New Giemsa method for the differential staining of sister chromatids.

Abstract

IF human lymphocytes1 or Chinese hamster2 cells are treated with the base analogue 5-bromodeoxyuridine (BrdU) in the latter part of the S period, Giemsa stained chromosomes exhibit a pattern of condensed and extended segments along their length. This phenomenon has been attributed to a delay in the spiralisation pattern of the late replicating regions along the chromosomes. Other experiments3 with Chinese hamster ovary (CHO) cells have shown that after two rounds of replication in the presence of BrdU or IdU, sister chromatids stain differentially with Giemsa, allowing the identification of the two chromatids, and the observation of sister chromatid exchanges (SCEs) without recourse to autoradiography. The chromatid with the bifilarly substituted DNA (BrdU substituted in both strands of DNA) is less condensed and stains more weakly with Giemsa than the unifilarly substituted sister chromatid. The yield of SCEs is approximately that observed by autoradiography.