structure of such chromosomes, it is necessary to know how these units are organized in the compact body of chromosomes. Intensive study of the ultrastructure of chromosomes began in the mid-1950s, which is associated with the introduction of electron microscopy into cytology. There are 2 hypotheses for the organization of chromosomes.

one). Uninemnaya the hypothesis states that there is only one double-stranded DNP molecule in the chromosome. This hypothesis has morphological, autoradiographic, biochemical and genetic confirmations, which makes this point of view the most popular today, since at least for a number of objects (Drosophila, yeast fungi) it is proven.

2). Polynemic the hypothesis is that several double-stranded DNP molecules are combined into a bundle - lameness , and, in turn, 2-4 chromonemes, twisting, form a chromosome. Almost all observations of chromosome polynemy were made using a light microscope on botanical objects with large chromosomes (lilies, various onions, beans, tradescantia, peony). It is possible that the phenomena of polynemy observed in the cells of higher plants are characteristic only of these objects.

Thus, it is possible that there are several different principles structural organization chromosomes of eukaryotic organisms.

In interphase cells, many sections of chromosomes are despiralized, which is associated with their functioning. They're called euchromatin. It is believed that the euchromatic regions of chromosomes are active and contain the entire main complex of genes of a cell or organism. Euchromatin is observed in the form of fine granularity or is not distinguishable at all in the nucleus of the interphase cell.

During the transition of a cell from mitosis to interphase, certain zones of various chromosomes or even entire chromosomes remain compact, spiralized and stain well. These zones are called heterochromatin . It is present in the cell in the form of large grains, lumps, flakes. Heterochromatic regions are usually located in the telomeric, centromeric, and perinucleolar regions of chromosomes, but may also be part of them. internal parts. The loss of even significant sections of heterochromatic regions of chromosomes does not lead to cell death, since they are not active and their genes temporarily or permanently do not function.

The matrix is ​​a component of the mitotic chromosomes of plants and animals, released during the despiralization of chromosomes and consisting of fibrillar and granular structures of a ribonucleoprotein nature. Perhaps the role of the matrix lies in the transfer of RNA-containing material by chromosomes, which is necessary both for the formation of nucleoli and for the restoration of the karyoplasm proper in daughter cells.

chromosome set. Karyotype

The constancy of such features as the size, location of the primary and secondary constrictions, the presence and shape of satellites, determines the morphological individuality of chromosomes. Due to this morphological individuality, in many species of animals and plants it is possible to recognize any chromosome of the set in any dividing cell.

The totality of the number, size and morphology of chromosomes is called karyotype of this type. A karyotype is like the face of a species. Even in closely related species, chromosome sets differ from each other either in the number of chromosomes, or in the size of at least one or more chromosomes, or in the shape of the chromosomes and in their structure. Therefore, the structure of the karyotype can be a taxonomic (systematic) feature that is increasingly used in the taxonomy of animals and plants.

Graphic image karyotype is called idiogram.

The number of chromosomes in mature germ cells is called haploid (denoted by n ). Somatic cells contain a double number of chromosomes - diploid set (2 n ). Cells with more than two sets of chromosomes are called polyploid (3n, 4n, 8n, etc.).

The diploid set contains paired chromosomes that are identical in shape, structure and size, but have different origin(one maternal, one paternal). They're called homologous.

In many higher diploid animals, there are one or two unpaired chromosomes in the diploid set, which differ in males and females - this genital chromosomes. The rest of the chromosomes are called autosomes . Cases are described when the male has only one sex chromosome, and the female has two.

In many fish, mammals (including humans), some amphibians (frogs of the genus Rana ), insects (beetles, Diptera, Orthoptera), the large chromosome is denoted by the letter X, and the small one by the letter Y. In these animals, in the karyotype of the female, the last pair is represented by two XX chromosomes, and in the male, by XY chromosomes.

Birds, reptiles, certain types fish, some amphibians (tailed amphibians), butterflies, the male sex has the same sex chromosomes ( WW -chromosomes), and female - different ( WZ chromosomes).

In many animals and humans, in the cells of female individuals, one of the two sex chromosomes does not function and therefore remains entirely in a spiralized state (heterochromatin). It is found in the interphase nucleus in the form of a lumpsex chromatinat the inner nuclear membrane. The sex chromosomes in the male body function both for life. If sex chromatin is found in the nuclei of the cells of the male body, this means that he has an extra X chromosome (XXY - Kleinfelter's disease). This may occur as a result of impaired spermatogenesis or oogenesis. The study of the content of sex chromatin in interphase nuclei is widely used in medicine for diagnosing human chromosomal diseases caused by imbalance of sex chromosomes.

Karyotype changes

Changes in the karyotype may be associated with a change in the number of chromosomes or with a change in their structure.

Quantitative changes in karyotype: 1) polyploidy; 2) aneuploidy.

polyploidy - This is a multiple increase in the number of chromosomes compared to the haploid. As a result, instead of ordinary diploid cells (2 n ) are formed, for example, triploid (3 n ), tetraploid (4 n ), octaploid (8 n ) cells. So, in an onion, the diploid cells of which contain 16 chromosomes, the triploid cells contain 24 chromosomes, and the tetraploid cells contain 32 chromosomes. Polyploid cells are different large sizes and increased resilience.

Polyploidy is widespread in nature, especially among plants, many species of which have arisen as a result of multiple doublings in the number of chromosomes. Majority cultivated plants eg soft wheat, multi-row barley, potatoes, cotton, most fruit and ornamental plants, are naturally occurring polyploids.

Experimentally, polyploid cells are most easily obtained by the action of an alkaloid. colchicine or other substances that disrupt mitosis. Colchicine destroys the spindle of division, due to which already doubled chromosomes remain in the plane of the equator and do not diverge towards the poles. After the termination of the action of colchicine, the chromosomes form a common nucleus, but already larger (polyploid). During subsequent divisions, the chromosomes will again double and diverge towards the poles, but a double number of them will remain. Artificially obtained polyploids are widely used in plant breeding. Varieties of triploid sugar beet, tetraploid rye, buckwheat and other crops have been created.

