Unscheduled dna synthesis in confluent and mitotically arrested populations of aging human diploid fibroblasts.

Unscheduled DNA synthesis, used as a measure of excision repair following exposure to ultra-violet irradiation, was determined in confluent and arrested human diploid fibroblasts and correlated with in vitro age. Confluent cultures exhibited identical levels of unscheduled DNA synthesis at all in vitro ages. Cells arrested by lowering the serum concentration of the incubation medium exhibited similar levels of unscheduled DNA synthesis as did confluent cells during the first one-third of the cells' characteristic in vitro lifespan. During the last two-thirds of the lifespan, however, arrested populations exhibited a 30 to 50% increase in the amount of detectable DNA repair. This apparent increase in ability to perform unscheduled DNA synthesis was not time or dose dependent and could not be attributed to alterations in precursor pools. It was postulated that the increase may be correlated with changes in DNA structure.