N-methyl-N-nitrosourea-induced DNA damage detected by the comet assay in Vicia faba nuclei during all interphase stages is not restricted to chromatid aberration hot spots

The genotoxic effect of the monofunctional alkylating agent N-methyl-N-nitr osourea (MNU) on root-tip nuclei of the field bean, Vicia faba, has been te sted by comparative application of three protocols of the comet assay. Whil e the alkaline denaturation/alkaline electrophoresis (A/A) procedure proved to be most sensitive at low doses, the alkaline denaturation/neutral elect rophoresis (A/N) procedure yielded an optimal dose-response curve within a wider dose range. With the neutral electrophoresis without alkaline denatur ation (N/N) procedure only minimal response was found. MNU-mediated single- strand breaks occurred in nuclei of all interphase stages. Detection of tan demly repeated FokI elements on comets by fluorescence in situ hybridizatio n showed an average involvement of these heterochromatin-specific sequences in MNU-mediated single-strand breaks. This, together with previous results , suggests that the pronounced clustering of chromosomal aberrations in het erochromatic regions after treatment with S phase-dependent mutagens is mai nly due to an error-prone interference of recombinative repair and replicat ion in damaged basic repeats of large tandem repeat arrays.