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

Cells of synchonized and of asynchronously growing cultures of a V79 Chinese hamster line were microirradiated with a low poweer 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 microirradation, 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 lesions rather than on their distribution in the chromatin. The results are consistent with the prediction of a “factor depletion model” wich assumes that in a given cell, GCS takes place both in irradiated and non-irradiated chromosomes of the total number of daughter strand-repair sites supasses a threshold value