In animals, complete polyploidy is very rare. For example, one of the species of frogs lives in the mountains of Tibet, the population of which in the plains has a diploid chromosome set, and the high-mountain populations have a triploid, or even tetraploid.

In humans, polyploidy leads to sharply negative consequences. The birth of children with polyploidy is extremely rare. Usually, the death of the organism occurs at the embryonic stage of development (about 22.6% of all spontaneous abortions are due to polyploidy). It should be noted that triploidy occurs 3 times more often than tetraploidy. If children with triploidy syndrome are still born, then they have anomalies in the development of external and internal organs, are practically unviable and die in the first days after birth.

Somatic polyploidy is more common. So, in human liver cells with age, dividing cells become less and less, but the number of cells with a large nucleus or two nuclei increases. Determination of the amount of DNA in such cells clearly shows that they have become polyploid.

Aneuploidy - this is an increase or decrease in the number of chromosomes, not a multiple of the haploid. Aneuploid organisms, that is, organisms in which all cells contain aneuploid sets of chromosomes, are usually sterile or non-viable. As an example of aneuploidy, consider some human chromosomal diseases. Kleinfelter syndrome: in the cells of the male body there is an extra X chromosome, which leads to a general physical underdevelopment of the body, in particular its reproductive system, and mental abnormalities. Down syndrome: an extra chromosome is contained in 21 pairs, which leads to mental retardation, anomalies of internal organs; the disease is accompanied by some external signs of dementia, occurs in men and women. Turner syndrome is caused by a lack of one X chromosome in the cells of the female body; manifested in the underdevelopment of the reproductive system, infertility, external signs of dementia. With a lack of one X chromosome in the cells of the male body, a lethal outcome is observed at the embryonic stage.

Aneuploid cells constantly arise in a multicellular organism as a result of a violation of the normal course of cell division. As a rule, such cells die quickly, however, under certain pathological conditions of the body, they successfully multiply. A high percentage of aneuploid cells is characteristic, for example, of many malignant tumors in humans and animals.

Structural changes in the karyotype.Chromosomal rearrangements, or chromosomal aberrations, result from single or multiple breaks in chromosomes or chromatids. Fragments of chromosomes at break points are able to connect with each other or with fragments of other chromosomes of the set. Chromosomal aberrations are of the following types. deletion is the loss of the middle portion of a chromosome. Difishencia is the detachment of the end portion of a chromosome. Inversion - detachment of a chromosome segment, turning it 180 0 and attachment to the same chromosome; this disrupts the order of the nucleotides. duplication detachment of a segment of a chromosome and its attachment to a homologous chromosome. Translocation detachment of a segment of a chromosome and its attachment to a non-homologous chromosome.

As a result of such rearrangements, dicentric and acentric chromosomes can be formed. Large deletions, divisions and translocations dramatically change the morphology of chromosomes and are clearly visible under a microscope. Small deletions and translocations, as well as inversions, are detected by a change in the inheritance of genes localized in the regions of chromosomes affected by the rearrangement, and by a change in the behavior of chromosomes during the formation of gametes.

Structural changes in the karyotype always lead to negative consequences. For example, the "cat's cry" syndrome is caused by a chromosomal mutation (deficiency) in the 5th pair of chromosomes in humans; manifests itself in the abnormal development of the larynx, which entails a "meow" instead of a normal cry in early childhood lagging behind in physical and mental development.

Chromosome reduplication

Doubling (reduplication) of chromosomes is based on the process of DNA reduplication, i.e. the process of self-reproduction of macromolecules nucleic acids, which ensures the exact copying of genetic information and its transmission from generation to generation. DNA synthesis begins with the separation of strands, each of which serves as a template for the synthesis of a daughter strand. The products of reduplication are two daughter DNA molecules, each of which consists of one parent and one child strand. An important place among the reduplication enzymes is occupied by DNA polymerase, leading the synthesis at a rate of about 1000 nucleotides per second (in bacteria). DNA reduplication is semi-conservative, i.e. during the synthesis of two daughter DNA molecules, each of them contains one "old" and one "new" strand (this method of reduplication was proved by Watson and Crick in 1953). Fragments synthesized during reduplication on the same strand are “crosslinked” by the enzyme DNA ligase.

Reduplication involves proteins that unwind the double helix of DNA, stabilize the untwisted sections, and prevent molecular entanglement.

DNA reduplication in eukaryotes occurs more slowly (about 100 nucleotides per second), but simultaneously at many points in one DNA molecule.

Since protein synthesis occurs simultaneously with DNA replication, we can speak of chromosome reduplication. Studies conducted back in the 1950s showed that no matter how many longitudinal strands of DNA the chromosomes of organisms of different species contain, during cell division, the chromosomes behave as if they consist of two simultaneously replicating subunits. After the reduplication that takes place in the interphase, each chromosome is double, and even before the start of division in the cell, everything is ready for an even distribution of chromosomes between the daughter cells. If division does not occur after reduplication, the cell becomes polyploid. During the formation of polytene chromosomes, chromonemes are replicated, but do not diverge, which results in giant chromosomes with a huge number of chromonemes.

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One of critical issues that have worried people at all times - the origin of mankind as a biological species.

With the development of such sciences as anthropology, paleontology, archeology, genetics, new data began to emerge, leading further and further from the original theories.

Carriers of heredity inside our body

The invention of the electron microscope made it possible to rise to a previously inaccessible level of science. The discoverers of the intracellular structure were in 1963 the professors of the Stockholm University Margit and Sylvain Nass.

