Barr bodies with an additional segment of the nucleus. Technical map of the practical lesson "Genetics of sex in humans
Often in gynecology and pediatric andrology, it becomes necessary to conduct a genetic examination of the patient. This method allows you to more reliably establish the causes of the disease that has arisen, select the necessary course of treatment and determine the prognosis in the development of the disease.
Sex chromatin is an important indicator of normal sexual development at the cellular level. Its diagnosis is necessary when we are talking about anomalies in the structure of the external genitalia. The study is also carried out in the detection of diseases caused by chromosomal pathologies and in the presence of signs of impaired sexual development. The reason for this may be the absence of menstruation in girls and the underdevelopment of the ovaries or a decrease in their function in boys.
Sex chromatin was first discovered by scientist J. Bar in 1949. While examining a cat, he noticed a chromatin formation on the periphery of the nucleus. Subsequently, this important feature of female somatic cells was revealed in most of the representatives of the order of mammals. In the cells of the male, such bodies were not found. The Barr body is a component of all women. And in some cases it has peculiar appendages (“drum sticks”). Somatic cells are integral parts human body. These include all cells of the body, except gametes. This discovery allowed us to expand our understanding of methods for determining sex in humans and mammals.
The presence of Barr bodies in the female body is explained by the presence of two X chromosomes in somatic cells. And only one of them is active. That is why in female cells there is always a Barr body. This theory later made it possible to identify anomalies in the development of the organism. So, a cell with three chromosomes will have two bodies, with four - three, and so on. Diagnosis of abnormal cell development can be applied to both men and women. Since 1953 this discovery has become actively used to determine the exact developmental disorders.
Sex chromatin, or Barr's body, looks like a dark mass of a rounded, triangular or rod-shaped shape, which is located near the inner surface of the nuclear membrane. This is an inactive X chromosome, which in its diameter does not exceed 1 micron. The detection of Barr bodies in 10-12% of the studied cells indicates positive sex chromatin. The material of the study is usually the epithelium of the buccal mucosa (inner surface), vagina, hair follicles, and if the indicator does not exceed 5%, this indicates a negative sex chromatin.
Sex chromatin causes differences in interphase nuclei in men and women. It is associated with the features of functioning and their structure. There are two types of Barr bodies: Y and X. The first variant is a structural component of the Y chromosome and is found mainly in men. For these purposes, a fluorochrome is used with the use of ultraviolet light. X-chromatin (Barr body) is an inactivated X chromosome. Its deactivation occurs as early as the first weeks of intrauterine development and persists during mitotic division for a long time.
It should be noted that sex chromatin is a dynamic structure. It can vary depending on the general condition of the body and cell metabolism. It is for this reason that it is not recommended to conduct an examination during the use of hormonal drugs, in the first days after birth, after taking antibiotics and other medicines.
Today, by the presence of sex chromatin, doctors determine the genetic sex of the unborn child, diagnose various clinical forms of gonadal dysgenesis and hermaphroditism. Analyzes for the detection of this component are widely used in forensic practice.
In 1949, M. Barr and C. Bertram found small bodies 0.8–1.1 μm in size in the nuclei of cat neurons, called Barr bodies. It turned out that Barr bodies are present in the interphase nuclei of somatic cells of female mammals and absent in males.
Similar structures were found in the nuclei of polymorphonuclear leukocytes in the form of "drumsticks" (drumsticks). Subsequently, it was found that these bodies are a condensed X chromosome, which undergoes inactivation even in early embryogenesis. In humans, there is a direct relationship between the number of sex chromatin bodies and the number of sex chromosomes: the number of sex chromatin bodies is one less than the number of X chromosomes. In a healthy woman, one body of sex chromatin can be found in somatic cells, for her this is the norm, for a man it is an anomaly. Only on the basis of this analysis can a doctor suspect Klinefelter's syndrome. If a woman does not have nuclei with sex chromatin, a karyotype with XO should be assumed, and if there are 2, 3 or more bodies, then karyotypes with XXX, XXXX, XXXXX chromosomes are possible. In the polymorphonuclear leukocytes of the blood of women, 1-6% of "drumsticks" are found, in men they are not normally found. The determination of sex chromatin HRP is used for diagnostic purposes of disorders of the sex X chromosomes.
Diagnostic value of the sex chromotype Due to the availability and simplicity of the method, the determination of sex chromatin has found wide application in examining patients with sex chromosome disorders, determining the sex of the fetus in prenatal diagnosis of sex-linked diseases, in forensic medicine to determine the gender of blood spots or parts of a corpse, in oncology for the appointment of targeted hormone therapy, in organ and tissue transplantation as a kind of marker of the sex of the donor and recipient.
Dermatoglyphics method. The method is designed to study the relief of the skin on the fingers, palms and soles of the feet. In these parts of the body there are epidermal protrusions - crests that form complex patterns. Each person has his own pattern of skin patterns, which makes it possible to use the method for identifying a person in forensic science.
