What is called a karyotype. What is a karyotype. His definition. Karyotyping is a necessary examination for infertility

Karyotype definition

The appearance of chromosomes changes significantly during the cell cycle: during the interphase, the chromosomes are localized in the nucleus, as a rule, despiralized and difficult to observe, therefore, cells in one of the stages of their division, the metaphase of mitosis, are used to determine the karyotype.

Procedure for determining the karyotype

For the procedure for determining the karyotype, any population of dividing cells can be used. To determine the human karyotype, as a rule, peripheral blood lymphocytes are used, the transition of which from the G0 resting stage to proliferation is provoked by the addition of the phytohemagglutinin mitogen. Bone marrow cells or a primary culture of skin fibroblasts can also be used to determine the karyotype. To increase the number of cells at the metaphase stage, colchicine or nocadazole are added to the cell culture shortly before fixation, which block the formation of microtubules, thereby preventing the chromatids from spreading to the poles of cell division and the completion of mitosis.

After fixation, preparations of metaphase chromosomes are stained and photographed; from micrographs form the so-called systematized karyotype- numbered set of pairs homologous chromosomes, the images of chromosomes are oriented vertically with their short arms up, their numbering is done in descending order of size, a pair of sex chromosomes is placed at the end of the set (see Fig. 1).

Historically, the first non-detailed karyotypes, which allowed classification by chromosome morphology, were obtained by Romanovsky-Giemsa staining, however, further detailing of the structure of chromosomes in karyotypes became possible with the advent of differential staining techniques for chromosomes. The most commonly used technique in medical genetics is the G-differential staining of chromosomes.

Classical and spectral karyotypes

Rice. Fig. 2. An example of translocation determination by a complex of transverse marks (stripes, classical karyotype) and by a spectrum of regions (color, spectral karyotype).

To obtain a classical karyotype, chromosomes are stained with various dyes or their mixtures: due to differences in the binding of the dye to different parts of the chromosomes, staining occurs unevenly and a characteristic banded structure is formed (a complex of transverse marks, eng. banding), reflecting the linear heterogeneity of the chromosome and specific for homologous pairs of chromosomes and their regions (with the exception of polymorphic regions, various allelic variants of genes are localized). The first chromosome staining method to obtain such highly detailed images was developed by the Swedish cytologist Kaspersson (Q-staining). Other stains are also used, such techniques have been common name differential staining of chromosomes:

• Q-staining- staining according to Kaspersson with acrichin mustard with a study under a fluorescent microscope. Most often used for the study of Y chromosomes (quick determination of genetic sex, detection of translocations between X and Y chromosomes or between Y chromosome and autosomes, screening for mosaicism involving Y chromosomes)
• G-staining- modified staining according to Romanovsky - Giemsa. The sensitivity is higher than that of Q-staining, therefore it is used as a standard method for cytogenetic analysis. Used to detect small aberrations and marker chromosomes (segmented differently than normal homologous chromosomes)
• R-staining- acridine orange and similar dyes are used, while staining parts of the chromosomes that are insensitive to G-staining. Used to reveal details of homologous G- or Q-negative regions of sister chromatids or homologous chromosomes.
• C-staining- used to analyze the centromeric regions of chromosomes containing constitutive heterochromatin and the variable distal part of the Y chromosome.
• T-staining- used to analyze telomeric regions of chromosomes.

Recently, the technique of the so-called. spectral karyotyping (fluorescent hybridization in situ, English Fluorescence in situ hybridization, FISH), consisting in the staining of chromosomes with a set of fluorescent dyes that bind to specific regions of the chromosomes. As a result of such staining, homologous pairs of chromosomes acquire identical spectral characteristics, which not only greatly facilitates the identification of such pairs, but also facilitates the detection of interchromosomal translocations, that is, movements of sections between chromosomes - translocated sections have a spectrum that differs from the spectrum of the rest of the chromosome.

Karyotype analysis

Comparison of complexes of transverse marks in classical karyotyping or regions with specific spectral characteristics makes it possible to identify both homologous chromosomes and their individual regions, which makes it possible to determine in detail chromosomal aberrations - intra- and interchromosomal rearrangements, accompanied by a violation of the order of chromosome fragments (deletions, duplications, inversions, translocations). Such an analysis has great importance in medical practice, making it possible to diagnose a number of chromosomal diseases caused by both gross violations of karyotypes (violation of the number of chromosomes) and a violation of the chromosomal structure or a plurality of cell karyotypes in the body (mosaicism).

Nomenclature

Fig.3. Karyotype 46,XY,t(1;3)(p21;q21), del(9)(q22): translocation (transfer of a fragment) between the 1st and 3rd chromosomes, deletion (loss of a section) of the 9th chromosome are shown. The marking of chromosome regions is given both by complexes of transverse marks (classical karyotyping, stripes) and by the fluorescence spectrum (color, spectral karyotyping).

To systematize cytogenetic descriptions, the International System for Cytogenetic Nomenclature (ISCN) was developed, based on differential staining of chromosomes and allowing a detailed description of individual chromosomes and their regions. The entry has the following format:

[chromosome number] [arm] [site number].[band number]

the long arm of a chromosome is denoted by the letter q, short - letter p, chromosomal aberrations are denoted by additional symbols.

Thus, the 2nd band of the 15th section of the short arm of the 5th chromosome is written as 5p15.2.

For the karyotype, an entry in the ISCN 1995 system is used, which has the following format:

[number of chromosomes], [sex chromosomes], [features].

Abnormal karyotypes and chromosomal diseases

Normal human karyotypes are 46,XX (female) and 46,XY (male). Violations of the normal karyotype in humans occur in the early stages of the development of the organism: if such a violation occurs during gametogenesis, in which the germ cells of the parents are produced, the karyotype of the zygote formed during their fusion is also impaired. With further division of such a zygote, all cells of the embryo and the organism that developed from it have the same abnormal karyotype.

However, karyotype disorders can also occur in the early stages of zygote fragmentation, the organism that has developed from such a zygote contains several cell lines (cell clones) with different karyotypes, such a plurality of karyotypes of the whole organism or its individual organs is called mosaicism.

As a rule, karyotype disorders in humans are accompanied by multiple malformations; most of these anomalies are incompatible with life and lead to spontaneous abortions in the early stages of pregnancy. However, a fairly large number of fetuses (~2.5%) with abnormal karyotypes endure until the end of pregnancy.

Some human diseases caused by karyotype abnormalities,

Karyotypes	Disease	A comment
47,XXY; 48,XXXY;	Klinefelter syndrome	X chromosome polysomy in men
45X0; 45X0/46XX; 45,X/46,XY; 46.X iso (Xq)	Shereshevsky-Turner syndrome	Monosomy on the X chromosome, including mosaicism
47,XXX; 48,XXXX; 49,XXXXXX	Polysomy on the X chromosome	Most common trisomy X
47,XX, 21+; 47,XY, 21+	Down syndrome	Trisomy on the 21st chromosome
47,XX, 18+; 47,XY, 18+	Edwards syndrome	Trisomy on the 18th chromosome
47,XX, 13+; 47,XY, 13+	Patau Syndrome	Trisomy on the 13th chromosome
46,XX, 5p-	crying cat syndrome	deletion of the short arm of the 5th chromosome
46 XX or XY, 15r-.	Prader-Willi syndrome	Anomaly 15 chromosomes

Karyotype of some biological species

Each species of organisms has a characteristic and constant set of chromosomes. The number of diploid chromosomes varies from organism to organism:

Hominid karyotype

see also

• Theory of heredity

Notes

Links

• Barbara J. Trask, Human Cytogenetics: 46 Chromosomes, 46 Years and Counting. Nature reviews, October 2002, vol. 3, pp. 769-778 (full text of the review on the site of the author's laboratory at the Fred Hutchinson Cancer Research Center)

Chromosomes
Main	Karyotype Ploidy Meiosis Mitosis
Classification	Autosome Gonosome Microchromosome Polytene chromosome Lampbrush chromosomes
Structure	Chromatids Cohesin Sister chromatid exchange

Karyotype , a set of features of the chromosome set, characteristic of each biological species. These signs include:

• number,
• size and shape of chromosomes
• position on the chromosomes of the primary constriction (centromere),
• the presence of secondary constrictions,
• alternation of heterochromatic and euchromatic regions, etc.

The karyotype serves as a "passport" of the species, reliably distinguishing it from the karyotypes of other species. The constancy of all signs The species karyotype is provided by the exact processes of distribution of chromosomes among daughter cells in mitosis and meiosis (these processes can be disturbed by chromosomal mutations).

A karyotype is a complete set of chromosomes in human cells. The norm for the content of chromosomes in somatic (non-embryonic) human cells is 46 chromosomes, organized into 23 pairs. Each pair consists of one chromosome from the mother and one from the father.

Appearance of chromosomes changes significantly during the cell cycle: during the interphase, the chromosomes are localized in the nucleus, as a rule, despiralized and difficult to observe, therefore, cells in one of the stages of their division are used to determine the karyotype - metaphase of mitosis.

Chromosomes in the light microscope at the stage of metaphase are DNA molecules packaged with specific proteins dense supercoiled rod-shaped structures. Thus, a large number of chromosomes are packed into a small volume and placed in a relatively small volume of the cell nucleus. The arrangement of chromosomes seen under a microscope is photographed and assembled from several photographs. systematized karyotype- a numbered set of chromosome pairs of homologous chromosomes. In this case, the images of chromosomes are oriented vertically, with short arms up, and their numbering is carried out in descending order of size. A pair of sex chromosomes (X and Y for a man, X and X for a woman) are placed at the very end of the image of the set of chromosomes.

When studying the karyotype, which is usually carried out at the metaphase stage of the cell cycle, the following are used:

• microphotography,
• special methods of coloring chromosomes and other methods.

To obtain a classical karyotype, chromosomes are stained with various dyes or their mixtures: due to differences in the binding of the dye to different parts of the chromosomes, staining occurs unevenly and forms characteristic banded structure(a complex of transverse marks), reflecting the linear heterogeneity of the chromosome and specific for homologous pairs of chromosomes and their regions (with the exception of polymorphic regions, various allelic variants of genes are localized). The first chromosome staining method to obtain such highly detailed images was developed by the Swedish cytologist Kaspersson (Q-staining). Other dyes are also used, such techniques have received the general name differential staining of chromosomes.