It turned out that living cell herself is complex organism, which includes all kinds of formations that perform various functions. It turned out that the cellular elements of mitochondria containing chromosomes, which, in turn, contain a DNA molecule, are responsible for the transmission of hereditary information. This is the result of an ancient mutation: the capture of a free bacterium by an active cell and their subsequent symbiosis. This bacterium can no longer live on its own, but its capabilities have allowed the development of organisms of disproportionate size and complexity. It is in the mitochondria that chromosomes are contained - carriers of genetic information responsible for the transmission of traits to subsequent generations.

The scheme of transmission of heredity

The carriers of sex data are chromosomes. Chromosome X - female, Y - male.

Male sex cells - spermatozoa, can be carriers of one of two types of chromosomes: X and Y. Female sex cell- the egg cell always has only one type of chromosome: X.

That is, when the male and female germ cells merge, either a set of XX chromosomes is obtained - in this case, a girl is obtained, or XY, then a boy is obtained. Boys get the Y chromosome from their father because their mother does not have it.

An important feature of the structure of human germ cells

Mitochondria are transmitted only through female reproductive cells! AT male cells- human spermatozoa have only one mitochondria, and it is destroyed after fertilization. Therefore, the genetic material contained in this structure, each subsequent generation receives only from the mother. Thus, if we imagine the resulting pyramid, the progenitor of everything modern humanity is one particular woman who lived in ancient times in Africa. Scientists gave her the code name "Mitochondrial Eve".

The first carrier of the Y chromosome was one progenitor: Adam, and all men received this chromosome from him. There are no men without a Y chromosome, but if it is, this individual cannot be a woman. Hormones are just a background to this fact.

After the discovery was made, which reduced the origin of mankind to Adam and Eve, the Church became active, claiming that science has found confirmation of the literal interpretation of the Bible. The nuance is that with an immaculate conception, the child would have nowhere to receive the Y chromosome, and without options it would be a girl.

Probabilities of building a genetic pyramid

Question: When did our root genetic ancestors live? According to the content of mitochondria in the eggs modern women, scientists place Eve approximately 150 thousand years ago. The result of the study of male germ cells gave reason to "settle" Adam only 50 thousand years ago. The reason for this discrepancy may be polygamy, since the head of the clan eliminated possible rivals. Thus, the number of straight male lines has decreased.

At the same time, women successfully passed on their genetic makeup to their daughters.

These developments are carried out by a well-known Russian scientist, molecular geneticist Professor K. V. Severinov. [S-BLOCK]

Suppose we have a population in front of us, consisting of a certain number of individuals with different options mitochondrial DNA. Not all left offspring. Someone died before they could do it. In other representatives, the offspring did not survive. And someone was lucky, and his genetic descendants began to make up the largest percentage of the population. Thus, it is this gene set that will receive a sufficient number of carriers to continue in the next generations.

It is not certain that the fittest individuals survived. There is always an important factor of chance. Some populations died completely as a result of epidemics and natural disasters. As a result of these factors, the variability disappeared: there was only one basic genetic line, but on this basis new traits constantly appeared. This is due to the fact that over time, mutations occur that change the appearance and behavior.

The study of the genetic base gives scientists the opportunity to understand how deeply and in what geographical area the roots of a particular people go. The African ethnic groups of the Bushmen and Pygmies are considered closest to the original variants.

Result of mutations

The BBC TV channel conducted an experiment: it brought black Americans to Africa. These people looked extremely happy, kissed the ground, hugged passers-by. According to prof. K. V. Severinov, this is nothing more than a farce, despite all its touchingness. Humanity has 30 thousand genes, and in a particular mitochondria there are only 25 of them. With each sexual reproduction, the set changes, and not only as a result of adaptation, but also due to any failures. One and a half to two dozen generations who lived on earth with a completely different climate and way of life inevitably affected the worldview of their descendants, despite the surviving external signs. [S-BLOCK]

Therefore, “mitochondrial Eve” is a conditional set of genetic traits, which at some point in development turned out to be more successful than other contemporary variants. Thanks to this set, all modern humanity was formed.

Mitotic supercompactization of chromatin makes it possible to study the appearance of chromosomes using light microscopy. In the first half of mitosis, they consist of two chromatids connected to each other in the region of the primary constriction ( centromeres or kinetochore) a specially organized section of the chromosome common to both sister chromatids. In the second half of mitosis, chromatids separate from each other. They form single strands. daughter chromosomes, distributed among daughter cells.

Chromosome shapes (depending on the location of the centromere and the length of the arms located on both sides of it):

1) equal-arms, or metacentric(with centromere in the middle);

2) uneven shoulders, or submetacentric(with centromere shifted to one of the ends);

3) rod-shaped, or acrocentric(with a centromere located almost at the end of the chromosome);

4) telocentric (point)- very small, the shape of which is difficult to determine.

With routine methods of staining chromosomes, they differ in shape and relative size. When using differential staining techniques, unequal fluorescence or dye distribution along the length of the chromosome is detected, strictly specific for each individual chromosome and its homologue.

Thus, each chromosome is individual not only in terms of the set of genes contained in it, but also in terms of morphology and the nature of differential staining.

Chromosome shapes:

I- telocentric, II- acrocentric, III- submetacentric, IV- metacentric;

1 - centromere, 2 - satellite, 3 - short shoulder 4 - long shoulder, 5 - chromatids

By Denver classification of chromosomes, they are arranged in pairs as their size decreases, taking into account the position of the centromere, the presence of secondary constrictions and satellites. The practice of chromosome analysis widely includes methods of differential staining of chromosomes. When processing chromosomes with special dyes in a fluorescent microscope, striation along the length of the chromosomes is visible (for the first time Kaspersson carried out in 1968, processed with acrichiniprite, now there are other methods). Each pair of chromosomes is characterized by individual striation (as well as a fingerprint). Identification of chromosomes allows you to make an idiogram of the karyotype.

