Abstract
Exponentially growing primary embryonic Chinese hamster cells, in which the pattern of distribution of heterochromatin is well characterized, were X-irradiated and fixed at 1,2,3, and 4 h following irradiation. In one set of cells repair of damage was completely blocked by ara A. The frequencies of chromatid breaks and exchanges were evaluated for individual chromosomes. An analysis of observed and estimated expected frequencies of chromosomal aberrations indicated that in general, (a) the initial damage was higher in euchromatic regions than the heterochromatic regions and (b) the repair of DNA lesions (as evaluated by the frequencies of chromatid gaps and breaks) was more efficient in euchromatic regions than heterochromatic regions.
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