Types of differential staining of chromosomes

• G-staining - modified staining according to Romanovsky - Giemsa. The sensitivity is higher than that of Q-staining, therefore it is used as a standard method for cytogenetic analysis. It is used to detect small aberrations and marker chromosomes (segmented differently than normal homologous chromosomes).
• Q-staining - Kaspersson staining with acryquin mustard with a study under a fluorescent microscope. Most often used to study Y chromosomes (quick determination of genetic sex, detection of translocations between X and Y chromosomes or between Y chromosome and autosomes, screening for mosaicism involving Y chromosomes).
• R-staining - acridine orange and similar dyes are used, while staining parts of the chromosomes that are insensitive to G-staining. Used to reveal details of homologous G- or Q-negative regions of sister chromatids, or homologous chromosomes.
• C-staining - used to analyze the centromeric regions of chromosomes containing constitutive heterochromatin and the variable distal part of the Y chromosome.
• T-staining - used to analyze the telomeric regions of chromosomes.

Recently, the so-called spectral karyotyping (fluorescent hybridization FISH), which consists in staining chromosomes with a set of fluorescent dyes that bind to specific regions of chromosomes. As a result of such staining, homologous pairs of chromosomes acquire identical spectral characteristics, which not only greatly facilitates the identification of such pairs, but also facilitates the detection of interchromosomal translocations, that is, movements of sections between chromosomes - translocated sections have a spectrum that differs from the spectrum of the rest of the chromosome.

The results are presented as karyograms(systematized arrangement of chromosomes excised from a micrograph) or ideograms- a schematic representation of chromosomes arranged in a row as their length decreases.

Comparative analysis of karyotypes is used in karyosystematics to determine the evolutionary paths of karyotypes, to determine the origin of domestic animals and cultivated plants, to identify chromosomal abnormalities leading to hereditary diseases, etc.

A karyotype can be defined as a set of chromosomes of somatic cells, including structural features of chromosomes. In multicellular organisms, all somatic cells contain the same set of chromosomes, that is, they have the same karyotype. In diploid organisms, the karyotype is the diploid set of chromosomes in the cell.

The concept of a karyotype is used not so much in relation to an individual as in relation to a species. In this case, they say that karyotype is species specific, that is, each species of organisms has its own special karyotype. Although the number of chromosomes in different types may coincide, but in their structure they always have one or another difference.

Although the karyotype is primarily a species characteristic, it can vary somewhat among individuals of the same species. The most obvious difference is the unequal sex chromosomes in female and male organisms. In addition, various mutations can occur, leading to anomalies in the karyotype.

The number of chromosomes and the level of organization of a species do not correlate with each other. In other words, a large number of chromosomes does not indicate a high level of organization. So hermit crab has 254 of them, and Drosophila has only 8 (both species belong to arthropods); a dog has 78, and a human has 46.

Karyotypes of diploid (somatic) cells consist of pairs of homologous chromosomes. Homologous chromosomes are identical in shape and gene composition (but not in alleles). In each pair, one chromosome goes to the body from the mother, the other is paternal.

Karyotype study

Cell karyotypes are examined at the metaphase stage of mitosis. During this period of cell division, the chromosomes are maximally spiralized and are clearly visible under a microscope. In addition, metaphase chromosomes consist of two (sister) chromatids connected at the centromere.

The section of chromatid between the centromere and telomere (located at the end on each side) is called the shoulder. Each chromatid has two arms. The short shoulder is denoted by p, the long one by q. There are metacentric chromosomes (arms are approximately equal), submetacentric (one arm is clearly longer than the other), acrocentric (in fact, only arm q is observed).

When analyzing the karyotype, chromosomes are identified not only by their size, but also by the ratio of the arms. In all organisms of the same species, normal karyotypes for these traits (chromosome size, shoulder ratio) are the same.

Cytogenetic analysis involves the identification of all chromosomes of the karyotype. In this case, the cytological preparation is subjected to differential staining using special dyes that specifically bind to different DNA regions. As a result, the chromosomes acquire a specific striation pattern, which allows them to be identified.

Differential stain method was discovered in the 60s of the XX century and made it possible to fully analyze the karyotypes of organisms.

The karyotype is usually represented as an idiogram.(a kind of scheme), where each pair of chromosomes has its own number, and chromosomes of the same morphological type are combined into groups. In a group, chromosomes are arranged in size from largest to smallest. Thus, each pair of homologous karyotype chromosomes on the idiogram has its own number. Often only one chromosome from a pair of homologues is depicted.

For humans, many laboratory and farm animals, chromosome striation schemes have been developed for each staining method.

Chromosomal markers are bands that appear when stained. Bands are grouped into regions. Both bands and regions are numbered from centromere to telomere. Some bands may show localized genes.

Karyotype recording

A karyotype record carries a certain characteristic of it. First, the total number of chromosomes is indicated, then the set of sex chromosomes. In the presence of mutations, genomic mutations are indicated first, then chromosomal ones. The most common are: + (extra chromosome), del (deletion), dup (duplication), inv (inversion), t (translocation), rob (Robertsonian translocation).

Examples of recording karyotypes:

48, XY - normal male chimpanzee karyotype;

44, XX, del (5)(p2) - karyotype of a female rabbit, in which division of the second segment of the short (p) arm of the fifth chromosome occurred.

Human karyotype

The human karyotype consists of 46 chromosomes, which was accurately determined in 1956.

Prior to the discovery of differential coloration, chromosomes were classified according to their total length and their centromeric index, which is the ratio of the length of the short arm of the chromosome to its total length. Metacentric, submetacentric and acrocentric chromosomes were found in the human karyotype. Sex chromosomes have also been identified.

Later, the use of differential staining methods made it possible to identify all chromosomes of the human karyotype. In the 1970s, rules (standard) for their description and designation were developed. So autosomes were divided into groups denoted by letters, each of which included chromosomes with a certain number: A (1-3), B (4, 5), C (6-12), D (13-15), E (16- 18), F (19, 20), G (21, 22). The sex chromosomes are the 23rd pair.

A normal human karyotype is written like this:

46, XX - for a woman,

46, XY - for a man.

Examples of human karyotypes with anomalies:

47, XX, 21+ - a woman with an extra 21st chromosome;

45, XY, rob (13, 21) - a man who had a Robertsonian translocation of the 13th and 21st chromosomes.

Introduction ................................................ ................................................. .. 1

Chapter 1. Mitotic Chromosomes............................................... .............. 2

Chapter 2. Meiotic chromosomes .............................................. .............. 5

Chapter 3. Cytogenetic Method............................................... ............... 13

Chapter 4. Sex chromatin .............................................. ....................... 20

Chapter 5. Mosaicism ....................................................... ................................... 23

One of the key issues of human genetics is the question of the structure and functioning of the material foundations of heredity. Information on each of the three levels of organization of hereditary structures (genetic, chromosomal, genomic) has been accumulating in recent years with amazing speed, and one can hope that the time is not far off when a fairly complete picture of human heredity will be drawn up. Even now, on this issue, a person can be attributed to the number of the best studied objects along with Drosophila, mice, and corn.

For a correct understanding of the significance of heredity in human pathology, it is necessary to have detailed information on three partially interconnected sections:

1) according to the morphological and chemical structure of chromosomes and the karyotype as a whole; 2) according to discrete traits of a person controlled by single genes (“inventory” of units of hereditary variability); 3) according to the "architectonics" of genes in chromosomes (linkage of genes and maps of chromosomes). For each of these sections, a lot of data has been accumulated, and their intensive development continues both in theoretical and applied (clinical) aspects.

The principles and main sections of general cytogenetics were formed during the 1920s and 1930s mainly due to studies carried out on Drosophila and some plants. Cytogepetics of humans and mammals, occupying leading place in modern cytogenetics, developed later, mainly due to methodological difficulties.

The history of the development of human cytogenetics can be divided into three periods. The first covers the period from the last century to the mid-1950s and is now of purely historical interest. These were the search for methodological approaches to obtaining preparations of human chromosomes by the cytologists of that time, remarkable for their perseverance and diligence (AG Andres, 1934). Although our cytogeneticists A. G. Andres and M. S. Navashin correctly described the first 10 pairs of large chromosomes, even the total number of chromosomes in human cells was not reliably established. Their morphology also remained unknown.

The second period, initiated by the work of Tjio and Levan in 1956, was characterized by the emergence and rapid development of modern human cytogenetics. Quite quickly, all the main methodological methods of chromosome analysis were developed, fundamental information was obtained about the human karyotype, about the main features of the structure and functioning of its normal chromosomes. It was during this period that medical cytogenetics was born, which opened up a new area of ​​human pathology, due to a change in the number or structure of chromosomes.

The third period in the development of human cytogenetics began in the 1970s. It can rightfully be considered the beginning of the modern stage in the development of the science of the cytological foundations of human heredity. A number of methodological innovations ensured the transition of cytogenetics to a qualitatively new level. The possibility of studying the individuality of human chromosomes and even their segments was realized. This immediately raised medical cytogenetics to a new level. It became possible to comprehensively investigate the morphology, function, chemical features of the structure and supramolecular organization of human chromosomes. The development in the same years of methods for genetic mapping of human chromosomes ensured the solution of the most difficult problem - the creation of genetic maps of chromosomes.

Thus, modern human cytogenetics is rich in factual material, a branched independent area of ​​human genetics. At present, the problem of identifying all elements of the human karyotype in the analysis at the stage of mitosis has been solved based on the use of differential chromosome stains.

Chromosomes as individual structures become available for research after a significant shortening and thickening, which they experience during the preparation of the cell for division. For somatic cells, this division is mitosis, for generative cells, first mitosis, and then meiosis.

Chapter 1. Mitotic chromosomes.

Basic information about the human chromosome set as a whole and about individual chromosomes was obtained as a result of studying chromosomes in the metaphase of mitosis. At this stage of mitosis, it is clearly seen that the diploid set of human chromosomes consists of 46 elements: 22 pairs of autosomes and one pair of sex chromosomes (XX in women and XY in men). On standard stained preparations, the shape of metaphase chromosomes is determined by the location of the primary constriction, which is formed due to decondensation of the centromeric region functioning in metaphase. Additional constrictions, called secondary constrictions, may exist on individual chromosomes. In the case of localization of such a constriction at the end of the chromosome, the distal segment of the chromosome separated by it is called a satellite.

In terms of shape and overall size, all human autosomes are easily divided into 7 groups, denoted by Latin letters from A to G (Fig. 8). In addition, all autosomes are numbered in order of decreasing total length (from 1 to 22).

