Dichromatic fluorescence of differentially condensed chromosomes stained with acridine orange

Abstract

Metaphase chromosomes of the Chinese hamster, differentially condensed by the action of: 1) 5-bromodeoxyuridine, 2) colcemid, 3) “cold shock” were stained with acridine orange in concentrations of between 1.5×10⁻⁷ and 3×10⁻⁵ g/ml at pH values of between 4.1 and 8.5. The stretched segments of the chromosomes gave a reddish-orange fluorescence, whereas the condensed segments gave a green fluorescence. Distribution of the color along the length of chromosome depended chiefly on the acridine orange concentration and on the exposure in ultraviolet light and was unchanged by variations in the pH of the dye and the molarity of the buffer in which it was made up. The phenomenon is evidently unconnected with variations in denaturation of DNA but is most probably due to structural-chemical differences between euchromatin and heterochromatin.