The induction of DNA strand breaks at specific sites by N-nitroso-N-ethylurea depends on the phases of the cell cycle

Synchronized root meristems of Pisum sativum were treated at each phase of the cell cycle with 6.25 mM N-nitroso-N-ethylurea. DNA extracted from treated cells and run in agarose gel electrophoresis exhibits a series of discrete fragments with length below 2500 bp and a significant number of unspecific single-stranded breaks (or alkali-labile sites). Experiments with micrococcal nuclease indicated that the nucleosomal organization of the chromatin is not responsible for the generation of the discrete fragments: it seems that their appearance is associated with a preferable attack of the mutagen at specific sites, characteristics for the plant genome. Moreover, a cell cycle dependent release of the discrete fragments was found with maximum at G1-S and minimum at mitosis. The model experiments designed to clarify this observation suggest that it might be determined from the cell cycle dependent fluctuation in the accessibility of the chromatin DNA and/or the process of excision-repair.