The formation and repair of single-strand breaks in DNA of cultured mammalian cells treated with UV-light, methylating agents or 4-nitroquinoline-1-oxide.

The technique of sedimentation in alkaline sucrose was used to examine the formation and repair of single-strand (SS) breaks in cultured mammalian cells that were treated with methyl methanesulfonate (MMS), methyl nitrosourea (MNUA), 4-nitroquinoline-1-oxide (4NQO) or UV-light. The SS breaks induced by MMS and 4NQO were largely repaired by HeLa cells during a 5-h post-treatment incubation. The SS breaks induced by MNUA and UV-light were not repaired by HeLa cells. L-cells were not able to repair the SS breaks induced by any of the agents, which correlates with the deficiency of these cells for repair synthesis of DNA. The following conclusions are discussed. MNUA and UV-light produce modifications in DNA which are not repaired but are translated into SS breaks in alkali. MMS produces SS breaks intracellularly but these are not derived from a simple depurination of methylated purines. 4NQO produces a modification in DNA which is translated into an SS break in alkali but which can be removed by an intracellular process.