X-Ray-Induced Inhibition of DNA Synthesis in Chinese Hamster Ovary, Human HeLa, and Mouse L Cells

An X-ray dose of 500 rad inhibits DNA synthesis in Chinese hamster ovary (CHO) cells, in HeLa S3 cells, and in mouse L cells. In no case can the inhibition we observed be explained on the basis of a pool effect or any other process that causes deficient use of exogenously supplied DNA precursors. In all cases the inhibition is mediated by a decrease in replicon initiation processes. Since this inhibition is observed at doses too low to affect individual replicons, we hypothesize that a single hit within a cluster of replicons in some manner blocks the initiation of all replicons within the cluster.