Evidence that the replication point is the site of sister chromatid exchange

DNA synthesis in Chinese hamster cells was blocked partially by treating the cells with either fluorodeoxyuridine (FUdR) or cycloheximide (CHM) for various lengths of time. Analyses of the population kinetics and measurement of incorporation of labeled nucleosides during the FUdR block strongly suggested that the number of growing points was accumulated by the treatment while the rate of chain growth was greatly reduced. No evidence for such an accumulation was obtained in the CHM-treated cells. To study the relation between DNA replication and sister chromatid exchange (SCE), bromodeoxyuridine-labeled cells were exposed to blue fluorescent light while DNA synthesis was blocked. The frequency of SCE induced by the light treatment appeared to increase as the number of growing points increased, implying that the site of exchange is confined to the replication forks. The induction of SCE by fluorescent light was inhibited completely by CHM-treatment. The reason for this finding remains to be elucidated.