Unscheduled DNA Synthesis in Diploid Yeast after X-irradiation in Different Phases of the Cell-cycle

Summary

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.