Based on a number of criteria, 22 pairs of human chromosomes are classified, the sex chromosomes of the 23rd pair are distinguished separately (International Denver Classification, 1960). For identification, a morphometric method and a centromeric index are used. The classification and nomenclature of uniformly colored human chromosomes was developed at international meetings convened in Denver (1960), London (1963) and Chicago (1966). According to the recommendations of these conferences, chromosomes are arranged in decreasing order of their length. All chromosomes are divided into seven groups, which were designated by letters of the English alphabet from A to G. All pairs of chromosomes were proposed to be numbered with Arabic numerals. Group A (1-3) - the largest chromosomes. Chromosomes 1 and 3 are metacentric, 2 are submetacentric.

Group B (4-5) - two pairs of large submetacentric chromosomes.

Group C (6-12) - submetacentric chromosomes, medium size. The X chromosome is similar in size and morphology to chromosomes 6 and 7.

Group D (13-15) - acrocentric chromosomes of medium size.

Group E (16-18) - middle chromosomes (16, 17 - metacentric, 18 - acrocentric).

Group F (19-20) - small metacentrics, practically indistinguishable from each other.

Group G (21-22) - two pairs of the smallest acrocentric chromosomes. The Y-chromosome stands out as an independent one, but in terms of morphology and size it belongs to the G group.

At the same time, the chromosomes of different groups are well distinguished from each other, while within the group they cannot be distinguished, with the exception of group A. Each human chromosome contains only its own sequence of bands, which makes it possible to accurately identify each chromosome and more high precision determine in which segment the restructuring occurred. The transverse striation of chromosomes is the result of uneven condensation of hetero- (highly coiled DNA) and euchromatin (relaxed DNA) throughout the entire length of the chromosome, reflecting the order of the genes in the DNA molecule.

The human karyotype in normal and with deviations is indicated as follows:

46,XY - normal male karyotype

46, XX - normal female karyotype

47, XX+G - karyotype of a woman with an extra chromosome from group G

Currently, there are DNA markers (or probes) for many even smaller segments of almost all pairs of chromosomes. With the help of such DNA probes, it is possible to accurately assess the presence or absence of a specific, even very small, segment in the chromosome.

The ability to identify chromosomes makes it possible to detect chromosomal abnormalities, both at the level of somatic cells and primary germ cells. These anomalies occur in three cases per 100 pregnancies. Anomalies in large chromosomes are not compatible with life and cause spontaneous miscarriages at different times. Down's disease is widely known, when an extra 21st chromosome is present in the karyotype: 2n + 1 (+21). The birth rate of children with trisomy on the 21st chromosome is high, 1:500, and continues to grow due to the unfavorable ecological environment, leading to non-disjunction of 21 pairs of chromosomes.

Morphology of chromosomes

Light microscopy. In the first half of mitosis, they consist of two chromatids connected to each other in the region of the primary constriction ( centromeres or kinetochore) a specially organized section of the chromosome common to both sister chromatids. In the second half of mitosis, chromatids separate from each other. They form single strands. daughter chromosomes, distributed among daughter cells.

equilateral, or metacentric (with a centromere in the middle),

unequal, or submetacentric (with a centromere shifted to one of the ends),

rod-shaped, or acrocentric (with a centromere located almost at the end of the chromosome),

point - very small, the shape of which is difficult to determine

The totality of all structural and quantitative features of a complete set of chromosomes, characteristic of cells of a particular type of living organisms, is called a karyotype.

The karyotype of the future organism is formed in the process of fusion of two germ cells (sperm and egg). At the same time, their chromosome sets are combined. The nucleus of a mature germ cell contains a half set of chromosomes (for humans - 23). A similar single set of chromosomes, similar to that in germ cells, is called haploid and is denoted - n. When an egg is fertilized by a sperm in a new organism, a karyotype specific for this species is recreated, which includes 46 chromosomes in humans. The total composition of the chromosomes of an ordinary somatic cell is diploid (2n). In a diploid set, each chromosome has another paired chromosome similar in size and location of the centromere. Such chromosomes are called homologous. Homologous chromosomes are not only similar to each other, but also contain genes responsible for the same traits.

The karyotype of a woman normally contains two X chromosomes, and it can be written - 46, XX. The male karyotype includes X and Y chromosomes (46, XY). All other 22 pairs of chromosomes are called autosomes.
Autosome groups:

Group A includes 3 pairs of the longest chromosomes (1, 2, 3rd);

Group B combines 2 pairs of large submetacentric chromosomes (4 and 5).

Group C, which includes 7 pairs of medium submetacentric autosomes (from the 6th to the 12th). According to morphological features, the X chromosome is difficult to distinguish from this group.

The middle acrocentric chromosomes of the 13th, 14th and 15th pairs are in group D.

Three pairs of small submetacentric chromosomes make up group E (16, 17 and 18).

The smallest metacentric chromosomes (19 and 20) make up the F group.

The 21st and 22nd pairs of short acrocentric chromosomes are included in the G group. The Y chromosome is morphologically very similar to the autosomes of this group.

23. The chromosome theory of T. Morgan.

Chromosomal theory of heredity- the theory according to which the transfer of hereditary information in a number of generations is associated with the transfer of chromosomes, in which genes are located in a certain and linear sequence.

1. The material carriers of heredity - the genes are located in the chromosomes, are located in them linearly at a certain distance from each other.
2. Genes located on the same chromosome belong to the same linkage group. The number of linkage groups corresponds to the haploid number of chromosomes.
3. Traits whose genes are on the same chromosome are inherited in a linked fashion.
4. In the offspring of heterozygous parents, new combinations of genes located on the same pair of chromosomes can occur as a result of crossing over during meiosis.
5. The frequency of crossing over, determined by the percentage of crossover individuals, depends on the distance between genes.
6. Based on the linear arrangement of genes on a chromosome and the frequency of crossing over as an indicator of the distance between genes, maps of chromosomes can be built.

The works of T. Morgan and his collaborators not only confirmed the importance

chromosomes as the main carriers of the hereditary material represented by individual genes, but also established the linearity of their location along the length of the chromosome.

