Abstract
In 5-bromodeoxyuridine (BUdR)-substituted human chromosomes stained with 4''-6-diamidino-2-phenylindole (DAPI), differential staining is suppressed totally by the H+-donor cysteamine (concentration 0.08 M). Differential staining may appear because the double BUdR-substituted chromatid is disintegrated via a photosensitive dye-visible light system. Cysteamine may prevent the production of strand breaks in DNA and, consequently, differential staining in BUdR-substituted chromosomes. Differential staining with DAPI causes irreversible changes in the double BUdR-substituted chromatid; this can be explained by the above-mentioned mechanism.