The Time Sequence of Events During Excision Repair in Tetrahymena Pyriformis

The time sequence of events during excision repair of DNA in Tetrahymena pyriformis was investigated after sublethal dose of u.v. radiation. Buoyant‐density analysis of the DNA from repairing cells grown in medium containing 5′‐bromodeoxyuridine makes it possible to distinguish repair synthesis from normal synthesis. Analysis of the DNA synthesized at various times after irradiation clearly indicates that repair synthesis starts very quickly after irradiation and is completed within 3 to 4 hr. Immediately after irradiation, normal DNA synthesis is greatly depressed but gradually resumes until it predominates after 3 to 4 hr. The molecular weight of DNA strands is reduced and the net rate of DNA synthesis is depressed immediately after irradiation. Both of these parameters are restored to their pre‐irradiation values by 3 to 4 hr after irradiation. During the repair period the majority of the pyrimidine dimers are removed. All of the data indicate that repair begins immediately after irradiation and is completed in 3 to 4 hr (about two thirds of a generation period).