The proof of the connection of the material substrate of heredity and variability with chromosomes was, on the one hand, the strict correspondence of the patterns of inheritance of traits discovered by G. Mendel to the behavior of chromosomes during mitosis, during meiosis and fertilization. On the other hand, T. Morgan's laboratory was found special type inheritance of traits, which was well explained by the relationship of the corresponding genes to the X chromosome. We are talking about sex-linked inheritance of eye color in Drosophila.

The concept of chromosomes as carriers of gene complexes was expressed on the basis of the observation of the linked inheritance of a number of parental traits with each other during their transmission in a number of generations. Such a linkage of non-alternative traits was explained by the placement of the corresponding genes in one chromosome, which is a fairly stable structure that preserves the composition of genes in a number of generations of cells and organisms.

According to the chromosomal theory of heredity, the set of genes

belonging to the same chromosome, forms clutch group. Each chromosome is unique in the set of genes it contains. The number of linkage groups in the hereditary material of organisms of a given species is thus determined by the number of chromosomes in the haploid set of their germ cells. During fertilization, a diploid set is formed, in which each linkage group is represented by two variants - paternal and maternal chromosomes, carrying the original sets of alleles of the corresponding gene complex.

The idea of ​​the linearity of the location of genes in each chromosome arose on the basis of the observation of a frequently occurring recombination(interchange) between maternal and paternal complexes of genes enclosed in homologous chromosomes. It was found that the frequency

recombination is characterized by a certain constancy for each pair of genes in a given linkage group and is different for different pairs. This observation made it possible to suggest a relationship between the frequency of recombination and the sequence of genes in the chromosome and the process of crossing over occurring between homologs in prophase I of meiosis (see Section 3.6.2.3).

The idea of ​​a linear distribution of genes explained well the dependence of the frequency of recombination on the distance between them in the chromosome.

The discovery of linked inheritance of non-alternative traits formed the basis for the development of a methodology for constructing genetic maps chromosomes using the hybridological method of genetic analysis.

Thus, at the beginning of the XX century. the role of chromosomes as the main carriers of hereditary material in the eukaryotic cell was irrefutably proven. This was confirmed by studying the chemical composition of chromosomes.

24. Division of somatic cells. Har-ka phases of mitosis.

The division of a somatic cell and its nucleus (mitosis) is accompanied by complex multiphase transformations of chromosomes: 1) in the process of mitosis, each chromosome is duplicated on the basis of complementary replication of the DNA molecule with the formation of two sister filamentous copies (chromatids) connected at the centromere; 2) subsequently, sister chromatids are separated and equivalently distributed over the nuclei of daughter cells.

As a result, the identity is maintained in dividing somatic cells chromosome set and genetic material.

Separately, it should be said about neurons - highly differentiated postmitotic cells that do not undergo cell divisions throughout their lives. The compensatory capabilities of neurons in response to the action of damaging factors are limited by intracellular regeneration and DNA repair in the non-dividing nucleus, which largely determines the specificity of neuropathological processes of hereditary and non-hereditary nature.

Mitosis- complex division of the cell nucleus, the biological significance of which lies in the exact identical distribution of the daughter chromosomes with the genetic information contained in them between the nuclei of the daughter cells, as a result of this division, the nuclei of the daughter cells have a set of chromosomes identical in quantity and quality to that in the mother cell.

Chromosomes- the main substrate of heredity, they are the only structure for which an independent ability to reduplication has been proven. All other organelles of the cell capable of reduplication carry it out under the control of the nucleus. In this regard, it is important to maintain the constancy of the number of chromosomes and evenly distribute them among the daughter cells, which is achieved by the entire mechanism of mitosis. This method of division in plant cells was discovered in 1874 by the Russian botanist I. D. Chistyakov, and in animal cells - in 1878 by the Russian histologist P. I. Peremezhko (1833-1894).

In the process of mitosis (Fig. 2.15), five phases proceed in succession: prophase, prometaphase, metaphase, anaphase and telophase. These phases, immediately following each other, are connected by imperceptible transitions. Each previous condition leads to the next one.

In a cell entering into division, the chromosomes take the form of a ball of many thin, weakly spiralized threads. At this time, each chromosome consists of two sister chromatids. The formation of chromatids occurs according to the matrix principle in the S-period of the mitotic cycle as a result of DNA replication.

At the beginning prophase, and sometimes even before its onset, the centriole is divided into two, and they diverge towards the poles of the nucleus. At the same time, the chromosomes undergo a process of twisting (spiralization), as a result of which they are significantly shortened and thickened. Chromatids move away from each other somewhat, remaining connected only by centromeres. A gap appears between the chromatids. By the end of prophase, a radiant figure forms around the centrioles in animal cells. Most plant cells do not have centrioles.

By the end of prophase, the nucleoli disappear, the nuclear membrane dissolves from the lysosomes under the action of enzymes, and the chromosomes are immersed in the cytoplasm. At the same time, an achromatic figure appears, which consists of threads stretching from the poles of the cell (if there are centrioles, then from them). Achromatic filaments are attached to the centromeres of chromosomes. A characteristic figure resembling a spindle is formed. Electron microscopic studies have shown that the threads of the spindle are tubules, tubules.

In prometaphase in the center of the cell is the cytoplasm, which has a slight viscosity. The chromosomes immersed in it are sent to the equator of the cell.

AT metaphase Chromosomes are in an ordered state at the equator. All chromosomes are clearly visible, due to which the study of karyotypes (counting the number, studying the shapes of chromosomes) is carried out precisely at this stage. At this time, each chromosome consists of two chromatids, the ends of which have diverged. Therefore, on metaphase plates (and idiograms from metaphase chromosomes), the chromosomes are A-shaped. The study of chromosomes is carried out precisely at this stage.

