Effects of X-Ray Irradiation on Deoxyribonucleotide and DNA Synthesis in Cultured Mouse Embryo Cells

Cultured secondary mouse embryo cells were irradiated in G1-phase or in S-phase with doses of X-rays which resulted in a depression of DNA synthesis. Pools of dTTP and dATP, as well as the activities of the enzymes ribonucleotide reductase and thymidine kinase were measured at different times after irradiation. After irradiation of G1-cells the pool sizes of both deoxyribonucleotides and the activity of ribonucleotide reductase—but not of thymidine kinase—were depressed in parallel with DNA synthesis, as compared to the controls. On the other hand, irradiation in S-phase resulted in an immediate small increase in the pool sizes of the deoxyribonucleotides. Ribonucleotide reductase activity was not affected at early time points after irradiation. Our results indicate that the decreased DNA synthesis observed after X-ray irradiation was not caused by a depletion of deoxyribonucleotide pools.