Inhibition and Recovery of Dna Synthesis in Uv‐irradiated Chinese Hamster V‐79 Cells†

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.