Chromosome aberration types in Chinese hamster (CHO) cells X-irradiated in S and G1

The different types of chromosome aberrations found after a single dose of 2.5 Gy of hard X-rays were scored in Chinese hamster (CHO) cells irradiated in early G1, late G1, early S and late S phase. Chromatid-type aberrations were found throughout S, and chromosome-type aberrations were found in early S. Sister unions were found in early S and G1, and were more common in early than in late G1. Sister unions manifested themselves as an increase in the number of apparent acrocentric chromosomes, with an accompanying apparent increase in chromosome number. It is suggested that previous studies have failed to observe sister unions in this cell type because of the difficulty of distinguishing between sister unions and acrocentric chromosomes.