Sister chromatid exchange.

Sister chromatid exchange (SCE) refers to the interchange of DNA between replication products. The technique for detecting such exchanges takes advantage of the semiconservative nature of DNA synthesis. 5'-bromodeoxyuridine (BrdU) is incorporated into the newly synthesized DNA. Using standard culturing techniques, Colcemid is added to the culture and conventional cytogenetic preparations are made. Differential staining with Hoechst dye and Giemsa allows the newly synthesized DNA within a chromatid to be recognized, since BrdU incorporation results in much weaker staining. SCEs represent a point of DNA template exchange during strand synthesis, visualized as asymmetric chromatid staining or "harlequin" chromosomes.