AT anaphase each chromosome splits longitudinally along its entire length, including in the region of the centromere, more precisely, there is a divergence of chromatids, which then become sister, or daughter, chromosomes. They have a rod-shaped shape, curved in the region of the primary constriction. The spindle threads shorten, move towards the poles, and behind them, the daughter chromosomes begin to diverge towards the poles. Their divergence is carried out quickly and all at the same time, as "on command". This is well shown by film frames of dividing cells. Violent processes also occur in the cytoplasm, which resembles a boiling liquid on film.

AT telophase daughter chromosomes reach the poles. After that, the chromosomes despiralize, lose their clear outlines, and form around them. nuclear membranes. The nucleus acquires a structure similar to the interphase of the mother cell. The nucleolus is restored.

25. Human germ cells, their structure. Types of structure of the oocyte.

To participate in sexual reproduction in parental organisms are produced gametes - cells specialized to provide generative function.

The fusion of maternal and paternal gametes results in

emergence zygotes - cell, which is a daughter individual at the first, earliest stage of individual development.

In some organisms, a zygote is formed as a result of the union of gametes that are indistinguishable in structure. In such cases, one speaks of isogamy.

In most species, according to structural and functional characteristics, germ cells are divided into maternal(eggs) and paternal(spermatozoa). Normally, eggs and sperm are produced different organisms- female (females) and male (males). In the division of gametes into eggs and spermatozoa, and individuals into females and males, the phenomenon sexual dimorphism(Fig. 5.1; 5.2). Its presence in nature reflects the differences in the tasks solved in the process of sexual reproduction by the male or female gamete, male or female.

Human male germ cells - spermatozoa, or sperm, about 70 microns long, have a head, neck and tail.

The spermatozoon is covered with a cytolemma, which in the anterior section contains a receptor that provides recognition of egg receptors.

The head of the spermatozoon includes a small dense nucleus with a haploid set of chromosomes. The anterior half of the nucleus is covered with a flat sac that forms the cap of the spermatozoon. The acrosome is located in it (from the Greek asgo - top, soma - body),

consisting of a modified Golgi complex. The acrosome contains a set of enzymes. In the nucleus of a human spermatozoon occupying

the bulk of the head contains 23 chromosomes, one of which is sexual (X or Y), the rest are autosomes. The tail section of the spermatozoon consists of an intermediate, main and terminal parts.

When examining spermatozoons under an electron microscope, it was found that the protoplasm of its head has not a colloidal, but a liquid crystalline state. This ensures the resistance of spermatozoons to adverse influences. external environment. For example, they are less damaged by ionizing radiation compared to immature germ cells.

All spermatozoa carry the same name (negative) electric charge which prevents them from sticking together.

A person releases about 200 million spermatozoa

Oocytes or oocytes(from lat. ovum - egg), ripen in an immeasurably smaller amount than spermatozoa. In a woman during the sexual cycle of 24-28 days), as a rule, one egg matures. Thus, during the childbearing period, about 400 mature eggs are formed.

The release of an oocyte from an ovary is called ovulation. The oocyte that comes out of the ovary is surrounded by a crown of follicular cells, the number of which reaches 3-4 thousand. It is picked up by the fringes of the fallopian tube (oviduct) and moves along it. Here the maturation of the germ cell ends. Egg has a spherical shape, larger than that of sperm, the volume of the cytoplasm, does not have the ability to move independently.

Structure. The human egg has a diameter of about 130 microns. Adjacent to the cytolemma is a shiny, or transparent, zone and then a layer of follicular cells. The nucleus of the female germ cell has a haploid set of chromosomes with an X-sex chromosome, a well-defined nucleolus, and there are many pore complexes in the karyolemma. During the period of oocyte growth, intensive processes of mRNA and rRNA synthesis take place in the nucleus.

In the cytoplasm, the protein synthesis apparatus (endoplasmic reticulum, ribosomes) and the Golgi apparatus are developed. The number of mitochondria is moderate, they are located near the yolk nucleus, where there is an intensive synthesis of the yolk, the cell center is absent. The Golgi apparatus in the early stages of development is located near the nucleus, and in the process of maturation of the egg, it shifts to the periphery of the cytoplasm.

Oocytes are covered, which perform a protective function, provide the necessary type of metabolism, in placental mammals they serve to introduce the embryo into the uterine wall, and also perform other functions.

The cytolemma of the egg has microvilli located between the processes of the follicular cells. Follicular cells perform trophic and protective functions.

Oocytes are much larger than somatic cells. The intracellular structure of the cytoplasm in them is specific for each animal species, which ensures specific (and often individual) developmental features. The eggs contain a number of substances necessary for the development of the embryo. These include nutrient material (yolk).

Oocyte classification is based on the signs of the presence, quantity and distribution of the yolk (lecithos), which is a protein-lipid inclusion in the cytoplasm used to nourish the embryo.

There are yolkless (alecital), low yolk (oligolecital), medium yolk (mesolecithal), multiyolk (polylecital) eggs.

In humans, the presence of a small amount of yolk in the egg is due to the development of the embryo in the mother's body.

The polarity of the oocytes. With a small amount of yolk in the egg, it is usually distributed evenly in the cytoplasm and the nucleus is located approximately in the center. These eggs are called isolecithal(from Greek. isos - equal). Most vertebrates have a lot of yolk, and it is unevenly distributed in the cytoplasm of the egg. it anisolecithal cells. The bulk of the yolk accumulates at one of the poles of the cell - vegetative pole. These eggs are called telolecital(from Greek. telos - the end). The opposite pole, to which the active cytoplasm free from yolk is pushed, is called animal. If the yolk is still immersed in the cytoplasm and is not isolated from it as a separate fraction, as in sturgeons and amphibians, the eggs are called moderately telolecithal. If the yolk is completely separated from the cytoplasm, as in amniotes, then this sharply telolecithal eggs.

