Unscheduled DNA synthesis in diploid yeast after X-irradiation in diffent phases of the cell-cycle.

The uptake of radioactive 5'-dTMP into the DNA of diploid yeast cells was measured in the G1 and S-phase of the cell-cycle. In control cells, the uptake is zero in G1 and increases with time in the S-phase. Cells irradiated in early G1 show an uptake (unscheduled DNA synthesis) which is higher than if irradiation is performed later in G1. An analysis which takes into consideration the incomplete synchronization of the cell population shows that, at the end of G1, no uptake would be present in an ideally-synchronous population. At the end of G1 the shoulder in the dose-effect curve for cell survival also disappears. This provides additional evidence that the shoulder in a dose-effect curve might be due to repair reactions in living cells.