A historical overview of bromo-substituted DNA and sister chromatid differentiation

Abstract

The thymidine analogue 5-bromo-2′-deoxyuridine (BrdU) has been widely used to make sister chromatid differentiation (SCD) evident in metaphase chromosomes of cells grown for two cycles in BrdU and, thus, containing varying amounts of the thymidine analogue. A direct consequence was the possibility of making sister chromatid exchange (SCE) evident without using autoradiographic procedures. The latter phenomenon was fi rst discovered in 1953, and its frequency is considered a reliable marker of pathological cell situations, as well as an indicator of mutagenic compounds. Several experimental procedures were found which produced SCD, such as the use of fl uorochromes like 33258 Hoechst or acridine orange, whose observation under fl uorescence microscopy was directly recorded by photos or stained with Giemsa to make chromosome preparations permanent. Other treatments followed by Giemsa staining required the use of saline hot solutions, acid solutions, nuclease attack and specifi c monoclonal antibodies. Basically two molecular mechanisms were invoked to explain the different affi nity of Giemsa stain for differential BrdUsubstituted chromatid DNA. The fi rst implied debromination of chromatid DNA, whose occurrence would be greater in chromatids containing an amount of BrdU greater than that present in sister chromatids. The second mechanism, although not denying the importance of DNA debromination, postulated that chromatin structural organization, in terms of DNA–protein and/or protein–protein DNA interaction, is responsible for SCD production.