Previous studies, using caffeine, have indicated that a deoxyribonucleic acid (DNA) synthesis period (S phase)-dependent potentiation of ultraviolet light (UV) damage occurs in mouse L cells, but the question remains whether this potentiation is due to a peculiar interaction of caffeine with irradiated cells or whether it is due to interference with a recovery process which can be demonstrated in the absence of the drug. To answer this question synchronized populations of L cells were exposed to various first exposures of UV. As a function of time after this initial exposure survival curves of cell colony-forming ability were determined. Results indicate that cells surviving the initial exposure delivered in G1 phase, which pass through S phase, show no effect of this exposure when irradiated in G2 phase. That is, the survival curve for such cells in G2 is the same as for cells irradiated in G2 phase which have not been irradiated in G1 phase. Conversely, cells which are exposed to UV in G2 phase and are allowed to progress to G1 phase are more sensitive to UV than cells in G1 phase which are not preirradiated in G2 phase. If these same cells are allowed to pass through S phase, they recover completely from the previous G2 irradiation. These results are consistent with the hypothesis that UV-irradiated cells recover from damage during their first passage through S phase.

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