Effect of Various Doses of X-Rays on the Number of Cells Synthesizing Deoxyribonucleic Acid

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Abstract
The number of cells synthesizing DNA (observed by means of autoradiographs as uptake of P32 into DNA in a form not removed by acid hydrolysis) in Vicia faba root meristems is reduced by moderate doses of X-rays to about 60% of the normal value during the subsequent 12 hours. Reduction is the same after doses of 50r to 200r. This result is interpreted to be due to a greater radiosensitivity to delay or inhibition of DNA synthesis of cells which are in approximately one-third of the cell cycle at the time of irradiation (the first part of interphase) than of cells at other stages of the cycle. It is suggested that many of the results reported in the literature, in which the amount of tracer incorporated into DNA at early times after moderate doses of radiation has been observed biochemically to be roughly 1/3-2/3 that in controls, could be explained on the hypothesis that only a part of the irradiated cell population is primarily affected.