On the Mechanism of Radiation Effect on DNA Synthesis

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Abstract
Large doses (>500 rads) of X-irradiation directly inhibit DNA synthesis in human bone marrow cells in vitro. Small doses (< 300 rads) do not inhibit DNA synthesis in cells which already have started DNA synthesis. In cells, however, which are in the presynthetic period at the time of radiation, such small doses will produce a 40 to 50% depression of the number of cells entering the subsequent synthetic period in a given time, without affecting the rate of subsequent DNA synthesis. It is suggested that unless two populations are involved, the damage suffered by the presynthetic cells results in a half-speed progress through the presynthetic period. It appears that a system connected with but not identical with DNA synthesis is more radio-sensitive than the process of DNA synthesis. In studies of radiation effects on DNA synthesis a clear differentiation has to be made between the process of DNA synthesis (synthetic period) and those of the presynthetic period, since different mechanisms maybe affected.