26. Reproduction of the living. Classification of methods of reproduction.

Reproduction, or reproduction, is one of the main properties that characterize life. Reproduction refers to the ability of organisms to produce their own kind. The phenomenon of reproduction is closely connected with one of the features that characterize life - discreteness. As you know, a holistic organism consists of discrete units - cells. The life of almost all cells is shorter than the life of an individual, therefore the existence of each individual is maintained by cell reproduction. Each type of organisms is also discrete, that is, it consists of separate individuals. Each of them is mortal. The existence of the species is supported by reproduction (reproduction) of individuals. Consequently, reproduction is a necessary condition for the existence of a species and the continuity of successive generations within a species. The classification of forms of reproduction is based on the type of cell division: mitotic (asexual) and meiotic (sexual). Reproduction forms can be represented as the following scheme

Asexual reproduction. In unicellular eukaryotes, this is a division based on mitosis, in prokaryotes, the division of the nucleoid, and in multicellular organisms, vegetative (Latin vegetatio - grow) reproduction, i.e., parts of the body or a group of somatic cells.

Asexual reproduction of unicellular organisms. In unicellular plants and animals, the following forms are distinguished asexual reproduction: division, endogony, multiple division (schizogony) and budding.

Division characteristic of unicellular (amoeba, flagella, ciliates). First, the mitotic division of the nucleus occurs, and then an ever-deepening constriction occurs in the cytoplasm. In this case, daughter cells receive an equal amount of information. Organelles are usually evenly distributed. In a number of cases, it has been found that division is preceded by their doubling. After division, the daughter individuals grow and, having reached the size of the maternal organism, proceed to a new division.

Endogony - internal budding. With the formation of two daughter individuals - endodyogony - the mother gives only two offspring (this is how toxoplasma reproduces), but there may be multiple internal budding, which will lead to schizogony.

schizogony , or multiple division, is a form of reproduction that has developed from the previous one. It is also found in unicellular organisms, for example, in the causative agent of malaria - malarial plasmodium. With schizogony, multiple nuclear division occurs without cytokinesis, and then the entire cytoplasm is divided into particles that separate around the nuclei. One cell produces many daughter cells. This form of reproduction usually alternates with sexual reproduction.

budding consists in the fact that a small tubercle initially forms on the mother cell, containing the daughter nucleus, or nucleoid. The kidney grows, reaches the size of the mother and then separates from it. This form of reproduction is observed in bacteria, yeast fungi, and from unicellular animals - in sucking ciliates.

sporulation found in animals belonging to the type of protozoa, the class of sporozoans. Spore is one of the stages life cycle, serving for reproduction, it consists of a cell covered with a membrane that protects against adverse conditions external environment. Some bacteria after the sexual process are able to form spores. Bacterial spores serve not for reproduction, but for experiencing adverse conditions and differ in their biological significance from spores of protozoa and multicellular plants.

Vegetative propagation multicellular glands During vegetative reproduction in multicellular animals, a new organism is formed from a group of cells that separates from the parent organism. Vegetative reproduction occurs only in the most primitive of multicellular animals: sponges, some coelenterates, flat and annelids.

In sponges and hydra, due to the reproduction of groups of cells on the body, protrusions (kidneys). The kidney includes ecto- and endoderm cells. In hydra, the kidney gradually increases, tentacles form on it, and, finally, it separates from the mother. The ciliary and annelids are divided by constrictions into several parts; missing organs are restored in each of them. Thus, a chain of individuals can be formed. In some intestinal cavities, reproduction occurs by strobilation, which consists in the fact that the polyploid organism grows quite intensively and, upon reaching known sizes begins to divide by transverse constrictions into daughter individuals. At this time, the polyp resembles a stack of plates. Formed individuals - jellyfish come off and begin independent life. In many species (for example, coelenterates), the vegetative form of reproduction alternates with sexual reproduction.

sexual reproduction

Sexual process. Sexual reproduction is characterized by the presence of a sexual process that provides the exchange hereditary information and creates conditions for the emergence of hereditary variability. As a rule, two individuals participate in it - female and male, which form haploid female and male sex cells - gametes. As a result of fertilization, i.e., the fusion of female and male gametes, a diploid zygote is formed with a new combination of hereditary traits, which becomes the ancestor of a new organism.

Sexual reproduction, compared with asexual reproduction, ensures the appearance of hereditarily more diverse offspring. Forms of the sexual process are conjugation and copulation.

Conjugation- a peculiar form of the sexual process, in which fertilization occurs by mutual exchange of migrating nuclei moving from one cell to another along the cytoplasmic bridge formed by two individuals. During conjugation, there is usually no increase in the number of individuals, but there is an exchange of genetic material between cells, which ensures the recombination of hereditary properties. Conjugation is typical for ciliary protozoa (for example, ciliates), some algae (spirogyra).

Copulation (gametogamy)- a form of the sexual process in which two sex-different cells - gametes - merge and form a zygote. In this case, the gamete nuclei form one zygote nucleus.

There are the following main forms of gametogamy: isogamy, anisogamy and oogamy.

At isogamy mobile, morphologically identical gametes are formed, but physiologically they differ into “male” and “female”. Isogamy is found in many algae.

At anisogamy (heterogamy) mobile, morphologically and physiologically different gametes are formed. This type sexual process is characteristic of many algae.

When oogamy gametes are very different from each other. The female gamete is a large immobile egg, containing a large supply of nutrients. Male gametes - spermatozoa- small, most often mobile cells that move with the help of one or more flagella. Seed plants have male gametes sperm- do not have flagella and are delivered to the egg using a pollen tube. Oogamy is characteristic of animals, higher plants, and many fungi.

27. Ovogenesis and spermatogenesis.

spermatogenesis. The testis consists of numerous tubules. A transverse section through the tubule shows that it has several layers of cells. They represent successive stages in the development of spermatozoons.

