Unscheduled DNA synthesis after partial UV irradiation of the cell nucleus. Distribution in interphase and metaphase.

Cells of an euploid strain of the Chinese hamster synchronized in the GI phase were microirradiated in the nucIeus with a laser UV microbeam (A=257 nm) and pulse-labelled with [3H]thymidine. In autoradiographs of cells fixed immediately after the pulse unscheduled DNA synthesis (UDS) was found restricted to tbe microirradiated part of the nueleus. Tbe rate of UDS varied with the UV energy applied and the post-irradiation incubation time. In other experiments chromosome preparations were established after an additional chase and a subsequent growth period. In 28 mitotic cells autoradiographic label was found concentrated on a few chromosomes which lay adjacent to each other in one part of the metaphase plate. The distribution of label on the chromosomes could clearly be distinguished from patterns wh ich originate from semi-conservative DNA synthesis within S phase. The label on chromosomes of microirradiated cells thus represents UDS. Dur findings support the following ideas on the arrangement of interphase chromosomes: (1) Decondensed interphase chromosomes may occupy rather compact territories. (2) Chromosomes do not necessarily exhibit a elose and permanent association with their respective homologues.