Denver classification of chromosomes as a basis for karyotyping

All living organisms contain a certain set of genetic material in the nuclei of cells. In eukaryotic cells it is represented by chromosomes. For convenience of the account and scientific researches the karyotype is systematized by means of various techniques. Let's get acquainted with the methods of ordering genetic material on the example of human chromosomes.

Classification of human chromosomes

Karyotype is a chromosome set (diploid), located in any of the body's somatic cells. It is characteristic of the given organism and is the same in all cells, with the exception of sex.

Chromosomes in karyotype are:

• Autosomes, do not differ in individuals of different sexes;
• Sex (heterochromosomes), differ in structure in individuals of different sexes.

The cells of the human body contain 46 strands of DNA, 22 of them are autosomes and one is genital. This is a diploid 2n set of genetic material. The pair of heterochromosomes in women is designated XX, in men XY, the karyotype designation, respectively, 44 + XX and 44 + XY.

In the gametes (gametes) there is a haploid or single 1n DNA set. Egg cells contain 22 autosomes and one X chromosome, spermatozoa - 22 autosomes and one of the heterochromosomes, X or Y.

Why is chromosome identification and classification necessary?

Denver and Parisian classification systems of hereditary material, widely used in the scientific community, are designed to unify and generalize the notion of a karyotype. A general approach is needed for the correct presentation and interpretation of research results in the field of genetics, karyosystematics, and selection.

Schematically, the karyotype is represented by an ideogram - a sequence of systematized and descending chromosomes. The ideogram reflects not only the dimensions of spiraled DNA, but also some morphological characteristics, as well as features of their primary structure (regions of hetero- and euchromatin).

Using the analysis of these graphs, the degree of kinship between different systematic groups of organisms is established.

In a karyotype, there may be pairs of autosomes, practically identical in size, which makes it difficult to locate and number them correctly. Let us consider what parameters take into account the Denver and Parisian classification of human chromosomes.

The results of the conference in Denver, 1960

In that year, a conference on human chromosomes was held in Denver, USA. Various approaches to the systematization of chromosomes (in terms of size, position of centromeres, areas with different degrees of spiralization, etc.) have been 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:

1. All human autosomes are numbered in order from 1 to 22 as their length decreases, and the X and Y symbols are assigned to the sex chromatids.
2. Chromosomes of the karyotype are divided into 7 groups, taking into account the position of the centromere, the presence of satellites and secondary constrictions on the chromatids.
3. 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 expressed as a percentage.

Denver classification of chromosomes is universally recognized in the world scientific community.

Groups of chromosomes and their characteristics

The Denver classification of chromosomes includes seven groups in which the autosomes are arranged in order of numbering, but are distributed unevenly in number. This is due to the characteristics by which they are divided into groups. More about this in the table.

As you can see, the Denver classification of chromosomes is based on the analysis of morphology without any manipulation of DNA.

Paris Classification of Human Chromosomes

This classification, introduced since 1971, is based on the methods of differential chromatin staining. As a result of routine coloring, all chromatids acquire their own pattern from light and dark bands, so that they are easily identified within groups.

When processing chromosomes with different dyes, individual segments are identified:

• Q-segments of chromosomes fluoresce as a result of application of the acrylic-mustard dye.
• G-segments appear after staining using Giemsa method (coincide with Q-segments).
• The coloration of R-segments is preceded by controlled thermal denaturation.

To indicate the locations of genes on chromosomes, additional designations are introduced:

1. The long arm of the chromosome is indicated by the lowercase letter q, the short one by the lowercase p.
2. Inside the shoulder, up to 4 regions are identified, which are numbered from the centromere to the telomeric end.
3. The numbering of bands within the regions also goes in the direction from the centromere.

If the position of the gene in the chromosome is known precisely, its coordinate is the band index. When the localization of the gene is less certain, it is indicated as being in a long or short arm.

For an accurate mapping of chromosomes, the study of mutagenesis and hybridization by any one technique is indispensable. Denver classification of chromosomes and Parisian in this case are inextricably linked and complement each other.