Abstract— DNA replication in UV‐irradiated Chinese hamster V‐79 cells was analyzed by measuring the incorporation of 14C‐thymidine into acid precipitable form and by DNA fiber autoradiography. As expected, UV exposure resulted in a rapid deceleration in the rate of thymidine incorporation, reaching a minimum rate 60–75 min following exposure to 2–10Jm‐2. After an additional 1–2h the rate of thymidine incorporation began to recover slowly, approaching the control rate within 10h following exposure to 10 Jm‐2 or less. The mechanism of the inhibition and recovery in thymidine incorporation was examined by DNA fiber autoradiography. The results indicate that UV radiation produces lesions in DNA which temporarily block chain growth of DNA that is synthesized during the first 5 h after exposure. Within 10 h after exposure, the lesions have either been repaired or modified (or the replicative enzymes altered) so that DNA chain growth is no longer impeded.

No abstract

No abstract

No abstract

The expression of the transient depression in the rate of DNA synthesis normally observed after exposure of randomly-dividing Chinese hamster V-79 or Chinese hamster CHO cells to ionizing radiation can be postponed or diminished by a post-irradiation treatment with 1.0 to 1.0 mM adenine or 1.5 mM caffeine. Caffeine may exert its effect by creating additional sites for replication in irradiated cells. Cells treated with caffeine or adenine for 2 or 4 hours after exposure to 3000 rad of 300 kVp X-rays exhibit depressed synthesis only after the removal of caffeine or adenine. These alterations in the timing of the X-ray-induced depression of the rate of DNA synthesis have no effect on X-ray-induced cell killing. Although a 4 hour post-irradiation treatment of randomly-dividing Chinese hamster V-79 cells with 1.0 or 2.0 mM caffeine potentiates X-ray-induced cell killing, this reduction in survival is due primarily to effects on cells in S-phase.