Induction of chromosome shattering by ultraviolet light and caffeine: the influence of different distributions of photolesions.

Cells of synchronized and of asynchronously growing cultures of a V79 Chinese hamster line were microirradiated with a low power laser-UV-microbeam of wavelength 257 nm. Ultraviolet light was either focused onto a small part of the nucleus (mode I) or distributed over the whole nucleus (mode II). Following microirradiation, the cells were incubated for 7-20 h with caffeine (1-2 mM) until chromosome preparation was performed. After both modes of microirradiation, shattering of the entire chromosome complement (generalized chromosome shattering, GCS) was observed. It is suggested that the probability by which GCS is induced depends on the total number of DNA lesions rather than on their distribution in the chromatin. The results are consistent with the prediction of a "factor depletion model" which assumes that in a given cell, GCS takes place both in irradiated and non-irradiated chromosomes if the total number of daughter strand-repair sites surpasses a threshold value.