Effects of counterstaining with DNA binding drugs on fluorescent banding patterns of human and mammalian chromosomes.

Pairs of fluorescent A-T specific dyes and nonfluorescent agents with similar or complementary base pair binding specificity were used to analyse the extent to which banding patterns in human chromosomes obtained by fluorescent staining can be modified by counterstaining. By testing a variety of different combinations of drugs, essentially three types of alterations were observed. Enhanced contrast of specific heterochromatic regions was obtained with pentamidine, or netropsin, in conjunction with the fluorescent stains Hoechst 33258, DAPI or DIPI, the resulting banding patterns being similar to that reported for distamycin A plus DAPI (DA-DAPI banding [21]. Uniform quenching of Hoechst 33258, DAPI or DIPI fluorescence was induced by counterstaining with stilbamidine or berenil. The combination of echinomycin with DAPI resulted in an improved contrast of DAPI banding on chromosome arms and pale fluorescence on major autosomal C band regions. In addition, a subdivision of the heterochromatic part of the Y chromosome may be discerned by this latter technique.