[2-colored fluorescence of differentially condensed chromosomes stained with acridine orange]

Metaphase chromosomes of the Chinese hamster differentially-condensed under the influence of a) 5-bromdeoxyuridine, b) colcemide, and c) cold, were stained with acridine-orange (AO) in concentrations of 1.5 X 10(-7) to 3 X 10(-5) g/ml at pH 4.1 to 8.5. It was found that stretched chromosomal segments fluoresced in the orange/red part of the spectrum, whereas normally condensed ones--were green. The colour distribution along the chromosome depended mainly on the AO concentration and the exposure in the UV-light, and was independent of pH and molarity of the buffer. Apparently this phenomenon cannot be attributed to the uneven denaturation of the chromosomal DNA, but rather depends on structural and/or chemical differences between euchromatin and heterochromatin.