Chromosome Banding Pattern
In 1912, Winiwater was a successful attempt to count the number of human chromosomes. He was proposed that human chromosomes are 48 in women and 47 in man; in this number men having one X-chromosome and women having two X chromosome. Painter, in 1923, while examining the testicular material of man, observed a heteromorphic pair of sex chromosomes and proposed the XY mechanism of sex determination in man. Tjio & Levan (1956), cultured somatic cells from fibroblasts of human embryos and counted the human chromosome number as 46. This chromosome number was confirmed by Ford and Hamerton while working with testicular material in the same year. Tjio and Levan provided greatly improved techniques for chromosome preparations. Moorhead et al., (1960), described a simple method of culturing lymphocytes from human blood.
Karyotyping Human Chromosomes
For karyotyping of human chromosomes, venous blood is taken and blood leucocytes are stimulated to divide (by mitosis) in vitro by the addition of phytohaemagglutinin. Colchicine is added to arrest cell division at the metaphase stage. It is further treated with a hypotonic saline solution which results in swelling of cells and dispersal and better clarity of chromosomes for counting and morphological study. Thereafter, the material is stained (e.g., with Giemsa technique) to demonstrate the banding patterns of chromosomes. Finally, a suitable metaphase spread is photographed through a high-power microscope. The individual chromosomes are cut out from the photograph. The chromosomes are then arranged in an orderly fashion in homologous pairs to produce a standard arrangement, the karyotype.
To characterize a chromosome in the karyotype, the following parameters are used:
1. Shape of chromosome
2. Length of chromosome
3. Centromeric index, i.e., this index is expressed in the form of the ratio of the short arm length to the total chromosome length:
Centromeric index = (Short arm length)/(Total chromosome length)
For example, the centromeric index in a metacentric chromosome is 0.5.
4. Proportion of the arms, ie., it is the ratio between the long arm and short arm of the chromosome. This ratio is 1:1 in a typical metacentric chromosome.
CLASSIFICATION OF HUMAN METAPHASE CHROMOSOME
Human metaphase chromosomes were first of all classified by a conference of cytogeneticists al Denver, Colorado in 1960 and is known as the Denver classification. To follow this classification, each of the 22 pairs of autosomes has been numbered from 1 to 22 according to their decreasing size. Patau (1960), divided the human chromosomes into the following seven groups designated A to G:
1. A group: 1 to 3 pairs – Metacentric
2. B group: 4 to 5 pairs – Submetacentric
3. C group: 6 to 12 pairs – Submetacentric
4. D group: 13 to 15 pairs – Acrocentric
5. E group: 16 to 18 pairs – Submetacentric (16 is metacentric)
6. F group: 19 to 20 pairs – Melacentric
7. G group: 21 to 22 pairs – Acrocentric
Group A consists of the longest metacentric chromosomes. Group G consists of the shortest acrocentric chromosomes. These chromosomes have Satellites that correspond to nucleolar organizers. Chromosomes of group D also contain satellites. In males, group G includes a variable Y chromosome which lacks the satellites.
The X chromosome is a member of group C and can be identified by special banding or staining methods.
Recently banding techniques reveals structural details of chromosomes. The main banding techniques are identified by letters such as Q, G, C, R, T, F, and N bands :
1. Q Banding. It uses fluorescent dyes (such as quinacrine mustard) and identifies the so-called Q bands.
2. G Banding. It uses Giemsa slain and identifies the G bands. With G banding three major types of chromatin can be recognized-euchromatin, centromeric, and intercalary heterochromatin. The Q and G bands are generally similar and correspond to intercalary heterochromatin.
3. C Banding. It stains specifically centromeric constitutive heterochromatin.
4. R Banding. It gives a pattern that is the reverse of that of Q and G banding.
5. T Banding. It stains telomeres of chromosomes.
In some banding, techniques use the Feulgen stain (F bands) and another selective stain the nuclear organizers (N bands) which are localized in the satellite chromosomes 13, 14, 15, 21, and 22. G banding is also an important tool in the analysis of mammalian, avian, reptilian, and amphibians chromosomes.
Clinical Importance of Chromosome Banding
Since banding patterns are unique and constant for each normal chromosome, in case of a large number of chromosomal abnormalities, such as loss of a very small part, insertion of an additional segment, and addition of the whole chromosome can be easily recognized, e.g., cat-cry syndrome due to loss of small part of chromosome 5; down syndrome due to an extra chromosome 21.