Relative Ultraviolet Sensitivity of Different Phases in the Cell Cycle of Chinese Hamster Cells Grown in Vitro

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Abstract
Chromosomal damage after UV irradiation of an in vitro mammalian cell line (Chinese hamster) in different phases of the life cycle was determined. Cells in DNA synthesis at the time of irradiation suffer about ten times the chromosomal damage of cells in G1 and G2 phases. Furthermore, for both G1- and S-phase cells which divide from 10 to 16 hours after irradiation, the greatest fraction of breaks (70 to 90%) is elicited as chromatid exchanges; whereas in G2- and S-phase cells which divide from 2 to 8 hours after irradiation, a smaller fraction of damage (about 40%) results in chromatid exchanges. The frequency of chromosome exchanges in irradiated cells is not significantly different from that in control cells. Cells in S-phase which were sampled 10 to 16 hours after irradiation sustained about 25 times as much damage as did cells in S-phase which were sampled prior to 8 hours, and in G1 cells only 19% of the exchanges were due to dicentrics. These findings are consistent with the hypothesis that the chromosome, although morphologically single, consists of two subunits which separate as the cell enters the S-phase.