Responses of Synchronous Populations of HeLa Cells to Ultraviolet Irradiation at Selected Stages of the Generation Cycle

Abstract

Synchronous HeLa S3 cultures were exposed to UV light at selected times in the generation cycle. Dose-survival curves show changes of both slope and extrapolation number. Sensitivity, measured as loss of colony-forming ability after doses of 70 ergs mm-2, increases late in G1, reaches a maximum in the middle of S, and decreases through the remainder of S and G2. The maximum occurs later than does the interphase maximum observed after X-irradiation with doses yielding about the same surviving fraction of cells. After doses up to 70 ergs mm"2, progression of cells from Gl to S or from G2 to mitosis is not blocked. Cells irradiated in G1 or S show a marked inhibition in the rate of incorporation of thymidine. The duration of DNA synthesis appears to be increased, resulting in a delay in cell division. Cells that die appear to do so largely before the 1st division after doses above 35 ergs mm-2. Incorporation of thymidine was detected in G1 and G2 cells after irradiation, the maximum amount measured after 3 hr. of incorporation following a dose of 140 ergs mm-2 cor responding to less than 1% of the DNA synthesized during a normal generation cycle. Incorporation of tritiated thymidine of high specific activity during this "unscheduled" synthesis decreased survival. Treatment with hydroxyurea, but not fluorodeoxyuridine, for 12 hr. after irradiation in Gl also decreases survival, but does not inhibit the "unscheduled" incorporation of thymidine.