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 methods. Let's get acquainted with the methods of ordering genetic material using the example of human chromosomes.
Classification of human chromosomes
Karyotype is a set of chromosomes (diploid) found in any of the body's somatic cells. It is characteristic of a given organism and is the same in all cells, except for the sex cells.
Chromosomes in the karyotype are:
- autosomes do not differ between individuals of different sexes;
- sexual (heterochromosomes), differ in structure in individuals of different sexes.
Cells of the human body contain 46 strands of DNA, of which 22 pairs of autosomes and one - sex. This is a diploid 2n set of genetic material. A pair of heterochromosomes in women is designated XX, in men - XY, the karyotype designation, respectively,44+XX and 44+XY.
In germ cells (gametes) there is a haploid or single 1n set of DNA. Eggs contain 22 autosomes and one X chromosome, sperm cells contain 22 autosomes and one of the heterochromosomes, X or Y.
Why do we need identification and classification of chromosomes
The Denver and Paris systems of classification of hereditary material, widely used in the scientific community, are designed to unify and generalize ideas about the karyotype. A common approach is needed for the correct presentation and interpretation of the results of research in the field of genetics, karyosystematics, and breeding.
Schematically, a karyotype is depicted using an ideogram - a sequence of systematized and arranged in descending order of chromosome size. The ideogram reflects not only the size of spiralized DNA, but also some morphological characteristics, as well as features of their primary structure (regions of hetero- and euchromatin).
By analyzing these graphs, the degree of relationship between various systematic groups of organisms is established.
A karyotype can contain pairs of autosomes that are almost identical in size, which makes it difficult to correctly position and number them. Let's consider what parameters the Denver and Paris classification of human chromosomes take into account.
Results of the Denver Conference, 1960
In the specified year in the city of Denver, USA, a conference on human chromosomes was held. On it, various approaches to the systematization of chromosomes (by size, positioncentromeres, areas with varying 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:
- All human autosomes are numbered in order from 1 to 22 as their length decreases, the sex chromatids are assigned the designations X and Y.
- 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, the 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.
The Denver classification of chromosomes is universally recognized in the world scientific community.
Chromosome groups and their characteristics
The Denver classification of chromosomes includes seven groups in which autosomes are arranged in numerical order, but distributed unevenly in number. This is due to the characteristics by which they are distributed into groups. More about this in the table.
| Chromosome group | Chromosome pair numbers | Features of the structure of chromosomes in the group |
| A | 1-3 | Long chromosomes, well distinguishable from each other. In the 1st and 3rd pairs, the position of the constriction is metacentric, in the 2nd pair - 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 |
Medium-sized chromosomes, all unequal arms are submetacentric, difficult to individualize. Identical in size and shape to the group's autosomes, replicating later than others. |
| D | 13-15 | Chromosomes in the group of medium size with almost marginal position of the primary constriction (acrocentric), have satellites. |
| E | 16-18 | Short chromosomes, in the 16th pair the equal arms are metacentric, in the 17th and 18th - submetacentric. |
| F | 19 and 20 | Short metacentric, almost indistinguishable from each other. |
| G |
21 and 22 Y chromosome |
Short chromosomes with satellites, acrocentric. They have slight differences in structure and size. Slightly longer than the other chromosomes of the group, with a secondary constriction on the long arm. |
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
Introduced since 1971, this classification is based on differential staining techniqueschromatin. As a result of routine staining, all chromatids acquire their own pattern of light and dark stripes, making them easily identified within groups.
When processing chromosomes with different dyes, separate segments are revealed:
- Q-segments of chromosomes fluoresce as a result of the application of the dye quinacrine mustard.
- G-segments appear after Giemsa staining (coincide with Q-segments).
- R-segment staining is preceded by controlled thermal denaturation.
Additional designations are introduced to indicate the locations of genes on chromosomes:
- The long arm of the chromosome is denoted by a lowercase letter q, the short arm is denoted by 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 the bands within the districts also goes in the direction from the centromere.
If the position of a gene in the chromosome is known exactly, its coordinate is the band index. When the localization of the gene is less certain, it is designated as being in the long or short arm.
For accurate mapping of chromosomes, the study of mutagenesis and hybridization, any one technique is indispensable. The Denver classification of chromosomes and the Parisian in this case are inextricably linked and complement each other.
