Distribution of radiation-induced G1 exchange and terminal deletion breakpoints in Chinese hamster chromosomes as detected by G banding.

A total of 255 chromosomal aberrations induced by X-rays in G1 phase of the cell cycle were scored in 600 G-banded metaphases prepared from Chinese hamster female cells. On the basis of a detailed analysis of these aberrations a total of 441 chromosomal breakpoints were mapped to the individual Chinese hamster chromosomes and their bands. More breakpoints were mapped to G-light (80.5%) than to G-dark (19.5%) bands. Chromosomal distribution of breakpoints revealed that chromosomes 5 and 8 had significantly more exchange breakpoints than expected on the basis of chromosomal length, whereas the X chromosome had significantly more terminal deletion breakpoints than expected. At the same time chromosomes 5 and 8 had a deficiency of terminal deletions, whereas the X chromosome had significant deficiency of exchange breakpoints. These results indicate that radiation-induced exchange and terminal deletion breakpoints, as observed in the first postirradiation metaphase, have different patterns of distribution in Chinese hamster chromosomes. Clustering of terminal deletions in the long arms of X chromosomes, which are entirely occupied by heterochromatin, suggests that chromosomal repair mechanisms responsible for rejoining of chromosomal breaks are less effective in heterochromatic than in other genomic regions.