The length of the same chromosome in mitosis varies considerably, since the process of natural condensation of the chromosome continues at the metaphase stage, which is greatly enhanced by colchicine. Therefore, an indicator of the relative rather than the absolute length of the chromosome serves for identification. However, its reliability is limited by the fact that chromosomes have different lengths, and in a given chromosome, the arms different sizes are reduced unequally: the shortening of the longer ones occurs faster than the short ones. This does not affect the above group characteristics, but prevents the identification of chromosomes close in size and shape within groups. Difficulties in the individual identification of chromosomes are also aggravated by the fact that differential condensation can also take place between homologous chromosomes, causing homologue heteromorphism. At present, the need to use the method of morphometry and the linear parameters of the chromosome determined with its help has actually disappeared due to the introduction into practice of chromosome analysis of differential coloration of chromosomes.

Analysis of spontaneous secondary constrictions, including satellite ones, does not significantly facilitate the recognition of individual chromosomes. With their help, autosome 9 can be most regularly identified, which often has a significant constriction in the pericentromeric region of the long arm. All ten human acrocentric chromosomes have a satellite constriction; aD- or G-chromosomes do not differ in this trait within the groups.

The morphological homogeneity of the chromosome in length, as it emerges from the microscopic study of metaphase chromosomes on routinely prepared and stained preparations, in fact turns out to be misleading. Methodological progress in the cytogenetics of humans and higher eukaryotes in general, which has taken place over the past 15-20 years, has led to the discovery of a deep linear differentiation of the chromosome in relation to both structure and function. This differentiation, which is individual for each chromosome, is relatively easy to detect in the metaphase of mitosis. Due to this, in modern human cytogenetics it is possible to identify all chromosomes not by separate and random features, but by the essential aspects of their structural and functional organization. In the practice of cytogenetic analysis, for this purpose, differential condensation of chromosomes, the chronology of DNA replication in chromosomes, or differential staining of chromosomes are studied (AF Zakharov, 1977).

The differential condensation of chromosome segments is one of its essential characteristics, most fully expressed in the interphase nucleus. Under natural conditions of the course of mitosis, chromosome regions that differ sharply in the degree of condensation during the interphase period look almost the same in the metaphase. Only with special methods of light or electron microscopy is it possible to detect an inhomogeneous linear structure of an outwardly homogeneous metaphase

chromosomes (Bahr and Larsen, 1974). Equalization of condensation cycles in different areas chromosomes can be inhibited artificially. For this purpose, 5-bromodeoxyuridine is especially successfully used (A. F. Zakharov, 1973, 1977;

Dutrillaux and Lejeune 1975). In the presence of this substance, the chromosomes enter metaphase unevenly compacted along their length. As a result of a thorough study of their morphology, it was shown that each human chromosome has a strictly constant and specific alternation of normally and weakly condensed regions and can be identified by this feature.

Intrachromosomal asynchrony of DNA replication is the second most important feature of the linear heterogeneity of the chromosome, which can be detected in the metaphase of mitosis. For a decade and a half, this feature of chromosomal organization has been available to study by the method of chromosome autography (under the editorship of A. A. Prokofieva-Belgovskaya, 1969; A. F. Zakharov, 1977; Giannelli, 1970, 1974). On the basis of this method, the fundamental patterns of reproduction of human chromosomes were revealed, among which the asynchrony of reproduction of different parts of the chromosome, the constancy and specificity of the order of reproduction for a given chromosome are the most important. However, the identification of individual chromosomes was less advanced by autoradiography than expected. On autographs, it is additionally possible to distinguish autosomes 4 and 5, 13, 14 and 15, 17 and 18. In female cells, one of the two X chromosomes differs in the late start and late end of DNA synthesis. Despite the limited data obtained by autoradiography, this technique proved to be extremely useful in improving the identification of abnormalities of these chromosomes and helped in the identification of several new independent syndromes in chromosomal pathology.

Significant progress in the study of the sequence of DNA synthesis along the length of each human chromosome is normal, its relationship with other characteristics of the chromosomal organization, its state in cases of numerical or structural changes in the chromosome set is currently taking place due to the use of thymidine analog as a precursor of DNA synthesis - 5- bromodeoxyuridine. The weakened ability to stain chromosome regions that included this precursor has armed cytogeneticists with an accurate method for studying the chronology of chromosomal reproduction, the possibilities of which are limited only by the resolution of light microscopy. The replication structure of all human chromosomes is revealed with the utmost clarity, and it can be described in clear morphological terms.

Each chromosome is made up of segments that replicate in different time. There is a clear alternation of areas with early and late replication. On the metaphase chromosome

such areas are clearly visible with a light microscope. The specificity of the replication structure of each chromosome consists of the individual size, number, and mutual arrangement of differing chromosomal regions (Fig. 9).

In contrast to the above two phenomena of uneven staining of chromosomes along the length caused by the inclusion of 5-bromodeoxyuridine in DNA, differential staining of chromosomes means the ability to selectively stain along the length of a chromosome that has not been modified in vivo by any influences. Differential staining of chromosomes in this case is provided by relatively simple temperature-salt effects on a fixed chromosome.

It is important to note that with all the variety of such treatments of chromosome preparations after fixation and the applied fluorochromic or nonfluorescent dyes, the detected linear heterogeneity of the chromosome is always the same. Its pattern changes only depending on the degree of compaction of the chromosome: in longer, weaker shortened chromosomes, further heterogeneity of those segments becomes noticeable, which looked homogeneously stained in highly condensed chromosomes. Differential staining can be observed either along the entire length of the chromosome (Q-, G- and R-segments), or in its centromeric region (C-segments).

The clearest idea of ​​the pattern of differential staining of chromosomes along the entire length can be obtained by staining preparations according to the G-method using the Giemsa stain (Fig. 10). On such preparations, the chromosomes look like cross-striated, differently colored segments (“banding”). The pattern of each pair of chromosomes is specific to it. The segments are not the same size. In small chromosomes of groups F and G, the pattern is formed by single segments, in large chromosomes there are many of them. The total number of stained and unstained segments in a normal chromosome set of medium degree of condensation, in accordance with the Parisian nomenclature, is 322. In prometaphase chromosomes, their number increases to 1000 or more.

At the Paris Conference on Nomenclature in Human Cytogenetics, a system for designating segments of normal chromosomes and chromosomes that have undergone various structural rearrangements was developed and has now entered the practice of cytogenetic analysis (ParisConference, 1971). On fig. 11 shows an example of this system for autosome 1.

Regardless of how the question of the nature of differential staining of chromosomes is resolved, cytological maps based on this phenomenon are of exceptional importance for the development of human cytogenetics. With their help, it is possible to attribute genetic markers not just to one or another chromosome arm, but to a certain region of the chromosome. In medical cytogenetics, it has become possible to identify the origin of abnormal chromosomes up to the exact description of regions.

The second type of differential staining of chromosomes reveals the specificity of the pericentromeric regions in human chromosomes. In different chromosomes, the sizes of C-segments are different, they are especially large in autosomes 1, 9, and 16. However, it is not possible to identify chromosomes similar in size and shape by this color. In the Y chromosome, C-chromatin is localized in the distal part of the long arm. In the same chromosome in different individuals, its content may vary.

Chapter 2. Meiotic chromosomes.

Meiosis combines a series of different processes by which primary germ cells differentiate into mature germ cells. At the beginning of this series, spermatogonia (oogonia) turn into primary spermatocytes (oocytes). The central event is the first meiotic division of the spermatocyte (oocyte), during which the chromosomes undergo particularly complex specific transformations during the prophase period. The first meiotic prophase is divided, as is known, into five stages: leptotene, zygotene, pachytene, diploten, and diakinesis. Unlike mitosis, whose prophase is practically not used in cytogenetic analysis, the prophase chromosomes of the first meiotic division are of great interest for human cytogenetics. Metaphase chromosomes of the first meiotic division, which are bivalents of homologous chromosomes, are less differentiated structures compared to metaphase mitotic chromosomes. Chromosomes of the second meiotic division are almost never used in human cytogenetics.

The course of meiosis in the male and female organisms differs significantly in several respects: the period of ontogenesis, the duration of individual phases, and the morphology of mitotic transformations.

In men, meiotic divisions begin at puberty and proceed continuously throughout the entire subsequent sexually mature state. This process, unlike female meiosis, is not cyclic. A large number of gametes simultaneously mature in the testicles, so the gonads of a sexually mature male can serve as a source of meiotically dividing cells at any time. On chromosome preparations, it is possible to simultaneously see various meiotic figures, from spermatogonial metaphases to metaphases of the second meiotic division. The duration of transformation from spermatogonia to spermatozoa takes about 8-9 weeks. The duration of the individual stages is very different, so cells of different stages occur with unequal frequency. The stages of pachytene and diakinesis, which are most important for cytogenetic analysis, are usually represented by a sufficient number of cells.

In the female body, meiosis occurs in two stages, separated by a large period of time. The first stage, including the formation of oogonia and the passage of the first meiotic division, takes place in the embryonic ovaries. By the time of the birth of the girl in the ovaries, all oogonia are differentiated into oocytes, and the latter have passed the stages of leptotene - pachytene and stopped at the stage of diplotene. Staying in this stage, called dictyoten, continues throughout the postnatal period of a woman's life. The subsequent development of the cell from the stage of dictyoten into a mature egg occurs cyclically, one cell per month, and ends with ovulation. The foregoing explains why the early stages of the first meiotic division in a woman can be analyzed only in the early embryonic period, and the subsequent stages are not available for study under normal conditions.

Basic information on the organization of human meiotic chromosomes was obtained from the study of testis cells. The following aspects of these studies can be distinguished.

Analysis of the linear structure of individual chromosomes. A characteristic feature of the structure of meiotic chromosomes, expressed mainly in the first stages of the prophase of meiosis, is their chromomeric structure (Fig. 12). From the data on the cytology of meiotic chromosomes of some plant species, the individuality of the chromomeric structure of each chromosome is well known (Cytology and Genetics of Meiosis by V. V. Khvostova and Yu. V. Bogdanov, 1975). Unfortunately, individual bivalents in the human chromosome set, both male and female, can be distinguished only in late pachytene, when they are significantly reduced and the chromomericity of their structure is significantly lost. Nevertheless, as a result of several attempts at pachytic analysis of chromosomes, the first data on the morphology of bivalents of acrocentric and some other chromosomes were obtained (under the editorship of A. A. Prokofieva-Belgovskaya, 1969; Hungeriord, 1973).

In the identification of pachytene bivalents, C- and Q-methods of differential staining were applied with some success (Goetz, 1975). Full agreement was found between the patterns of G-staining and the chromomeric structure of pachytenic chromosomes, as well as between the patterns of meiotic and mitotic chromosomes stained by the G-method (Lucianie. a., 1975).

