A type of DNA synthesis in mammalian cells that is stimulated by ultraviolet light has been studied by means of radioautography and density gradient centrifugation. The characteristics of this synthesis are: (a) it is not semiconservative; (b) it is enhanced by the presence of 5-bromodeoxyuridine in the DNA molecule; (c) the degree of stimulation is dose dependent; (d) there is less variability in the rate of incorporation of H3-thymidine during this synthesis than during normal DNA synthesis; (e) it occurs in cells that are not in the normal DNA synthesis phase (G1 and G2 cells). This kind of synthesis has been found in cultured cell lines from five different species; however, in some strains, the presence of bromouracil in the DNA is required before it can be demonstrated by radioautography.

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