Repair Replication in Chinese Hamster Cells After Damage From Ultraviolet Light*

After irradiation with u.v. light, Chinese hamster cells perform repair replication of their DNA. Small numbers of nucleosides are inserted into DNA, such that when BrUdR is used there is no detectable change in density. Repair replication begins immediately after irradiation, but it decelerates steadily and at least half is complete within 4 hr. Repair replication saturates above 200 ergs/mm2 at a level which represents 0.055 per cent replacement of all thymine sites in 4 hr. Repair replication in mammalian cells, in contrast to that in microorganisms, does not appear to replace pyrimidine dimers excised from DNA in acid soluble form, and neither repair nor semiconservative replication discriminates between BrUdR and TdR.