Chromosomal conjugation and formation of chiasmata. The study of diakinesis - metaphase I of meiosis in male cells showed that homologous conjugation is mandatory for all human chromosomes, including short ones. In one or another bivalent there are from 1 to 6 chiasmus; according to different authors, their total number per chromosome set ranges from 35 to 66 (Ford, 1973). The distribution of chiasmata in individual bivalents became possible to analyze after each bivalent could be identified based on sequential staining using the Q- and C-techniques (Hulten, 1974). According to Hulten (1974), the average frequency of chiasmata in individual autosomes is proportional to the chromosome length. It is not affected by numerical or structural abnormalities in other chromosomes. Apparently, chiasmata form in certain regions of each chromosome. Elucidation of the number and localization of chiasmata in each chromosome is important in their genetic mapping.

Identification of chromosomal abnormalities. The phenomenon of conjugation of homologous chromosomes in meiosis is used to identify many chromosomal rearrangements that affect the linear structure of the chromosome. Deletions, insertions, inversions, reciprocal translocations, duplications lead to a change in the configuration of the bivalent. Univalents, trivalents, etc. arise. In combination with the analysis of mitotic chromosomes, the study of the morphology of meiotic chromosomes in pachytene, diakinesis and meta-phase I was repeatedly carried out in cases of numerical or structural changes in autosomes, sex chromosomes in men with infertility (A. A. Prokofieva -Belgovskaya and V.K. Bordzhadze, 1971; Kjessler, 1966; Hulten, 1974, etc.). The submicroscopic or supramolecular organization of the chromosomal apparatus has been studied quite insufficiently. If about the structure of the chromosome at the level of light microscopy and about molecular structure Since extensive information has been accumulated on the hereditary material, the intermediate steps in the ultrastructural organization of the chromosome remain largely unknown. So far, there are no actual prerequisites to raise the question of the possible specifics of the ultrastructural organization of the human genetic apparatus.

The most valuable information on the fine structure of functioning chromosomes has come from the study of polytene chromosomes, which are a specific but natural model of the chromosomes of the interphase nucleus in dipteran cells, and the “lampbrush” chromosomes found in amphibian oocytes in meiotic prophase I. The large size of these chromosomes made it possible to carry out their careful study under a light microscope. As a result of these studies, provisions were formulated that are considered fundamental for the organization of eukaryotic chromosomes as a whole (I. I. Kiknadze, 1972).

In the interphase nucleus, the chromosome regions corresponding to euchromatin have a chromomeric structure. Each chromomere is a structural and functional unit of the chromosome as a longitudinally differentiated organelle. The differential transcription of these units is structurally provided by decondensation of the deoxyribonucleoprotein packaged in it, which is expressed in the form of puffs in polytene chromosomes, or loops in lampbrush chromosomes.

The method of studying the fine structure of interphase nuclei that do not have polytene chromosomes, as well as metaphase chromosomes, is electron microscopy (Yu. S. Chentsov, V. Yu. Polyakov, 1974). Unfortunately, based on the results obtained by this method, it has not yet been possible to form a complete picture of the ultrastructure of the interphase core. On electron diffraction patterns of ultrathin sections, the main detectable morphological unit is a thread in different sections with a diameter of 10 nm or less. On preparations of chromatin spread on the surface of the water meniscus, extended filaments about 23–25 nm in diameter are found.

Despite numerous studies of mitotic or meiotic chromosomes, data on their ultrastructure, which would make it possible to create a consistent model for the packing of an elementary chromosome strand during cell division, remain scarce. The greatest information has been obtained on the ultrastructure of specialized regions of chromosomes: the centromere region, the nucleolus, the synaptonemal complex in meiotic chromosomes. Electron microscopy data of whole isolated chromosomes were used for their identification, with special attention paid to human metaphase chromosomes (Bahr and Larsen, 1974). This method made it possible to detect an uneven packing density of elementary chromosome filaments along the length of the chromosomes, and the pattern of this unevenness turned out to coincide with the linear differentiation of the chromosome structure, detected under a light microscope. Elementary fibrils on electron diffraction patterns of whole spread chromosomes have a size of about 25-30 nm. A biochemical study of such fibrils and corresponding calculations give grounds to conclude that the nucleoprotein molecules in them are in a supertwisted state and that, in addition to histones, fibrils contain other proteins.

Sufficiently complete coverage of the issues of molecular genetics and chromosome organization in numerous special monographs and manuals (S. E. Bresler, 1973; I. P. Ashmarin, 1974; G. Stent, 1974, etc.) eliminate the need for a detailed consideration of these issues in this book. A relatively new molecular aspect of chromosomal organization arose in connection with the development of methods for fractionating the total DNA of the genome by the frequency of similar nucleotide sequences and methods for hybridizing nucleic acids on chromosomal preparations. These methods opened up the possibility of elucidating the localization of different DNA fractions in the chromosome set. Important discoveries made in this new field, borderline between molecular and cytological genetics, were: a) the discovery in the eukaryotic genome, in addition to DNA with unique sequences, of a large proportion of DNA with the same or similar nucleotide sequences, repeated many hundreds and thousands of times (G. P. Georgiev, 1973; S. A. Limborskaya, 1975); b) detection of uneven localization of DNA with different characteristics in the chromosome set: DNA with the largest number of repetitive sequences is localized in heterochromatic regions of chromosomes.

To date, DNA fractionation and determination of the chromosomal localization of fractions have been carried out on many types of organisms. Each species is characterized by its specific structure of the genome in relation to the composition of DNA and the specifics of their distribution over the chromosomes of the set. Many works in this direction have been performed on human cells. The results obtained in them are summarized by A. F. Zakharov (1977) and Jones (1973).

The DNA of the human genome can be fractionated into DNA with unique copies (about 64%) and DNA with repetitive sequences. According to the rate of renaturation, which reflects the repeatability of nucleotide sequences, the last fraction can be divided into DNA with a low (13.4%), intermediate (12.3%) and high (10.3%) rate of renaturation of DNA molecules. Thus, about 10% of all DNA in the human genome has a high repetition rate of identical sequences.

Using gradient ultracentrifugation, at least four types of so-called satellite DNAs were isolated from a group of highly repetitive DNAs. In addition to these types of DNA, in experiments with DNA-RNA hybridization, the chromosomal localization of DNA encoding the synthesis of 5S, 18S and 28S ribosomal RNAs was studied. At present, the distribution of different types of DNA in human chromosomes is as follows.

DNA with low and intermediate repeatability of nucleotide copies is found in all chromosomes, and it is localized along the entire length of their arms.

DNA with a high frequency of nucleotide copies is found mainly in the pericentromeric and partly telomeric regions. Satellite individual DNAs are unevenly distributed in different chromosomes. Thus, the Y-chromosome is especially rich in satellite DNA I and IV, chromosomes 1 and 16 contain the most satellite DNA II, and chromosome 9 - III. Ribosomal DNA 18S and 28S is contained almost exclusively in the short arms of all 10 acrocentric chromosomes. The distal part of the long arm of autosome 1 is a preferred site for pistrones encoding 5S RNA. It is possible that the in situ DNA-RNA hybridization method will be able to map not only polygenic loci, but also structural genes that are repeated a small number of times (Rotterdam. Conference, 1974).

The two most important features of the genetic organization of eukaryotes are the differential activity of structural genes and a large share the genes that regulate this process must be based on the corresponding structural organization of the chromosome. Decades of hard work by cytogeneticists have brought us much closer today to understanding how structure and function interact in the chromosome, how the chromosome performs its complex role of integrating the gene system.

The first fundamental feature of the structural and functional organization of the chromosome is the existence of two different functional types of chromosomal material - euchromatin and heterochromatin. Their main difference lies in transcriptional activity.

The lack of genetic activity in heterochromatin is due either to its lack of structural genes (structural heterochromatin) or to the temporary exclusion of the chromosome region carrying such genes from genetic transcription (facultative heterochromatin, heterochromatinization).

The second most important feature of the chromosomal organization is the linear dissection of the chromosome into sections consisting of different types of chromatin. Each chromosome is distinguished by its unique arrangement of hetero- and euchromatic regions.

The subdivision of chromatin according to genetic significance correlates well with the difference in chromatin types and according to a number of other characteristics: the state of condensation in the interphase nucleus and the chronology of condensation in the mitotic and meiotic cycle; DNA replication time;

in relation to coloring with fluorochromes or non-fluorescent dyes; sensitivity to the damaging effect of chemical mutagens; chemical features of DNA and, apparently, proteins that make up chromatin; phenotypic manifestations chromosomal rearrangements. Heterochromatin is characterized by a condensed state in the interphase nucleus, advanced condensation in the prophase of mitosis and meiosis, and the ability to lag behind in condensation spontaneously or under the influence of certain influences in the metaphase of mitosis. Compared to euchromatin, heterochromatic regions of chromosomes are reproduced in later segments of the S-period. With differential staining according to the G- and C-method, heterochromatic segments retain the ability to stain (G-segments) and even stain intensively (C-segments). In cytogenetics, the uneven distribution along the length of the chromosome of its structural damage induced by mutagenic substances is well known: it is heterochromatin regions that are characterized by increased damage. DNA with repeatedly repeated nucleotide sequences is characteristic of heterochromatin. Unlike euchromatin, which contains unique genes, an imbalance in which negatively affects the phenotype of an organism, changes in the amount of heterochromatin do not affect or significantly less affect the development of the organism's traits.

The interconnection of various structural and functional characteristics of the chromosome is the third fundamental feature of the chromosome organization. The issue of cause-and-effect relationships in the noted correlation complex is being actively studied. An answer must be obtained, in particular, to the question of whether the entire diversity of properties of different types of chromatin can be reduced to differences in the chemical characteristics of chromosomal DNA. However, regardless of progress in understanding these correlations, their phenomenology serves as the main tool for understanding the structural and functional dissection of each specific human chromosome. In the longitudinal differentiation of individual chromosomes in terms of condensation density, staining with certain dyes, the features of their DNA and other characteristics, there are not formal signs of identification of chromosomes or their regions, but signs that have a genetic meaning. This new field of human cytogenetics is under active development and, combined with advances in chromosome mapping, will take human cytogenetics to an even higher level. Of the information already available on this problem, the following are of interest to genetics.

