The replication of unsubstituted and 5-bromodeoxyuridine- or 5-chlorodeoxyuridine-substituted DNA regulates the rate of induction of sister chromatid exchanges.

The thymidine (dThd) analog 5-bromodeoxyuridine (BrdUrd) is widely used in studies of the induction of sister chromatid exchanges (SCEs), since growth in the presence of BrdUrd allows the subsequent differential staining of the chromosomes through the fluorescence-plus-Giemsa (FPG) method. However, the analog itself induces SCEs, an aspect of its use which is often not considered. We have studied the induction of SCE by BrdUrd and a second dThd analog 5-chlorodeoxyuridine (CldUrd). Growth of Chinese hamster ovary (CHO) cells for 2 rounds of replication in the presence of different concentrations of either analog results in increasing results in increasing SCE frequencies which are linearly proportional to the degree of analog substitution for dThd in the DNA. However, CldUrd causes 3 to 5 times the number of SCEs found with BrdUrd, at equivalent substitution for dThd. With both analogs the increase in SCE frequency is due to the replication of the analog-substituted DNA and not to the incorporation of analog into nascent DNA. This induction of SCE can be considered at the level of a single strand of DNA since the replication of bifilarly substituted DNA results in twice the number of SCEs that are induced by the replication of unifilarly substituted DNA.