Localization of chromosome breakpoints induced by DNase I in Chinese hamster ovary (CHO) cells

DNase I was electroporated into S-phase CHO cells and induced chromosome breakpoints were localized in G-banded metaphases. More than 75% of breakpoints mapped to Giemsa-light bands, 18% to Giemsa-dark bands and about 7% to band junctions. Chromosome breakpoint clusters produced by DNase I colocalized with chromosome breakpoints induced by the restriction endonucleases AluI and BamHI in the G1- and S-phases of the cell cycle in CHO cells. Digestion of metaphase spreads with AluI, BamHI and DNase I produced G-bands, indicating that G-light bands are more sensitive to endonuclease action. The possible role of nuclease-sensitive sites in active chromatin as selective targets for the induction of chromosome breakpoints by these endonucleases is discussed.