Heterochromatin stained by the C-stain method is found in all human chromosomes and is called structural heterochromatin. In all autosomes and the X chromosome, it occupies, as in most chromosomes of other biological species, a pericentromeric region. In the Y chromosome, it is localized in the distal part of the long arm. In different chromosomes, the amount of C-heterochromatin is different. Its especially large blocks, extending mainly to long arms, are contained in autosomes 1, 9, and 16; it is these regions that are known as the most regular secondary constrictions. Particularly small blocks of this chromatin are observed in autosome 2 and in the X chromosome. In acrocentric chromosomes, heterochromatin extends to short arms.

Apparently, the circumcentromeric heterochromatin is not the same in different chromosomes, which follows from a number of facts. This heterogeneity is already revealed by the different optimal time and pH of the alkaline range used in the C-staining technique, at which C-chromatin appears in different chromosomes. Heterogeneity is especially demonstrative when staining chromosomes with acrichin or acrichin mustard: the C-heterochromatin of autosomes 1, 9, and 16 does not fluoresce at all, while the heterochromatin of autosomes 3, 4, acrocentric chromosomes, and the Y chromosome glows extremely brightly. The genetic significance of the heterogeneity of human C-heterochromatin is not yet clear. The chemical basis of this heterogeneity is beginning to become clear. Experiments with hybridization of DNA with RNA on cytological preparations have established that differences in heterochromatin of different human chromosomes can be associated with features of the DNA structure. In all cases, this is DNA with repeated nucleotide sequences, but different chromosomes apparently contain different classes of DNA. Thus, of the well-characterized satellite DNAs, satellites I and IV are found in large numbers in the Y chromosome, satellite II is found in autosomal heterochromatin 1 and 16, and satellite III is found in autosomal heterochromatin 9. Structural heterochromatin of acrocentric chromosomes is the main carrier of ribosomal DNA.

In full accordance with the data of general cytogenetics on the weak negative effect of an imbalance in heterochromatic material on the development of an organism, there are data on the existence of significant polymorphism in the human population, due to the size of pericentromeric heterochromatin. The content of C-type structural heterochromatin varies especially strongly in autosomes 1, 4, 9, 13-15, 16, 21-22 and the Y-chromosome. The absence of phenotypic deviations from the norm in most carriers of such karyotypic variants allows us to consider them as variants of the norm. However, this problem has been put on the agenda quite recently. It requires thorough research on a large population material before reasonable boundaries of the chromosome norm are outlined, beyond which the heterochromatin imbalance becomes not indifferent for the organism.

There are many reasons to consider chromosomal regions stained positively by the G-method as a type of structural heterochromatin. In addition to the relationship to dyes, this idea is supported by the late replication of these regions, the formation of chromomeres by them in prophase meiotic chromosomes, and the ability to lag behind in mitotic condensation under the influence of 5-bromodeoxyuridine or cold. It is important to note that an imbalance in autosomes, especially rich in G-staining chromatin, entails the appearance of the least severe developmental anomalies for an individual - the carrier of such an imbalance. Thus, trisomies 13, 18, and 21 belong to this category of chromosomal anomalies. There are also reports that DNA with an average repetition of identical nucleotide sequences is localized in G-staining segments of chromosomes.

The questions that human cytogenetics faces regarding the structure, localization, and especially the genetic significance of structural heterochromatin are relatively new.

Progress in their resolution cannot be separated from progress in deciphering the nature of heterochromatin in eukaryotes in general.

In addition to structural heterochromatin, there is facultative heterochromatin, the appearance of which in the chromosome is due to the heterochromatinization of euchromatic regions under special conditions. There is reliable evidence of the existence of this phenomenon in human chromosomes on the example of genetic inactivation of one of the X chromosomes in somatic cells women. In humans and other mammals, this special case a phenomenon first discovered in Drosophila Muller in 1932 and called "gene dose compensation". For mammals, its essence lies in the evolutionarily formed mechanism of inactivation of the second dose of genes localized in the X chromosome, due to which, despite the unequal number of X chromosomes, the male and female organisms are equal in the number of functioning genes.

Formulated by Lyon (1961, 1974), the corresponding hypothesis, which received her name, consists of three main provisions:

1. In the somatic cells of a normal female body, one of the two X chromosomes is inactivated.

2. In different cells of the body, either the maternal or paternal X chromosome is inactivated.

3. Inactivation occurs in the early embryonic period and is persistently retained by this X chromosome in cell generations.

The Lyon hypothesis is based on a large number of genetic and cytological facts, including those obtained in humans, which have been continuously updated over the years since its nomination and information about which can be found in a number of reviews (A.F. Zakharov, 1968; Lyon, 1972, 1974 ; Ghandra and Brown, 1975, etc.).

Genetic facts are based on the fact that two cell populations are found in heterozygotes for traits linked to the X chromosome. In one of them, the action of the maternal X-chromosome gene is manifested, in the other, the paternal one, which is associated with the inactivation of the paternal or maternal alleles, respectively. When formulating her hypothesis, Lyon relied on cases of mosaic coloration of the coat of mice, which was due to inactivation in different parts of the body of either the wild gene or its mutant allele. In humans, detailed evidence of the existence in the body of heterozygous women of two populations of cells, each of which has one of the two alleles of the gene localized in the X chromosome inactivated, was obtained by studying the effects of genes for glucose-6-phosphate dehydrogenase, phosphoglycerate kinase, hypoxanthine-phosphoribosyltransferase, erythrocyte blood groups Xg (a), in the study of X-linked agammaglobulinemia and mucopolysaccharidosis (Hunter's syndrome), hemophilia. In heterozygotes for electrophoretic variants of glucose-6-phosphate dehydrogenase, it was confirmed that the human X chromosome is inactivated in the early embryonic period (Migeon and Kennedy, 1975). These findings must be kept in mind when interpreting data on X-linked hereditary diseases, especially in monozygotic twins.

Cytological evidence in favor of the Lyon hypothesis is also very strong, in that in normal female somatic cells, one of the two X chromosomes corresponds to the characteristics of a heterochromatinized chromosome. In the interphase nucleus, it is found in the form of the so-called Barr body (X-chromatin) - a densely condensed, intensely stained lump of chromatin. In prophase, this chromosome is ahead of its homologue, the second X chromosome, in the condensation cycle. Under conditions of experimental exposure to cold or 5-bromodeoxyuridine, one of the X chromosomes lags significantly behind in condensation, not differing in this respect from the structural heterochromatin of autosomes 1, 9, 16 and the Y chromosome. The second X chromosome is one of the most delayed in the beginning and end of DNA replication.

A study of numerous cases of anomalies in the X-chromosome system in humans shows that the phenomenon of gene dose compensation also applies to all cases of violations in the number of X chromosomes, leaving only one X chromosome in an active state in a somatic cell. Particularly demonstrative in this regard are X-polysomy, when the number of inactivated X chromosomes is equal to the number present in the cell minus one genetically functioning one.

As shown above, information about the human karyotype is constantly deepening, and more and more research is being carried out at the molecular level. The cytological study of the material foundations of human heredity is well complemented by the genetic analysis of discrete traits.

Chapter 3. Cytogenetic method.

Human genetics uses a variety of research methods that are also used in other areas of biology - genetics, physiology, cytology, biochemistry, etc. Anthropogenetics also has its own research methods: cytogenetic, twin, genealogical, etc.

Achievements in molecular biology and biochemistry made a great contribution to the development of genetics. At present, biochemical and molecular genetic research methods play a leading role in human genetics and medical genetics. However, the classical methods of human genetics, such as cytogenetic, genealogical and twin methods, are of great importance at the present time, especially in matters of diagnostics, medical genetic counseling and prognosis of offspring.

Let's get acquainted with the possibilities of the cytogenetic method.

The essence of this method is to study the structure of individual chromosomes, as well as the characteristics of the set of chromosomes of human cells in normal and pathological conditions. Lymphocytes, buccal epithelial cells, and other cells that are easy to obtain, culture, and subject to karyological analysis serve as a convenient object for this. It is an important method for determining sex and chromosomal hereditary diseases person.

The basis of the cytogenetic method is the study of the morphology of individual chromosomes of human cells. The modern stage of knowledge of the structure of chromosomes is characterized by the creation of molecular models of these the most important structures nucleus, the study of the role of individual components of chromosomes in the storage and transmission of hereditary information.

In Chapter 1, we looked at the components of chromosomes such as proteins and nucleic acids. Here we briefly dwell on the structure and morphology of chromosomes.

The structure of chromosomes.

The chromosomal theory of heredity was created by the American scientist T. G. Morgan. After conducting a large number of studies on the Drosophila fruit fly, Morgan and his students found that it was in the chromosomes that the heredity factors discovered by Mendel, which were called genes, were located. T. Morgan and his students showed that genes are arranged linearly along the length of the chromosome.

After it was proved that chromosomes are the main genophores (carriers of genes), the period of their most intensive study began. Advances in molecular biology and genetics have made it possible to understand some patterns in the structure and functioning of chromosomes in prokaryotes and eukaryotes, but much remains unknown here. In recent years, the chromosomes of eukaryotes, especially humans, have become the subject of study by various specialists, from geneticists to physicists.

It has now been established that the structure of the chromosome is based on chromatin - a complex complex of DNA, proteins, RNA and other substances that make up the chromosome (we discussed the structure of chromatin in detail in Chapter 1). It is assumed that the human chromosome includes one giant DNA molecule, RNA molecules, histones and acidic proteins, various enzymes, phospholipids, Ca 2+, Mg 2+ metals and some other substances. The method of stacking and mutual arrangement of the molecules of these chemical compounds in the chromosome is not yet known. A long strand of DNA cannot be arranged randomly on a chromosome. There is an assumption that the DNA strand is packed in a regular way and is associated with proteins.

F. Arrighi and co-authors (1971) found that unique sequences occupy more than 56% of the DNA of human chromosomes, highly repetitive - 12.4%, intermediate repeats - 8%. The total number of repetitive genes in the DNA of the human chromosome is 28%. The number of chromosomes in humans has long remained unclear. The fact is that it was difficult to determine the number of chromosomes in mammals, especially in humans. Chromosomes were small, very numerous, difficult to count. When the cells were fixed, they merged into lumps, which made it difficult to determine the true number of chromosomes. Therefore, the first researchers could not accurately and correctly calculate the number of chromosomes in human cells. A different number of chromosomes was called - from 44 to 50.

