All living organisms contain a certain set of genetic material in the nuclei of cells. In eukaryotic cells, it is represented by chromosomes. For the convenience of accounting and scientific research, the karyotype is systematized using various techniques. Let's get acquainted with the methods of ordering genetic material on the example of human chromosomes.
Human chromosome classification
A karyotype is a chromosome set (diploid) located in any of the somatic cells of the body. It is characteristic of a given organism and is the same in all cells, with the exception of reproductive cells.
Chromosomes in the karyotype are:
- autosomes do not differ in individuals of different sexes;
- sexual (heterochromosomes) differ in structure in individuals of different sexes.
The cells of the human body contain 46 strands of DNA, of which 22 are pairs of autosomes and one is reproductive. This is a diploid 2n set of genetic material. A pair of heterochromosomes in women is designated XX, in men - XY, the designation of the karyotype, respectively, 44 + XX and 44 + XY.
In germ cells (gametes) there is a haploid or single 1n DNA set. The eggs contain 22 autosomes and one X chromosome, sperm cells contain 22 autosomes and one of the heterochromosomes, X or Y.
Why identification and classification of chromosomes is needed
The Denver and Paris systems of classification of hereditary material, widely used in the scientific community, are designed to unify and generalize the idea of the karyotype. A general approach is needed for the correct presentation and interpretation of research results in the field of genetics, karyosystematics, and selection.
Schematically, a karyotype is depicted using an ideogram, a sequence of systematized and descending chromosome sizes. The ideogram reflects not only the size of the spiralized DNA, but also some morphological characteristics, as well as the features of their primary structure (hetero- and euchromatin regions).
By analyzing these graphs, the degree of kinship between various systematic groups of organisms is established.
The karyotype can contain pairs of autosomes that are almost identical in size, which makes it difficult to correctly position and number them. Consider what parameters take into account the Denver and Paris classification of human chromosomes.
Denver Conference Results 1960
In that year, a conference on human chromosomes was held in the city of Denver, USA. On it, various approaches to the systematization of chromosomes (in size, position of centromeres, regions with different degrees of spiralization, etc.) were combined into a single system.
The decision of the conference was the so-called Denver classification of human chromosomes. This system is guided by the principles of:
- All human autosomes are numbered in the order of 1 to 22 as their length decreases, and the designations X and Y are assigned to sex chromatids.
- Karyotype chromosomes are divided into 7 groups, taking into account the position of centromeres, the presence of satellites and secondary constrictions on chromatids.
- To simplify the classification, a centromeric index is used, which is calculated by dividing the length of the short arm by the entire length of the chromosome and is expressed as a percentage.
Denver classification of chromosomes is universally recognized in the global scientific community.
Chromosome groups and their characteristics
Denver classification of chromosomes includes seven groups in which autosomes are arranged in numerical order, but are unevenly distributed in number. This is due to the signs by which they are distributed in groups. More on this in the table.
Chromosome group | Numbers of pairs of chromosomes | Features of the structure of chromosomes in the group |
A | 1-3 | Long chromosomes, well distinguishable from each other. In 1 and 3 pairs the position of the waist is metacentric, in 2 pairs - submetacentric. |
B | 4 and 5 | Chromosomes are shorter than the previous group, the primary constriction is located submetacentrically (close to the middle). |
C | 6-12 X chromosome | Chromosomes of medium size, all inequalities are submetacentric, difficult to be individualized. Identical in size and shape to group autosomes, replication ends later than others. |
D | 13-15 | Chromosomes in the medium-sized group with almost edge position of the primary constriction (acrocentric) have satellites. |
E | 16-18 | Short chromosomes, in the 16th pair the metacentric equal arms, in the 17th and 18th - the submetacentric. |
F | 19 and 20 | Short metacentrics are practically indistinguishable from each other. |
G | 21 and 22 Y chromosome | Short chromosomes with satellites, acrocentric. They have slight differences in structure and size. A little longer than the other chromosomes of the group, on the long shoulder there is a secondary constriction. |
As you can see, the Denver classification of chromosomes is based on an analysis of morphology without any DNA manipulation.
Paris classification of human chromosomes
This classification, introduced since 1971, is based on the methods of differentiated chromatin staining. As a result of routine coloring, all chromatids acquire their own pattern of light and dark stripes, which makes it easy to identify within groups.
When processing chromosomes with various dyes, individual segments are detected:
- Q-segments of chromosomes fluoresce as a result of the use of the dye Akrikhin mustard.
- G-segments appear after staining according to the method of Giemsa (coincide with Q-segments).
- R-segment staining is preceded by controlled thermal denaturation.
To indicate the locations of genes on chromosomes, additional notations are introduced:
- The long arm of the chromosome is indicated by a lowercase letter q, the short - a lowercase p.
- Inside the shoulder, up to 4 regions are distinguished, which are numbered from the centromere to the telomeric end.
- The numbering of bands within the areas also goes in the direction from the centromere.
If the position of the gene in the chromosome is known exactly, its coordinate is the strip index. When the localization of a gene is less defined, it is designated as being in a long or short shoulder.
For exact mapping of chromosomes, the study of mutagenesis and hybridization, any one technique can not do. The Denver chromosome classification and the Paris one in this case are inextricably linked and complement each other.