Topographic examination of sister chromatid differential staining by Nomarski interference microscopy and scanning electron microscopy.

BrdU-substituted Chinese hamster chromosomes were treated with a hog Na2HPO4 solution and stained with Giemsa to produce sister chromatid differential staining (SCD). The process of SCD was examined with the Nomarski differential interference microscope and the scanning electron microscope. After the Na2HPO4 treatment alone, unifilarly BrdU-substituted (TB) chromatids appeared somewhat more severely collapsed than the bifilarly substituted (BB) chromatids. Subsequent Giemsa staining, however, brought about pronounced piling up of the Giemsa dye on the TB-chromatids but not on the BB-ones, causing highly distinct differential Giemsa staining as well as a marked differentiation in surface topography between the sister chromatids. Removal of the Giemsa dye from the differentially Giemsa stained chromosomes resulted in a disappearance of such a pronounced topographic differentiation.