Increased unscheduled DNA synthesis in aged human diploid fibroblasts

Using Hayflick's model, ultraviolet (UV) induced unscheduled DNA synthesis was compared in cells at a low, middle and high population doubling level (PDL). Concomitant DNA replication was prevented by arresting all cultures in the G₁ phase by lowering the serum concentration. After UV-irradiation cells at a high PDL incorporated 1.5–2 times more ³H] thymidine into DNA than cells at a low and middle PDL. These findings seem to indicate that the repair capacity of cells at a high PDL is more than those of cells at low and middle PDL. Alternatively, the higher incorporation might be explained by a difference in the pool sizes of precursors of cells at different PDL. These possibilities were examined by adding fluorodeoxyuridine to the system to reduce de novo synthesis of deoxythymidine triphosphate (dTTP), and measuring the pool size of dTTP and the specific activity of ³H] dTTP in cells at different PDL. The results indicated that the increased incorporation in fact reflects increased unscheduled DNA synthesis in cells at a high PDL.