In this article we will discuss about the study of a complete chromosomes.
For karyotype analysis, a sample of peripheral blood is drawn from an individual, the leukocytes separated and cultured for about 3 days. To stimulate growth and cell division phyto-haemaglutinin (PHA) is added. The dividing lymphocytes are arrested at metaphase stage by exposing them to colchicine.
The cells are then harvested and slides prepared. The metaphase spreads are photographed and the chromosomes cut out and rearranged according to size and location of centromeres. The study of a complete chromosome complement in this manner is called karyotype analysis. The karyotype of a normal human male is shown in Fig. 21.1.
The 23 pairs of human chromosomes are classified into 7 groups A through G. Group A includes chromosomes 1, 2 and 3, largest in size and metacentric. Group B has chromosomes 4 and 5, smaller than group A chromosomes, and sub-metacentric. Group C is largest containing chromosomes 6 through 12 and X, all of medium length and sub-metacentric.
Group D has medium sized acrocentric chromosomes 13, 14 and 15. Group E chromosomes 16, 17 and 18 are shorter and either meta-or sub-metacentric. Group F has shorter metacentric chromosomes 19 and 20. Group G contains the smallest acrocentric chromosomes 21 and 22 as well as Y.
The banding technique for chromosomes has proved useful in identifying abnormalities in chromosomes. It was first found by Caspersson and his associates in 1968 that metaphase chromosomes can be stained with fluorescent DNA binding agents such as quinacrine mustard to yield dark and light bands. Since then numerous staining methods have been found for producing specific banding patterns in chromosomes.