Induction of sister chromatid exchanges in Chinese hamster ovary cells by 5-bromodeoxyuridine

Sister chromatid exchange (SCE) induction in established cell lines has become a widely used criterion for the assessment of genotoxicity. Detection of SCE is only possible if a base-analogue such as bromodeoxyuridine (BrdU) is incorporated into the DNA; it is not known to what extent the observed background level of SCE is a consequence of the presence of the base-analogue, and to what extent it represents a 'natural' background. In this paper, experiments are described in which a number of different BrdU incorporation schemes are compared. The results suggest that under the normal SCE-test conditions, e.g. 5-10 microM BrdU the majority of the spontaneous SCE are of the natural (non-BrdU) origin. The remaining (BrdU-dependent) SCE seems to be a consequence of greater number of spontaneous lesions in the BrdU-containing DNA.