DNA lysis is involved in the simplified fluorescence plus Giemsa method for differential staining of sister chromatids.

The DNA labelling of the bifilarly 5-bromodeoxyuridine- to-substituted chromatid decreased relative to that of the unifilarly substituted chromatid with increasing duration of HB pretreatment (Hoechst 33258 plus black light at 55 degrees C). Sister chromatid differential staining was detected by Giemsa as well as a DNA-specific dye, ethidium bromide, after 4 s of HB pretreatment. The contrast of sister chromatid differential staining was improved with increased duration of HB pretreatment or by incubation with exonucleases. Hydrogen donors such as cysteamine, cysteine, and L-ascorbic acid inhibited the HB pretreatment, but this inhibition could be overcome by increasing the duration of HB pretreatment.