The Effect of Oxygen on the Delay in the Initiation of DNA Synthesis after Irradiation

The effect of oxygen on cell cycle delay by low doses of radiation on synchronized Harding Passey melanoma cells has been studied. Cells were irradiated 5 h after subculturing into fresh medium, and the delay before the start of S was measured. DNA synthesis was measured by frequent pulse labeling of the cells with radioactive thymidine to obtain the S-phase profile. The amount by which the irradiated cells S-phase profile had to be moved in time so that both the ascending and descending portions of the first S phase overlayed that of the controls was used as a measure of the delay. The magnitude of the delay was exponentially related to radiation dose and the effect of irradiating in the absence of oxygen was a dose multiplying factor of 2.5. This was similar to the oxygen effect on survival for cells irradiated under the same conditions.