Usually, chromosomes in cells are observed during mitosis at the stage of the metaphase plate. In the interphase nucleus, chromosomes are not visible under a light microscope. In 1912, G. Winivarter, studying the chromosomes in the spermatogonia and oogonia of the human gonads, removed during the operation, found that the male set of chromosomes (karyotype) contains 47 chromosomes, and the female - 48. In 1922, T. Painter repeated the research Winivarter and found that the male and female karyotypes contain 48 chromosomes each, but the female differs from the male in only two chromosomes. Women have 2 large sex chromosomes, while men have one large X chromosome and one small K chromosome. In subsequent years, this view was supported by other scientists. P. I. Zhivago and A. G. Andrea (1932) proposed the first classification of chromosomes depending on their length. Since the chromosomes are very close to one another and it is very difficult to study them, in subsequent years the exact number of chromosomes in humans has been the subject of controversy and discussion. However, an agreement was gradually reached between researchers on this issue, and for 30 years most cytogeneticists believed that in humans the diploid number of chromosomes is 48, and the haploid number is 24. Improved methods for studying chromosomes have made it possible to obtain more accurate information about the number of chromosomes in human cells. , as well as identify normal karyotype abnormalities responsible for some deformities. Two methods have proved particularly fruitful:

1. Treatment of cell culture with the alkaloid colchicine, which leads to the accumulation of dividing cells at the metaphase stage;

2. Treatment of cells with weak salt solutions, causing swelling, straightening of chromosomes, which facilitates their study.

In 1956, Swedish cytologists J. Tiyo and A. Levan prepared cell cultures from lung tissues taken from aborted human embryos and, using an improved cell processing technique, obtained unusually clear preparations in which 46 chromosomes were clearly visible.

A few months later, C. Ford and J. Hammerton in England found that diploid precursors of germ cells in the testes of men (spermatogonia) also have 46 chromosomes, and haploid ones (spermatocytes of the 1st division) - 23 chromosomes each.

After that, many cells from different human organs and tissues were studied, and everywhere the normal number of chromosomes turned out to be 46.

The female karyotype differs from the male in only one sex chromosome. The remaining 22 pairs are the same for men and women. These 22 pairs of chromosomes are called autosomes. A normal karyotype consists of 44 autosomes (22 pairs) and two sex chromosomes - XX in women and XY in men, that is, the female karyotype has two large sex chromosomes, and the male one has one large and one small.

In human germ cells there is a single (haploid) set of chromosomes - 23, and in somatic cells - a double (diploid) set - 46. These discoveries stimulated further study of chromosomes. Methods have been developed for studying chromosomes in a culture of peripheral blood lymphocytes and on other objects. At present, chromosomes are relatively easily examined in peripheral blood lymphocytes. Venous blood is placed in a special nutrient medium, phytohemagglutinin is added, which stimulates cells to divide, and placed in a thermostat for 72 hours. 6 hours before the end of incubation, colchicine is added here, which delays the process of cell division at the stage of the metaphase plate. Then the culture is placed in a hypotonic NaCl solution, in which the cells swell, which leads to a slight rupture of the shells of the nucleus and the transition of chromosomes into the cytoplasm. After that, the preparations are stained with nuclear dyes, in particular acetoorcein, and examined in a light microscope with immersion.

Considered under a microscope total chromosomes, photograph them, then cut out each chromosome from the photo with scissors and stick it on a blank sheet of paper in a row, starting from the largest (first) chromosome and ending with the smallest (twenty-second) and sex Y-chromosome. The luminescent technique allows you to quickly and easily conduct mass studies in order to identify patients with various types of chromosomal abnormalities. The set of quantitative (number of chromosomes and their sizes) and qualitative (chromosome morphology) features of a diploid set of a single cell is denoted by the term "karyotype". The structure of chromosomes changes depending on the stage of cell division (prophase, metaphase, anaphase, telophase).

Already in the prophase of mitosis, it can be seen that the chromosome is formed by two mutually intertwining threads of the same diameter - chromatids. In metaphase, the chromosome is already spiralized, and its two chromatids lie in parallel, separated by a narrow gap. Each chromatid is made up of two hemichromatids. As a result of mitosis, the chromatids of the maternal chromosome become sister chromosomes, and the semichromatids become their chromatids. Chromatids are based on chromonemes - the so-called thinner strands of DNP, consisting of protein and nucleic acids.

In interphase (the interval between two cell divisions), chromatin is closely associated with nuclear membranes and the nuclear protein matrix. It also forms large areas of despiralized strands of DNP. Then gradually the chromatin spiralizes, forming typical metaphase chromosomes. Their sizes vary from 2 to 10 microns.

Currently, the structural features of autosomes and sex chromosomes (on bone marrow cells, lymphocytes, fibroblasts, skin cells, regenerating liver) are being intensively studied.

Chromosomes contain structures called chromomeres. A chromomere is a coiled section of a chromonema. The spaces between chromomeres are represented by chromonemal filaments. The location of chromomeres on each chromosome is strictly fixed, hereditarily determined.

A chromomere is a relatively large genetic unit, comparable in length to the E. coli chromosome. The structure and function of the chromomere is the main mystery of modern genetics. It is assumed that some chromomeres are one genetic locus, where there is one structural gene and many regulatory genes. It is possible that several structural genes are located in other chromomeres.

Chromonemes and chromomeres are surrounded by a non-staining substance - a matrix. It is believed that the matrix contains deoxyribonucleic and ribonucleic acids, proteins.

Certain sections of chromosomes form nucleoli. Nucleoli are more or less despiralized sections of chromosomes surrounded by products of gene activity (ribosomes, RNA particles, etc.). Here is the synthesis of ribosomal RNA, as well as certain stages of the formation of ribosomes. It synthesizes most of the cell's RNA.

In the metaphase chromosome, several more formations are distinguished: the centromere, two arms of the chromosome, telomeres and a satellite.

The centromeric (meros - in Greek, part) region of the chromosome is a non-staining break in the chromosome, visible on the preparation of chromosomes. The centromere contains 2-3 pairs of chromomeres and has a complex structure. It is believed that it directs the movement of the chromosome in mitosis. Spindle threads are attached to the centromeres.

Telomeres - special structures at the ends of chromosomes - also have a complex structure. They contain several chromomeres. Telomeres prevent the terminal attachment of metaphase chromosomes to each other. The absence of telomeres makes the chromosome "sticky" - it easily attaches to other fragments of chromosomes.

Some regions of the chromosome are called euchromatic, while others are called heterochromatic. Euchromatic regions of chromosomes are genetically active regions; they contain the main complex of functioning nuclear genes. The loss of even the smallest fragment of euchromatin can cause the death of the organism. Heterochromatic regions of chromosomes are usually highly spiralized and, as a rule, are not genetically active. The nucleolar organizer is located in heterochromatin. The loss of even a significant part of heterochromatin often does not lead to death of the organism. Heterochromatic regions of the chromosome replicate later than euchromatic regions. It should be remembered that euchromatin and heterochromatin are not a substance, but a functional state of a chromosome.

If you arrange photographs of homologous chromosomes as their size increases, then you can get the so-called ideogram of the karyotype. Thus, an idiogram is a graphic representation of chromosomes. On the idiogram, pairs of homologues are arranged in rows in order of decreasing size.

In humans, on an idiogram, among 46 chromosomes, three types of chromosomes are distinguished depending on the position of the centromere in the chromosome:

1. Metacentric - the centromere occupies a central position in the chromosome, both arms of the chromosome are almost the same length;

2. Submetacentric - the centromere is located closer to one end of the chromosome, resulting in chromosome arms of different lengths.

Classification of human chromosomes according to the size and location of the centromere
Group of chromosomes	Karyotype number	Characteristics of chromosomes
A(1)	1,2,3	1 and 3 almost metacentric and 2 large submetacentric
AT 11)	4,5	large subacrocentric
C (III)	6-12	medium submetacentric
A(lV)	13-15	medium acrocentric
E(V)	16-18	small submetacentric
F(VI)	19-20	the smallest megacentric
G(VII)	21-22	the smallest acrocentric
X chromosome (belongs to group III	23	medium almost metacentric
Y chromosome	23	shallow acrocentric

3. Acrocentric - the centromere is located at the end of the chromosome. One shoulder is very short, the other is long. Chromosomes are not very easy to distinguish one from the other. Cytogenetics, in order to unify methods for identifying chromosomes, at a conference in 1960 in Denver (USA) proposed a classification that takes into account the size of chromosomes and the location of centromeres. Patau in the same year supplemented this classification and proposed to divide the chromosomes into 7 groups. According to this classification, the first group A includes large 1, 2 and 3 sub- and acrocentric chromosomes. To the second group B - large submetacentric pairs 4-5. The third group C includes medium subacrocentric (6-12 pairs) and the X chromosome, which is located between chromosomes 6 and 7 in size. Group D (fourth) includes medium acrocentric chromosomes (13, 14 and 15 pairs). To group E (fifth) - small submetacentric chromosomes (16, 17 and 18 pairs). To the group F(sixth) small metacentric (19th and 20th pairs), and group G (seventh) - the smallest acrocentric chromosomes (21st and 22nd pairs) and a small acrocentric sex Y-chromosome (Table 4).

There are other classifications of chromosomes (London, Paris, Chicago), in which the provisions of the Denver classification are developed, concretized and supplemented, which ultimately facilitates the identification and designation of each of the human chromosomes and their parts.

The acrocentric chromosomes of group IV (D, 13-15 pairs) and group VII (G, 21-22 pairs) on the short arm bear small additional structures, the so-called satellites. In some cases, these satellites are the cause of the adhesion of chromosomes to each other during cell division in meiosis, resulting in an uneven distribution of chromosomes. In one sex cell, there are 22 chromosomes, and in the other - 24. This is how monosomies and trisomies arise for one or another pair of chromosomes. A fragment of one chromosome can join a chromosome of another group (for example, fragment 21 or 22 joins 13 or 15). This is how translocation occurs. Trisomy of the 21st chromosome or translocation of its fragment is the cause of Down's disease.

Within these seven groups of chromosomes, on the basis of only external differences visible in a simple microscope, it is almost impossible to identify chromosomes. But when processing chromosomes of acryhini, moreover, and with the help of a number of other staining methods, they can be identified. Various

methods of differential staining of chromosomes according to the Q-, G-, C-technique (A. F. Zakharov, 1973) (Fig. 27). Let us name some methods for identifying individual human chromosomes. Various modifications of the so-called method are widely used. Q. For example, the QF method - using fluorochromes; QFQ method - using quinacrine; QFH method - using a special dye company "Hext" No. 33258, which reveals repeating sequences of nucleotides in the DNA of chromosomes (satellite DNA, etc.). Modifications of the GT trypsin method are a powerful tool for studying and individual characterizing chromosomes. Let us name, for example, the GTG method, which includes treatment of chromosomes with trypsin and Giemsa staining, the GTL method (treatment with trypsin and staining according to Leitman).

