The effects of ionizing radiation on DNA replication were investigated autoradiographically at the cellular and subcellular levels in mouse leukemic L5178Y cells. (1) The rate of DNA synthesis at the cellular level was depressed immediately after irradiation and the depression continued for subsequent 2 hr. The dose-response curve was biphasic. The <tex-math>$D_{37}$</tex-math> dose of radiosensitive and radioresistant components was ≤1300 and ≥10500 rads, respectively. (2) The rate of DNA chain growth as measured by DNA autoradiography was only slightly affected by radiation (<tex-math>$D_{37}=13,000$</tex-math> rads), whereas the initiation of DNA replication seemed to be much more sensitive than the former. Accordingly, it may be reasonable to assume that sensitive and resistant components found at the cellular level are attributed to radiation-induced inhibition of initiation and continuation (chain growth) of DNA replication, respectively. (3) The rate of DNA chain growth after 5-hr FUdR treatment (0.02 μM or less) was determined to be 0.54 μm/min in nonirradiated control cells.

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