The ability of cells from young and old cultures of WI38 human fibroblasts to undergo unscheduled DNA synthesis was studied. Following inhibition of semi-conservative synthesis in media lacking arginine and containing a hydroxyurea block, cells were irradiated with u.v. light and repair replication was measured by determination of specific activity of DNA and by autoradiography. Unscheduled DNA synthetic capacity was significantly reduced in old cultures. Quantitation of silver grains revealed that this reduction was a result of decreased repair activity by individual cells, which correlated well with a similar decrease in the proportion of dividing cells in the population. This observation that unscheduled DNA synthesis is reduced in older cells is in apparent contradiction to some earlier reports, and the failure of other investigators to identify aging changes in repair capacity is discussed in relation to differences in experimental procedure.
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