Sister chromatid differential staining by direct staining in Na2HPO4-Giemsa solution and the mechanism involved.

The direct staining of BrdU-substituted Chinese hamster chromosomes in a Na2HPO4-Giemsa solution without any pretreatments resulted in a B-dark type SCD istituted (TB) chromatids stained light. Detailed examinations of the staining process suggested that the Na2HPO4 solution acts to collapse chromosomes whereas the Giemsa dye works to reconstruct the collapsed chromosomes, and that during the reconstruction process preferential binding of the Giemsa dye to the BB-chromatids occurs to produce the B-dark SCD. It was revealed that not only the time but the temperature at which chromosome preparations are kept prior to use considerably affect the occurrence of SCD.