DNA damage in bromodeoxyuridine substituted SV40 DNA and minichromosomes following UVA irradiation in the presence of Hoechst dye 33258

Bromodeoxyuridine-substituted SV40 DNA and SV40 minichromosomes were prepared from infected CV1 cells, and exposed to USA light in the presence of different concentrations of Hoechst dye 33258. Following agarose gel electrophoresis, the yields of ssbs and dsbs were determined. In DNA these yields are dependent on the level of BrdU substitution (0.038 ssb and 0.00022 dsb per BrdU residue)kJm -2 ), and on the mode of dye binding (type I versus II). The yields of both ssbs and dsbs were found to be lower by factors of 1.25 and 2 respectively in minichromosomes. UVA irradiation in the presence of the thiol cysteamine results in lower strand break yields, presumably due to the repair of DNA radicals formed during photolysis