The Presence of DNA Breaks and the Formation of Chromatid Aberrations after Incorporation of ¹²⁵IdUrd May Be Necessary but Are Not Sufficient to Block Cell Cycle Progression in G₂ Phase

Cell progression into mitosis and chromatid aberration frequencies were compared in two Chinese hamster ovary (CHO) cell lines after incorporation of <tex-math>${}^{125}{\rm IdUrd}$</tex-math>. Asynchronous, exponentially growing populations of CHO K1 and the DNA repair-deficient, radiation-sensitive CHO irs-20 cells were compared after a 10-min exposure to 14.8 kBq/ml <tex-math>${}^{125}{\rm IdUrd}$</tex-math>. Essentially no differences were seen for either end point between the cells of the two cell lines. As the cells in S phase at the time of labeling entered the mitotic cell selection window, the number of mitotic cells of each cell line declined to approximately 60% of the respective unlabeled control. Chromosome analysis of the mitotically selected cells indicated an¹²⁵ I decay-dependent increase in the number of chromatid aberrations in cells of both cell lines. The appearance of aberrations together with the known rates of production and rejoining of DNA double-strand breaks show that cells are able to progress through G₂ phase and into mitosis in the presence of such breaks. The data suggest that DNA damage may be necessary, but is not sufficient to cause a radiation-induced blockade of cell progression through G₂ phase.