Skin patterns are examined in various directions: fingerprinting - fingertips, palmoscopy - palms and plantoscopy - the soles of the feet. Papillary lines on the finger pads form three types: arc, loop, curl. A quantitative indicator of dermatoglyphics is the ridge count. The arc pattern occurs with a frequency of 6%, the loop pattern is about 60%, and the whorl pattern occupies an average position of 34%.
Finger triradii are located at the base of the fingers. (a, b, c, d). Near the bracelet fold separating the hand from the forearm is the main triradius (t), angle formed at the junction of lines atd, normally not exceeding 57 o. It has been established that with chromosomal abnormalities, the angle atd is: with Down's syndrome 81 0 , Patau 108 0 , Shereshevsky-Turner syndrome 66 0 , Klinefelter's syndrome 42 0 . The method can be used in clinical genetics, as an additional method for predicting the manifestation of diseases with a hereditary predisposition
biochemical methods. This large group of methods is used to diagnose metabolic diseases, which are caused by changes in the activity of enzymes, and the cause of the changes are gene mutations. Biochemical methods are based on the determination of either the primary product of the activity of a mutant gene - a mutant protein, or the determination of intermediate metabolic products formed under the influence of defective enzymes. The methods are time-consuming to perform and cannot be used for mass studies.
V last years many countries have developed special programs called screening programs. For their implementation, the simplest and most accessible express diagnostic methods are used. Population-statistical method. The method is intended to study the distribution of individual genes in human populations, otherwise the study of the genogeography of hereditary diseases Based on the accumulated data in this regard, groups of genes of the gene concentration category are distinguished: genes that have a universal distribution, which include most of the known ones (phenylketonuria, color blindness, various forms of dementia ), genes that have a local distribution (sickle cell anemia, congenital dislocation of the hip). The population-statistical method makes it possible to calculate the genetic structure of a population from the frequencies of genes and genotype carriers in various population groups. For these purposes, the Hardy-Weinberg equation is used. Method of genetics of somatic cells. Somatic cell hybridization is a method of fusion of two different types of cells obtained from different people, as well as human cells with cells from mice, guinea pigs, Chinese hamsters, monkeys and other organisms. When such cells merge, a heterokaryon is formed - a hybrid cell containing both types of nuclei in one cytoplasm. After mitotic division, mononuclear cells are formed from binuclear heterokaryons. They form a synkaryon, a hybrid cell with the chromosomes of both parental cells. For example, human-mouse hybrid cells have 43 pairs of chromosomes, 23 pairs from humans and 20 pairs from mice. With further reproduction, the gradual disappearance of chromosomes of one species occurs. This method allows you to build genetic maps X human rhomosome.
- The Barr body (X-sex chromatin) is an inactive X-chromosome folded into a dense (heterochromatic) structure, observed in the interphase nuclei of somatic cells of female placental mammals, including humans. It stains well with basic dyes.
Any of the two X chromosomes in the genome can be inactivated at the beginning of embryonic development; the choice is made randomly. An exception in the mouse is the cells of the germinal membranes, also formed from the tissue of the embryo, in which only the paternal X chromosome is inactivated.
Thus, in a female mammal, heterozygous for any trait determined by the X-chromosome gene, different alleles of this gene work in different cells (mosaicism). A classic visible example of such mosaicism is the coloration of tortoiseshell cats - in half of the cells the X chromosome with the "red" is active, and in the other half - with the "black" allele of the gene involved in the formation of melanin. Tortoiseshell cats are extremely rare and have two X chromosomes (aneuploidy).
In humans and animals with aneuploidy, having 3 or more X chromosomes in the genome (see, for example, Klinefelter's syndrome), the number of Barr bodies in the nucleus of a somatic cell is one less than the number of X chromosomes.
Related concepts
Chromosomal rearrangements (chromosomal mutations, or chromosomal aberrations) are a type of mutations that change the structure of chromosomes. The following types of chromosomal rearrangements are classified: deletions (loss of a chromosome section), inversions (change in the order of genes of a chromosome section to reverse), duplications (repetition of a chromosome section), translocations (transfer of a chromosome section to another), as well as dicentric and ring chromosomes. Isochromosomes are also known, bearing two identical arms. If restructuring changes...
The process of implementing a genetically determined program for the formation of a specialized cell phenotype, reflecting their ability to perform certain profile functions. Differentiation changes the function of the cell, its size, shape and metabolic activity.
Linked inheritance is the phenomenon of correlated inheritance of certain states of genes located on the same chromosome.
Conjugation in ciliates is the sexual process of ciliates, accompanied by the transfer of nuclei between partner cells during their direct contact. The presence of such a peculiar sexual process is a unique feature of ciliates. The sexual process in ciliates, unlike the sexual process in the usual view, is not accompanied by the formation of gametes, therefore they do not have a zygote. In addition, the conjugation of ciliates is not accompanied by reproduction, that is, an increase in the number of cells, therefore, conjugation ...
