The effects of pretreatment of synchronized CHO cells with 5-BUdR and TdR on chromosomal damage and cell death.

Chinese hamster ovary (CHO) cell populations were exposed independently to thymidine (TdR) or 5-bromodeoxyuridine (5-BUdR) for either one cell generation (approximately 18 h) or two (36 h) and subsequently synchronized in mitosis either by chemical arrest with colcemid or by mechanical shaking. Upon subsequent X-irradiation at various stages in the cell cycle, the synchronized populations were analyzed for cellular lethality and chromosomal damage. This investigation confirmed that each radiation response was "age-dependent" and essentially similar for both methods of synchrony. Cells irradiated in mitosis were found to be most radio-sensitive while cells irradiated during the DNA synthesis stage were most radio-resistant. A comparative analysis revealed that the two responses of radiation were more pronounced for cells labelled with 5-BUdR than they were for TdR-labelled cells. A further enhancement of radiation responses was noted for cells that were grown in the presence of the halogenated pyrimidine analog for two cell generations, thus re-affirming that the nucleus is a primary target for radiation damage. The comprehensive conclusion sought with regard to the degree of correlation between chromosomal damage and loss of cellular reproduction integrity awaits such information as the molecular organization of mammalian chromosomes and the mechanisms by which aberrations are produced within these chromosomes.