Reversal of DNA polarity as revealed by sister chromatid exchanges in ring chromosomes

DNA polarity at sister chromatid exchange (SCE) sites were studied in ring chromosomes from Chinese hamster cells. If the polarity of DNA strands was conserved. a double-length, symmetric dicentric ring chromosome with symmetric twin SCEs appeared after two cell cycles when a SCE occurred in S1. Indeed, approximately two-thirds of double-length. symmetric ring chromosomes belonged to this class. One-third of them, however, did not show symmetric SCEs, suggesting that polarity was not always conserved. SCE counts at centromeric regions were not high enough to account the frequency (about 1/3) of apparently inverted rejoining sites. The hypothesis that the polarity of rejoining sites is either conserved or inverted and that illegitimate rejoinings are partially repaired could explain the results. Telomere-like structures or intermediate structures during double-strand repair processes may contribute to the inverted polarity.