The outer layer (reproduction zone) is spermatogonia- round cells they have a relatively large nucleus and a significant amount of cytoplasm. During the period embryonic development and after birth, before puberty, the spermatogonia divide by mitosis, thereby increasing the number of these cells and the testis itself. The period of intense division is called the period breeding

After the onset of puberty, part of the spermatogonia also continues to divide mitotically and form the same cells, but some of them move to the next growth zone located closer to the lumen of the tubule. Here there is a significant increase in cell size due to an increase in the amount of cytoplasm. At this stage they are called primary spermatocytes.

The third stage in the development of male gametes is called ripening period. During this period, two rapidly advancing divisions occur one after the other. From each primary spermatocyte, two secondary spermatocyte and then four spermatids, having an oval shape and much smaller sizes. Cell division during the maturation period is accompanied by a rearrangement of the chromosome apparatus (meiosis occurs; see below). Spermatids move to the zone closest to the lumen of the tubules, where they form spermatozoa.

In most wild animals, spermatogenesis occurs only at certain times of the year. In the intervals between them, the tubules of the testes contain only spermatogonia. But in humans and most domestic animals, spermatogenesis occurs throughout the year.

Ovogenesis. The phases of oogenesis are comparable to those of spermatogenesis. This process also has breeding season when intensively divided oogonia- small cells with a relatively large nucleus and a small amount of cytoplasm. In mammals and humans, this period ends before birth. formed by this time primary oocytes remain unchanged for many years. With the onset of puberty, periodically individual oocytes enter a period growth cells increase, yolk, fat, pigments accumulate in them.

In the cytoplasm of the cell, its organelles and membranes, complex morphological and biochemical transformations take place. Each oocyte is surrounded by small follicular cells that provide its nutrition.

Next comes ripening period. during which two successive divisions occur associated with the transformation of the chromosome apparatus (meiosis). In addition, these divisions are accompanied by an uneven division of the cytoplasm between daughter cells. When the primary oocyte divides, one large cell is formed - secondary oocyte, containing almost all of the cytoplasm, and a small cell called primary polocyte. At the second division of maturation, the cytoplasm is again unevenly distributed. One large secondary oocyte and a secondary polocyte are formed. At this time, the primary polocyte can also divide into two cells. Thus, one secondary oocyte and three polocytes (reduction bodies) are formed from one primary oocyte. Further, an egg is formed from the secondary oocyte, and the polocytes dissolve or remain on the surface of the egg, but do not take part in further development. The uneven distribution of the cytoplasm provides the egg cell with a significant amount of cytoplasm and nutrients that will be required in the future for the development of the embryo.

In mammals and humans, the periods of reproduction and growth of eggs take place in follicles (Fig. 3.5). A mature follicle is filled with fluid, inside it is an egg cell. During ovulation, the wall of the follicle bursts, the egg enters the abdominal cavity and then, as a rule, into the fallopian tubes. The period of egg maturation takes place in the tubes, and fertilization takes place here.

In many animals, ovogenesis and egg maturation occur only during certain seasons of the year. In women, one egg usually matures monthly, and for the entire period of puberty - about 400. maturation and give the egg cells. This means that various unfavorable factors to which the female organism is exposed during life can affect their further development; toxic substances(including nicotine and alcohol) that enter the body can penetrate into oocytes and further cause disturbances in the normal development of future offspring.

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 of performance by the cell of its functions and periods of rest. At this time, the further cell fate is uncertain: the cell may begin to divide (enter mitosis) or begin to prepare to perform specific functions.

Chromosomes(ancient Greek khr^tsa - color and agar - body) - nucleoprotein structures in the nucleus of a eukaryotic cell, visible during cell division (mitosis or meiosis). These formations are a high degree chromatin condensation. When stretched, the length of a chromosome can be up to 5 cm.

In the early interphase (phase G () in each of the future chromosomes there is one DNA molecule. In the synthesis phase (S) DNA doubles. In the late interphase (phase G-,), each chromosome consists of two identical DNA molecules interconnected in the area centromeric sequence.

Before the division of the cell nucleus begins, the chromosome begins to spiralize, or pack, forming thick chromatin threads, or chromatids, each of which contains one identical DNA molecule. The significant thickness of the chromosome at the metaphase stage allows you to finally see it in a light microscope (Fig. 3.2).

For general acquaintance and better understanding of the following material in Fig. 3.3 shows diagrams of mitosis and meiosis.

Rice. 3.2.

Rice. 3.3.

cell karyotype- a set of features of a complete set of chromosomes, inherent in the body, mind or cell line. A karyogram is a visual representation of the complete set of chromosomes (Fig. 3.4).

Compilation of a karyotype (Fig. 3.4) is carried out as follows. For dividing cells with chromosomes, an image is obtained (photograph, etc.), and then the homologous chromosomes in the image are paired and lined up in size.

Chromosomes are treated with special dyes that stain eu- and heterochromatia differently (loose and densely packed chromatin) - Giemsa stain on i-bands and etc.

There are two international classifications of human autosomes (non-sex chromosomes).

Denver classification(1960, USA) - individual principle estimates of autosomes by their size and shape (groups from A to O; Fig. 3.4).

Parisian classification(1971) - autosomes are identified by eu- and heterochromic regions specific for each pair (staining; stripes).

Number of chromosomes in karyotypes.

At first, they limited themselves to the study of plant and insect chromosomes with a small number of large

Rice. 3.4. Human cell karyotype according to the Denver classification

chromosomes. Mammals usually have a significant number of relatively small chromosomes.

From the 1920s to the mid 1950s. it was widely believed that a person has 48 chromosomes (in the beginning, only 37 chromosomes were found).

Until the 1950s it was believed that Caucasoids (representatives of the white race) have 48 chromosomes, and Mongoloids have a set of X0 (without a male Y-chromosome!) And 47 chromosomes (Guttmap B. et al., 2004). However, in 1956, Tijo and Levan (J.-H. Tjio, A. Levan) from Sweden proved that the true number of normal chromosomes in humans is 46.

In primates, the number of chromosomes is comparable to the number of chromosomes in humans (rhesus monkeys have 42; chimpanzees, gorillas and orangutans have 48).