CHROMOSOME TERMINAL DELETION FORMATION IN CHINESE-HAMSTER OVARY CELLS

To investigate the fate of unrejoined DNA double-strand breaks, the fr equency of Co-60 gamma-ray- and restriction-enzyme-induced terminal ch romosome deletions, a marker of unrejoined breaks, was determined in C HO-K1 and in xrs-5 cells. The xrs-5 cell is a DNA double-strand break repair-deficient derivative of CHO-K1. Terminal deletion frequency was small in both CHO-K1 and xrs-5 cells when cells were irradiated or tr eated with restriction enzyme while in the G(1) phase of the cell cycl e. In contrast, previous studies have shown that treatment of cells in G(2) leads to large deletion frequencies, especially in xrs-5 cells. Cell cycle analyses show large G(2) blocks in irradiated xrs-5 cells w ith only partial recovery over a 24-96-h period. These results suggest that most CHO cells with unrejoined breaks are blocked in G(2) and, t herefore, do not contribute to chromosome mutation frequencies. The sm all frequencies of terminal deletions that are found in these cells ma y reflect either an inefficiency in the G(2) checkpoint mechanism or, perhaps, a modification of broken ends that allows passage through G(2 ).