Known methods with the treatment of chromosomes with acetate salts and Giemsa dye, methods using barium hydroxide, acridine orange and others.

Chromosomal DNA is detected using the Feulgen reaction, staining with methyl green, acridino orange, dye No. 33258 from the Hekst company. Acridino-orange dye with single-stranded DNA forms dimer associates and gives red luminescence, with double-stranded helical DNA forms one-dimensional associates and luminesces green.

By measuring the intensity of red luminescence, one can judge the amount free places in DNP and chromatin, and the ratio green - red luminescence - about the functional activity of chromosomes.

Histones and acidic proteins of chromosomes are detected at different pH by staining with bromphenode blue, strong green, silver, immunoluminescent method, RNA staining with hallucyanin alum, Hext dye No. 1, acridino orange when heated to 60 °.

Electron microscopy, histoautoradiography and a number of other methods are widely used.

In 1969, the Swedish biologist T. Kaspersson and his collaborators showed that chromosomes stained with mustard quinacrine and illuminated under a microscope with the longest wavelength of the ultraviolet spectrum begin to luminesce, with some parts of the chromosomes glowing brighter, others weaker. The reason for this is different chemical composition surface of the chromosome. In subsequent years, researchers discovered that the ends of the human Y chromosome glowed brighter than any other human chromosome, making the Y chromosome easy to spot on a slide.

Acryhiniprit preferentially binds to GC-pairs of DNA. Individual disks of heterochromatic regions fluoresce. Remove the DNA - the glow disappears. Maps of fluorescent chromosomes were compiled. Of the 27 species of mammals, only humans, chimpanzees, gorillas, and orangutans have glowing Y chromosomes. The glow is associated with gene repeats that appeared in evolution 20 million years ago.

So, normally in human somatic cells there are 46 chromosomes (23 pairs), and in sex cells - 23 chromosomes, one chromosome of each pair. When a sperm cell and an egg cell fuse, the number of chromosomes doubles in the zygote. Thus, each somatic cell of the human body contains one set of paternal chromosomes and one set of maternal chromosomes. If a person has 46 chromosomes, then in various monkeys the number of chromosomes is 34, 42, 44, 54, 60, 66.

Under the action of ultrasound or high pressure it is possible to break the DNA strands that make up the chromosome into separate fragments. By heating DNA solutions to a temperature of 80-100°,

you can cause DNA denaturation, the divergence of the two strands that make it up. Under certain conditions, the disconnected DNA strands can reassociate into a stable double-stranded DNA molecule (DNA reassociation or renaturation). DNA denaturation and renaturation can also be obtained on preparations of fixed chromosomes, by processing them accordingly. If after that the chromosomes are stained with the Giemsa dye, then a clear transverse striation is revealed in them, consisting of light and dark stripes. The location of these bands on each chromosome is different. Thus, Giemsa discs can also identify each of the 23 pairs of chromosomes.

These and other methods, especially the hybridization of somatic cells of various animals and humans, are used for mapping chromosomes, i.e., for determining the position of different genes in one or another chromosome. Currently, about 200 genes have been mapped in human autosomes and sex chromosomes.

At the end of 1975, the following number of genes was localized in different human chromosomes (AF Zakharov, 1977): 1 chromosome - 24 genes; 2 chromosomes - 10, 3-2, 4-3, 5-3, 6-14, 7-4, 8-1, 9-8, 10-5, 11-4, 12-10, 13-3, 14 -3, 15-6, 16-4, 17-14, 18-1, 19-4, 20-3, 21-4, 22-1; Y-chromosome - 2; X chromosome - 95 genes.

Chapter 4. Sex chromatin.

In 1949, M. Barr and C. Bertram, studying cat neurons, drew attention to the fact that the interphase cell nucleus contains an intensely stained body, and it is present only in the nuclei of female cells and absent in males. It has been found in many animals, and always only in females. This little body is called the sex chromatin, or Barr body. In a number of vertebrates and in humans, it appears in early ontogenesis at the gastrula stage, but before development gonads (sex glands). The location, shape and structure of sex chromatin is not affected by sex hormones, therefore, it is not a secondary sexual characteristic. Between the number of sex chromatin bodies and the number X- chromosomes in the nucleus are directly connected. Sex chromatin in interphase nuclei is due to the spiralization of one of the X chromosomes, the inactivation of which is a mechanism that equalizes the balance of sex chromosome genes in the cells of males and females (i.e., this is one of the mechanisms for dose compensation of genes).

In 1961, several researchers simultaneously suggested that one of the X chromosomes in normal women is relatively inactive genetically. In 1961, the English researcher M. Lyon put forward a hypothesis about the mechanisms of inactivation of one of the X chromosomes in the cells of the female body. The main points of this hypothesis are as follows:

1. One of the two X-chromosomes of a woman's cells is inactive.

2. An inactive chromosome may be of the paternal or maternal organism.

3. Inactivation occurs in early embryogenesis and persists during further reproduction and development of the cell line. This process of X-chromosome inactivation is reversible in a number of generations:

XX*->- wow ->XX* etc. (here the asterisk denotes the helical X chromosome). The Portuguese geneticist Serra proposed to call this type of reversible changes in the genetic material treption (from the Greek treptos - change).

The spiralized X chromosome in the cell forms the sex chromatin or Barr body. If women have several X chromosomes in the cell nucleus, then there are several Barr bodies in the cells, only one X chromosome remains active. The X chromosome is not completely inactivated, part of the short arm remains genetically active. Inactivation of the X chromosome to a certain extent depends on the stage of the cell cycle and the physiological state of the organism. By the presence of an excess or absence of a Barr body, some types of hereditary diseases can be diagnosed (for example, Klinefelter's syndrome, Shereshevsky-Turner syndrome). Cells that do not contain sex chromatin (chromatin-negative cells) are found in individuals with a set of chromosomes 45, XO (Shereshevsky-Turner syndrome);

46,XY(normal men); 47, XYY(Klinefelter syndrome with two Y chromosomes). Usually, in the cells of a normal male body, a certain number of Barr pseudobodies (condensed sections of autosomes) and spiralized Y chromosomes are found, therefore, when diagnosing various chromosomal diseases, it is necessary to be able to distinguish these formations from the typical sex chromatin formed by a spiralized extra X chromosome. Barr's body is found at chromosome set 46,XX (normal women); 47, XXY and 48, XXYU (classic Klinefelter's syndrome). Two Barr bodies are found in a person with three X chromosomes, (47, XXX); three X chromosomes and one Y (48, XXXY, Klinefelter's syndrome); 49, XXXXY (Klinefelter's syndrome). Three Barr bodies occur in 48, XXXX and 49, XXXXY karyotypes (severe Klinefelter's syndrome).

In polyploid cells, the number of sex chromatin bodies corresponds to ploidy. According to the Gardner formula, the number of Barr bodies (B)

equals B = X - , where X - the number of X chromosomes, R - cell ploidy. In non-polyploid cells, the number of sex chromatin bodies is equal to the number of X chromosomes minus one. (IN = X - 1).

Structural changes in chromosomes

Chromosomes can undergo various structural changes. Particularly important are the loss of individual fragments of chromosomes (division) or the transfer of a section of one chromosome to another (translocation). Translocation is denoted by the Latin letter /, in brackets next to it is written the index of the group or the number of the donor chromosome, the designation of the transferred site. The same designations are indicated for the recipient chromosome, for example 46, XXt (Wed+ + B4 q-). Letters in brackets R And q indicate the chromosome arms affected by the translocation. The short arm of a chromosome is denoted by the letter R, long - letter q, the satellite is indicated by the letter s, etc. An increase in the length of the arm is indicated by a plus sign, and a decrease by a minus sign (both of them are placed after the chromosome symbol).

The appearance of one extra chromosome in the karyotype leads to trisomy. A multiple increase in the number of all chromosomes is called polyploidy (there may be triploids, tetraploids, etc.). The loss of one of a pair of homologous chromosomes leads to a condition called monosomy. Changes in the number or structure of chromosomes are called chromosomal aberrations.

Consider the most frequent species structural disorders of chromosomes - deletions and translocations. With a deletion, the total number of chromosomes is not changed. However, some chromosome lacks genetic material, which causes various changes in the phenotype. The most common deletion is the 5th and 18th autosomes and the X chromosome. Deletions lead to the development of various hereditary diseases and syndromes.

In 1963, J. Lejeune described the "cat cry" syndrome. The cry of such children resembles the "meow of a cat." Children have a sharp underdevelopment of the larynx, a round moon-shaped face, microcephaly, micrognathia, a Mongoloid incision of the eyes, low-lying deformed auricles, muscular hypotension, and mild secondary sexual characteristics. These children are mentally retarded. In the karyotype of children, a deletion of the short arm of the 5th pair of chromosomes is noted.

The division of the long and short arms of the 18th chromosome is accompanied by various violations structures of the face, skeleton, internal organs. Children have mental retardation, malnutrition, hypotension, microcephaly, underdevelopment of the face, low rough voice, underdevelopment of the external genitalia, middle ear, atresia of the external auditory canal and other anomalies.

With a deletion of the short arm of the 18th chromosome, patients also have various defects in the skeleton, internal organs, and mental retardation.

A deletion of the short arm of the X chromosome can be interpreted as a partial monosomy of the X chromosome. Described in women who have growth retardation, ovarian underdevelopment without severe somatic anomalies. Although sex chromatin is detected in them, however, its size is much smaller than normal.

In chronic myeloid leukemia, shortening of the short arm of the 21st chromosome (the so-called Philadelphia chromosome) is noted. However, this chromosome is found only in blood cells and bone marrow punctate. Other cells have a normal karyotype.

As a result of two terminal shortages, followed by the connection of broken ends, ring chromosomes are formed. Therefore, this violation of the structure of chromosomes is actually a special case of a deletion. The clinical picture of patients - carriers of ring chromosomes - resembles that of the deletion of the corresponding chromosome. So, with the ring chromosome of group B (5th pair), the clinical picture of the "cat's cry" syndrome develops, and with the ring X chromosome, the clinical picture is close to Shereshevsky-Turner syndrome.

Translocations are structural rearrangements in which genetic material is exchanged between chromosomes. Possible different kinds translocations: reciprocal, in which there is a mutual exchange of fragments; non-reciprocal, when the genetic material of one chromosome is transferred to another, and finally centric connections. The last translocations between acrocentric chromosomes are the most common. In this case, only a small fragment of the short arms of the acrocentric chromosomes is lost. Most of these rearrangements can be considered balanced, since they do not cause serious deviations in the phenotype of the translocation carrier. However, the offspring of such carriers have clinically pronounced defects characteristic of an abnormal set of chromosomes.

