Unscheduled DNA synthesis in human diploid cells of different donor ages.

Human diploid fibroblast-like cells from two donor of widely different ages were tested for their ability to perform unscheduled DNA synthesis (UDS) after exposure to ultraviolet irradiation. Cultures were assayed at various times throughout their in vitro lifespans as both confluent and mitotically arrested populations. Cells from both donors maintained their ability to perform UDS throughout their lifespans with arrested populations exhibiting increased levels. The appearance of elevated levels of UDS in arrested cells was directly related to the age of the donor. These results indicate that the loss of ability to repair DNA damage is not a primary cause of in vitro senescence. They do suggest, however, that the regulation of DNA repair synthesis is effected by increasing age.