Spontaneous sister-chromatid exchanges in Chinese hamster cells in vivo and in vitro

The incidence of sister-chromatid exchanges (SCEs) was studied in Chinese hamster cells. The cells used were an established cell line (Don), an aneuploid secondary culture still exhibiting contact inhibition of growth, a primary culture, bone-marrow cells in vivo, a Don-derived clone having ring chromosomes, and endoreduplicated Don cells. The frequency of SCEs in the presence of bromodeoxyuridine (BUdR) increased in the following order: bone-marrow cells less than Don less than secondary culture cells less than primary culture cells. Marked increases in BUdR concentration induced only slight increases of SCEs. Some ring chromosomes showed Moebius strip and concatenated ring structures, indicating spontaneous occurrence of SCEs in the absence of BUdR. The incidence of spontaneous SCEs observed in ring chromosomes approximates that of SCEs observed in ordinary rod chromosomes in the presence of low dose levels of BUdR. SCEs occurring in the first and second cell cycles were separately counted in endoreduplicated cells. The ratio of single SCEs, which occurred in the second cell cycle, to twin SCEs, which occurred in the first cell cycle, was about 2, when the count of single SCEs was corrected for the induction effect. This implies that uninemy chromatids retain the polarity of DNA when SCEs occur, that bifilarly and trifilarly BUdR-substituted DNA strands give equal numbers of SCEs and that incidence of SCEs is independent of the length of S in the Don cells used here.