Several kinds of cultured mammalian cells have been examined after exposure to ultraviolet light to determine if repair replication and unscheduled DNA synthesis occur. All kinds thus far examined, with the exception of cells derived from patients with Xeroderma pigmentosum, show both phenomena. Cells of human origin exhibit both phenomena to a greater extent, however, than do cell lines derived from rodents. There is at least a rough correlation of the extent of repair replication with the extent of unscheduled synthesis suggesting that both are manifestations of the same process. Both phenomena also occur after exposure to visible light of HeLa cells with bromouracil-substituted DNA. The results are discussed in terms of the possible role of these phenomena in the repair of damaged DNA.

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