DNA repair in late-passage human cells.

Monolayer cultures of WI-38 cells from passages 18 to 60 were exposed to ultraviolet radiation, or to N-acetoxy-AAF, or to gamma-rays. The cultures were incubated in 3H-dThd, and unscheduled DNA synthesis--a measure of DNA repair--was measured radioautographically. Late passage cultures showed less repair, and many cells showed none. Double-label radioautographs indicated that there is an excellent correlation between cells that do scheduled synthesis and cells that do unscheduled synthesis. We interpret our results as indicating that failure of repair is not a causal event in the failure of late passage cells to divide, but that as cells age the ability to do the many coordinated steps in repair declines.