Structure of chromosomes
The nucleus of each somatic cell of the human body contains 46 chromosomes. The set of chromosomes of each individual, both normal and pathological, is called a karyotype. Of the 46 chromosomes that make up the human chromosome set, 44 or 22 pairs are autosomal chromosomes, the last pair is the sex chromosomes. In women, the constitution of the sex chromosomes is normally represented by two X chromosomes, and in men by the X and Y chromosomes. In all pairs of chromosomes, both autosomal and sexual, one of the chromosomes is derived from the father and the second from the mother. Chromosomes of one pair are called homologues, or homologous chromosomes. In the sex cells( spermatozoa and ovules) contains a haploid set of chromosomes, that is, 23 chromosomes. Spermatozoa are divided into two types, depending on whether they contain the X or Y chromosome. All eggs normally contain only the chromosome X.
Chromosomes are clearly visible after special coloration during cell division, when the chromosomes are maximally helical. Moreover, in each chromosome, a constriction is revealed, which is called a centromere. The centromere divides the chromosome by a short arm( denoted by the letter "p") and a long arm( denoted by the letter "q").The centromere determines the movement of the chromosome during cell division. By position, the chromosome centromeres are classified into several groups. If the centromere is located in the middle of the chromosome, then this chromosome is called metacentric, if the centromere is located closer to one end of the chromosome, it is called acrocentric. Some acrocentric chromosomes have so-called satellites, which in a nondividing cell form nucleoli. The nucleoli contain numerous copies of the rRH K. Moreover, submetacentric chromosomes are distinguished when the centromere is not located in the middle of the chromosome, but is shifted to one of the ends, but not as significantly as in the acrocentric chromosomes.
The ends of each arm of the chromosome are called telomeres. It has been established that telomeres play an important role in maintaining the stability of chromosomes. The telomeres contain a large number of repeats of a sequence of nucleotides, the so-called tandem repeats. Normally, during cell division, there is a decrease in the number of these repeats in telomeres. However, each time they are completed with a special enzyme called telomerase. A decrease in the activity of this enzyme leads to a shortening of telomeres, which is believed to be the cause of cell death, and normally accompanies aging.
Before the appearance of methods for differential chromosome coloring, they were distinguished by the substrate, the position of the centromere, and the presence of satellites. Allocated 5 groups - from A to G, which are fairly well separated from each other. However, within the groups, the differentiation of chromosomes represented certain difficulties. This changed when methods for differential chromosome coloring were developed. A prominent contributor to the development of these methods was the Russian scientist AF Zakharov.
Chromosome preparations can be prepared from any nuclear fissile somatic cells. They are often obtained for peripheral blood lymphocytes. Lymphocytes are isolated from venous blood and transferred to a small amount of nutrient medium with the addition of phytohemagglutinin. Phytohemagglutinin stimulates the division of lymphocytes. The cells are then cultured at 37 ° C for three days, after which colchicine is added to the lymphocyte culture, which stops cell division at the metaphase stage, when the chromosomes are most condensed. The cells are transferred to a slide, a hypotonic NaCl solution is added to them. Cells burst, and the chromosomes flow out of them. Then follows the fixation and coloring of the chromosomes.
In recent years, new methods for visualizing chromosomes or their parts have appeared. The methods are a combination of cytogenetic and molecular genetic methods. They are all based on the ability of single-stranded DNA to bind to the complementary sequence of genomic DNA localized in chromosomes. Single-stranded DNA, which in this case is a DNA probe, is loaded with a special dye, and after joining with genomic DNA the probe is easily detected on a chromosomal preparation( the so-called metaphase plate) when it is microscopized in ultraviolet light. This method is called "fluorescence in situ hybridization."
All methods of coloring chromosomes can reveal their structural organization, which is expressed in the appearance of transverse striation, different in different chromosomes, as well as some other details.
Several different color methods are used to identify individual chromosomes. The most commonly used method is the chromosome coloring of the Giemsa dye. The chromosome preparations with this coloring method are first treated with trypsin, which removes the proteins contained in the chromosome. Then a Giemsa dye is applied to the preparation, which reveals in the chromosomes a pattern of light and dark segments characteristic for each of them. Usually, up to 400 segments can be counted on a haploid set. If the chromosomes are first heated before staining the Giemsa, then the pattern of the bands is preserved, but their color changes to the opposite one, i.e., the dark bands become light, and vice versa. This method of coloring is called reverse banding, or R-method. If, prior to the application of the Giemsa dye, the chromosome preparation is first treated with acid and then with alkali, then centromeres and other regions rich in heterochromatin, containing highly repeating DNA sequences, are stained. High-resolution methods for differential chromosome coloring have also been developed. They allow us to identify up to 800 transverse bands on the haploid set of chromosomes.
Transverse strips that are identified by differential coloration are called segments. The character of the arrangement of segments along the length of the chromosomes is different, which makes it possible to conduct a sufficiently accurate identification of each chromosome in a karyotype. A form of representation of an ideal karyotype with a typical pattern of bands on each chromosome has been developed. This form is called an ideogram.
For the convenience of describing the karyotype, a special system is proposed, in which the shoulders of the chromosome are first distinguished: p - short and q - long, - and centromeres - n .Each shoulder is divided into regions, and the count goes from the centromere. Each region is divided into segments, the account of which also begins with a segment located closer to the centromere.
The material from which the chromosomes are constructed is called chromatin. It consists of DNA and the surrounding histones and other proteins. That part of the chromatin, which is poorly colored by special dyes for chromosomes, is called euchromatin, and the one that is intensely stained is heterochromatin. It is believed that euchromatic regions of chromosomes contain actively expressed genes, heterochromatin regions, on the contrary, include inactive genes and non-expressing DNA sequences.
The molecular structure of chromosomes is quite complex. The function of this structure is to pack DNA so that it fits in the chromosome. If genomic DNA were represented as an ordinary double-stranded spiral, then it would extend to 2 m. When packing DNA, the same principle of the spiral is used, but it is represented by several levels. As a result of complex packaging, the initial length of the DNA molecule decreases by a factor of 10,000.