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.