Effects of cycloheximide on X-ray induction of chromatid exchanges in synchronous Chinese hamster cells

Abstract

Chinese hamster cells growing on plastic were synchronized by shaking loose mitotic cells. X-irradiation of mitotic cells produced chromosome type exchanges only, but when metaphase cells were irradiated following a 2-h treatment with Colcemid prior to selecting the metaphase cells, with or without the simultaneous addition of cycloheximide (CHM) (25 μg/ml), 50% of the exchanges were of the chromatid type. When mitotic cells synchronized without Colcemid were plated into medium containing CHM (5 μg/ml), they divided and acquired the radioresistance characteristic of G 1 cells. However, CHM added to cells in early G 1 (1 h after plating) prevented the transition from radiation-induced chromosome exchanges to radiation-induced chromatid exchanges, which normally occurred as untreated cells entered the late G1 and early S phases. These results indicate that the decrease in radiosensitivity associated with the transition from the mitotic phase to the G1 phase is not dependent on protein synthesis. Furthermore, the hypothesis is supported that the chromosome consists of two strands, and that the association between these strands can be loosened without protein synthesis when the cell approaches mitosis. However, inhibition of protein synthesis when the cell is in early G 1 prevents the dissociation between these strands which occurs normally as the cell progresses through G 1 into the S phase.