Cajal body (TC) (eng. Cajal body, CB) is a formation in the nucleus of a cell that is present in some nuclear organisms. The typical size of Cajal bodies is 1-2 microns, and one cell can contain from 0 to 10 TCs. Many cell types do not have TK, but TK is found in the nuclei of neurons and cancer cells. The main function of Cajal bodies is the processing of small nuclear and small nucleolar RNAs, as well as the assembly of ribonucleoprotein complexes.
Paraspeckles, or paraspeckles, are a class of nuclear bodies located in the interchromatic space of the cell nucleus in mammalian cells. They are composed of proteins and RNA and are formed by the interaction of a long non-coding RNA known as NEAT1/Men ε/β and proteins of the DBHS (Drosophila Behavior Human Splicing) family, namely P54NRB/NONO, PSPC1 and PSF/SFPQ. Paraspeckles play an important role in the regulation of gene expression, ensuring the retention in the nucleus of RNA molecules containing ...
Epistasis is a type of gene interaction in which the expression of one gene is influenced by another gene (s) that is not allelic to it. A gene that suppresses the phenotypic manifestations of another is called epistatic (inhibitor, suppressor); a gene whose activity is changed or suppressed is called hypostatic.
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 the development of the 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). 6
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 V = 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. (V = 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 results in a condition called monosomy. Changes in the number or structure of chromosomes are called chromosomal aberrations.
Let's consider the most frequent types of 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, 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 structural disorders of the face, skeleton, and 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. Various types of translocations are possible: 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.
Sex chromosomes (gonosomes, heterosomes) differ both in structure (length, position of the centromere, amount of heterochromatin) and in the content of genes.
Chromosome X- this is a medium-sized submetacentric chromosome, included in group C). It is present in the somatic cells of individuals of both sexes: in a double copy in women with a 46,XX karyotype and in one copy in men with a 46,XY karyotype; and also in one copy in all eggs and 50% of spermatozoa. The X chromosome is rich in euchromatic regions and contains 1336 genes, including: somatic genes, feminization regulatory genes, structural feminization genes, structural masculinization genes. Thus, the X chromosome is mandatory in the somatic cell karyotype of both female and male sexes.
Chromosome Y is a small acrocentric chromosome, belongs to group G; 2/3 of the distal arm q are represented by heterochromatin and are genetically inactive. The Y chromosome is represented by one copy in all somatic cells of male individuals with a 46,XY karyotype and in 50% of spermatozoa. It contains 307 genes, among which are: masculinization regulatory genes (SRY + Tdf), fertility genes (AZF1, AZF2), several structural somatic genes and pseudogenes.
Morphological and genetic differences between X and Y chromosomes, as well as differences in the number of X chromosomes in the karyotype, caused genetic inequality between the sexes (women have a double dose of X chromosome genes compared to men. However, this inequality does not manifest itself, due to the compensation mechanism, as a result which only one X chromosome remains functional in the somatic cells of both men and women, namely:
In cells 46,XX, only one X chromosome is active;
In cells 46,XY, X and Y chromosomes are active;
In cells 47,XXX, only one X chromosome is active;
In cells 47,XXY, only one X chromosome and one Y chromosome are active;
In cells 48,XXXY, only one X and one Y chromosome is active;
By heterochromatinization of one of the two X chromosomes and females, X sex chromatin is formed, and as a result of heterochromatinization of the 2/3q Y chromosome, Y sex chromatin is formed in males.
Sex chromatin X:
Represents an inactivated X chromosome in the form of facultative heterochromatin, in somatic cells 46,XX;
It is detected in the interphase nuclei of somatic cells in the form of a Barr body about 1 µm in size;
The Barr test is used to determine the number of X chromosomes in the karyotype in normal and in the case of gnosomal aneuploidies;
The number of xp.X = the number of Barr bodies + 1 (active xp.X);
46,XX - 1 Barr body;
46,XY - no Barr body;
47,XXX - 2 Barr bodies;
47,XXY - 1 Barr body;
45,X - Barr body is absent;
48,ХХХХ - 3 Barr bodies.
Sex chromatin Y:
2/3 of the Y q arm of the Y chromosome is present, in the form of constitutive heterochromatin, in somatic cells 46,XY and spermatozoa 23,Y;
It is detected in the interphase nuclei of cells in the form of a body F (fluorescent) about 1 µm in size;
The F test is used to identify the Y chromosome (prenatal sex determination);
Number xp.Y = number of bodies F;
46,XX - there is no body F;
46,XY - 1 body F;
47, XYY - 2 bodies F;
47,XXY - 1 body F;
48,XXYY - 2 bodies F;
46,X,i(Yp) – there is no F body;
46,X,i(Yq) – 1(0.5 µm) F body.
