Postreplication repair in human cells: on the presence of gaps opposite dimers and recombination.

Human cells prelabeled with [32P]phosphate were exposed to UV and then pulse-labeled with [3H]thymidine. The DNA from these cells was subsequently treated with T4 endonuclease V, an enzyme which specifically nicks DNA at positions adjacent to pyrimidine dimers. Sedimentation in alkaline sucrose gradients revealed that both the DNA made before and that made after irradiation contained nuclease-sensitive sites, indicating that a recombinational process between these DNAs might be occurring during postirradiation incubation. Sedimentation in neutral sucros gradients showed that the molecular weight of native DNA remained unchanged for both DNAs upon endonuclease treatment, indicating that gaps opposite dimers are not necessarily formed after irradiation.