The local irradiation of regenerating liver 1 hour after the administration of tritiated thymidine revealed no loss of labeled DNA after 3000 R and 6000 R of x-radiation. This is six to twelve times the$D_{37}$ dose (500 R) of the initial radiosensitive component of the biphasic dose response found for the reduction of DNA synthesis following irradiation. The reduction of both autoradiographic and biochemical results to approximately 80% of control values after 12,000 R, in two independent experiments, suggests that thymidine-labeled-DNA degradation may be beginning to occur at this dose. The extensive degradation of DNA found in Escherichia coli (up to 50%) has not been found in mammalian cells, either in vivo or in vitro. Differences in repair mechanisms, or differences in the status of the DNA, may be the reasons for the differences in degrees of DNA degradation found after irradiation in bacterial and mammalian cells. Therefore, the present experimental results fail to substantiate the hypothesis that substantial degradation of DNA occurs in mammalian cells in the radiation dose range which results in a significant depression of the rate of DNA synthesis.

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