It is known that Down's disease can be observed both in trisomy of the 21st autosome, and in the translocation of a fragment of this chromosome to others. Such patients have 46 chromosomes, but one of the chromosomes is actually double, since a fragment of the 21st chromosome is still attached to it, and as a result, such a rearrangement turns out to be unbalanced. In the parents of these patients, the karyotype included 45 chromosomes, but one of the chromosomes was actually double (with a translocation). When an egg containing this chromosome is fertilized, the normal sperm in the zygote will actually have three 21st chromosomes, which is phenotypically manifested by Down's disease.

The 21st chromosome most often translocates to the 15th or other chromosomes of the D group (13th, 14th) in women, or to the 22nd in men. In this case, young healthy parents may have a child with Down's disease, in contrast to trisomy 21, which is more common in children born to elderly mothers. It is practically impossible to determine the presence of a translocation in an individual before the birth of a child with Down's disease without examining the karyotype, since the phenotype of these carriers is not much different from the phenotypes of individuals with normal genotypes. Therefore, in all these cases, the study of the karyotype is of particular importance.

The mechanism of development of Down's disease during translocation in one of the parents can be represented as follows. In translocation, an individual's karyotype consists of 45 chromosomes, as one chromosome is enlarged. Translocation affects all cells, including oogonia and spermatogonia. During the formation of germ cells (gametes), 23 chromosomes fall into one gamete, and 22 into the other. But the translocated chromosome can end up both in a gamete with 22 chromosomes and in a gamete with 23 chromosomes. Thus, 4 variants of gametes are theoretically possible: 23 normal chromosomes, 23 with translocation, 22 normal chromosomes and 22 with translocation. If the translocation is denoted by an apostrophe, then the following series of gametes will be obtained: 23 23 1 22 22 1 .

If these gametes are fertilized by a normal gamete of the opposite sex, then we get the following combinations: 1) 23 + 23 = 46 chromosomes (normal karyotype); 2) 23 1 + 23 = 46 1 chromosomes, but actually 47 chromosomes (in this case, Down's disease will develop); 3) 22 + 23 = 45 chromosomes (such a zygote is not viable and dies); 4) 22 1 +23 = 45 1 chromosomes (in this case, an individual is born with a translocation, like one of his parents).

The chances of giving birth to a child with Down's disease (with a translocation in one of the parents) are 33%. This is a very big risk, and in this case, further childbearing is not desirable, especially since there is a risk of translocation in grandchildren. If a child with Down syndrome caused by trisomy 21 is born to parents with a normal karyotype, then the chances of giving birth to the same child again are very small. However, not in all cases at the birth of a child with Down's disease due to the translocation of the 21st chromosome, the translocation is present in the somatic cells of the mother. In about half of mothers, the karyotype is normal, and the translocation occurred during meiosis, preceding the formation of the egg from which the organism of the sick child developed.

Chapter 5. Mosaicism.

This is a condition where cells with normal and abnormal karyotypes are mixed in the body, say, 46/47 or 46/45. It arises due to non-disjunction of chromosomes at the initial stages of embryonic development. Mosaicism gives erased, mild symptoms of the disease compared with patients who have a changed karyotype in all cells. A person with a mosaic variant of Down's disease may have only some of the physical signs of the disease. The development of intelligence is not disturbed. With mosaicism 45XO / 46XX, Shereshevsky-Turner syndrome is more mildly expressed. These patients may develop ovarian tissue and ovulate. With the 46XY/47XXY karyotype, Klinefelter's syndrome is more mildly expressed. Among patients, female and male mosaics with the indicated karyotypes are more common than "pure" cases of Shereshevsky-Turner syndrome or Klinefelter's syndrome. With age, the clone of abnormal cells is gradually eliminated, and therefore it is difficult to establish mosaicism in the elderly, although in the embryonic and early postembryonic period it was sufficiently pronounced and could lead to the development of phenotypic signs of the disease. The fewer abnormal cells in the body, the weaker the signs of the disease. This can explain the erased and rudimentary forms of these diseases.

With blood diseases, a multiple (polyploidy) or non-fold (aneuploidy) increase in the number of chromosomes can occur. However, it is observed only in blood cells, while in other somatic cells the karyotype is normal.

List of used literature.

1. Berdyshev G.D., Krivoruchko I.F. Human genetics with the basics of medical genetics. - Kyiv: Vishcha school, 1979.

2. Bochkov N.P. Human genetics. - Moscow, 1973.

3. Vogel F. Motulsky A. Human genetics: the history of the human chromosome, formal genetics. Moscow: Mir, 1989.

4. Stern, Kurt Fundamentals of human genetics. Moscow: Medicine, 1965

5. McKusick V. Human genetics. Moscow: Mir, 1967.

Human genetics with the basics of medical genetics. Berdyshev G.D., Krivoruchko I.F. p.5-21

Human genetics: the history of the human chromosome, formal genetics. Vogel F. Motulsky A. pp.11-19

Human genetics: the history of the human chromosome, formal genetics. Vogel F. Motulsky A. pp.23-31

Human genetics. Bochkov N.P. With. 44

Human genetics. McKusick. V. p.10, 13-22

Fundamentals of human genetics. Stern, Kurt. p.41-60

Human genetics. Bochkov N.P. p.90

A karyotype is a diploid set of chromosomes of a given type of organism, characterized by a constant number, size and shape of chromosomes. The human karyotype has 46 chromosomes or 23 pairs. Paired chromosomes are called homologous, they have the same length and shape, contain allelic genes. Chromosomes are 40% DNA, 40% histone proteins and 20% non-histone proteins. The complex of all chemical substances that make up the chromosomes is called chromatin. Chromosomes can be in cells in two structural and functional states - spiralized and despiralized. During interphase, they are in a despiralized state. In a spiralized state, they are in the period of mitosis. The maximum spiralization of the chromosome is reached in the metaphase of mitosis. The metaphase chromosome consists of two chromatids connected in the region of the primary constriction (centromere). Some chromosomes have secondary constrictions and satellites. The centromere divides the chromatid into two arms. The short arm is usually denoted by the letter p and the long arm by the letter q.

The structure of the metaphase chromosome.

secondary

constriction p

centromere

In 1960, the English geneticist Patau proposed to classify human chromosomes based on the relative length and position of the centromere (centromeric index). The centromeric index is the ratio of the length of the short arm to the length of the entire chromosome. In accordance with the centromere index, metacentric (constriction in the middle), submetacentric (one arm is longer than the second), and acrocentric chromosomes (with a disproportionately very short one arm) are distinguished.

metacentric submetacentric acrocentric

In 1960, at the international genetic symposium in Denver (USA), the International (Denver) classification of human chromosomes was adopted. Patau developed the basic principles of classification. The classification takes into account the length and shape of the chromosomes. All pairs of autosomes are numbered with Arabic numerals from 1 to 22 in decreasing order of their length. The sex chromosomes are denoted by the Latin letters X and Y and are placed at the end of the layout. Women normally have XX sex chromosomes, while men have XY.

All pairs of autosomes are divided into 7 groups in accordance with the length and shape of the chromosomes. Groups are denoted by Latin letters from A to G. Groups are clearly distinguished from each other.

Group A(1,2,3 pairs) the longest metacentric (1,3) and submetacentric (2) chromosomes. Chromosome 1 is the largest metacentric chromosome, the centromere is located in the middle. The largest submetacentric chromosome is chromosome 2. Chromosome 3 is almost 20% shorter than chromosome 1 and therefore easily identified. Absolute length from 11 µm (1 pair) to 8.3 µm (2 pairs).

Group B(4 and 5 pairs) long submetacentric chromosomes. They do not differ from each other without differentiated staining. The absolute length is 7.7 µm.

Group C(6 - 12 pairs). Medium-sized chromosomes, submetacentric. With standard (routine) staining, the X chromosome cannot be distinguished from other chromosomes in this group. It is similar in size to chromosomes 6 and 7. Absolute length from 7.7 µm (6 pairs) to 5.8 µm.

GroupD(13 - 15 pairs). These acrocentric chromosomes are very different in shape from all other human chromosomes. All three pairs on the short arm contain satellites. The length of the proximal portions of the short arms varies, satellites may be absent or very large, may fluoresce brightly, or may not give fluorescence. Absolute length from 4.2 µm.

Group E(16 - 18 pairs). Relatively short submetacentric chromosomes. The absolute length is 3.6-3.5 µm.

GroupF- (19 - 20 pairs) small metacentric chromosomes. In preparations with routine staining, they look the same, but with differential staining they differ sharply. The absolute length is 2.9 µm.

GroupG(21 - 22 pairs) - two pairs of the smallest acrocentric chromosomes. On the short shoulder they have a satellite. The variability of their short arms is as significant as in group D chromosomes. The absolute length is 2.9 µm.

The Y chromosome is a small acrocentric chromosome 2.8 µm long. Usually (but not always) larger than the G group chromosomes, and the chromatids of its long arm tend to lie parallel to each other. In this it differs from the chromosomes of group G, in which the chromatids of the long arms form a wide angle.

The X chromosome is similar in routine staining to influenza C chromosomes, but differs when differentiated staining is used. X - submetacentric chromosome, 6.8 µm long.

Classification of chromosomal diseases.

More than 1000 diseases have been described so far. About a hundred of them have a clear clinical picture and are called syndromes. All chromosomal diseases can be divided into three groups depending on the nature of the change in the karyotype.

Chromosomal diseases.

Polyploidy Associated diseases Associated diseases

with a change in number and with a change in number and

structures of autosomes structures of sex

chromosomes.

Depending on the percentage of affected cells, there are full chromosomal diseases and mosaic. Complete ones are the result of a generative mutation in the parents (i.e., mutations occur during the formation of germ cells in the parents). All embryonic cells have an altered karyotype. Mosaic are the result of a somatic mutation that occurs in the embryo itself during the embryonic period of development. Therefore, some of the patient's cells have a normal set of chromosomes, and some of the cells are altered.

Allocate also sporadic chromosomal diseases (a consequence of a new mutation) and inherited(inherited from parents with a balanced mutation or from parents with a chromosomal disease). Cases of the birth of children in patients with Klinefelter's syndromes, polysomy X in women, polysomy Y in men, and women with Down syndrome are described. Men with Down syndrome are infertile because they they have impaired spermatogenesis.