The distribution of quinacrine on chromosomes as determined by X-ray microanalysis. II. Comparison of heterochromatic and euchromatic regions of mouse chromosomes.

The distribution of quinacrine and protein sulphur has been compared with that of DNA in euchromatic and heterochromatic regions of mouse chromosomes stained with the fluorescent dye quinacrine, using X-ray microanalysis. Heterochromatin tends to bind relatively more quinacrine than euchromatin, and contains a greater concentration of sulphur. Measurements of quinacrine fluorescence, when compared with quinacrine binding, show that the excitation of fluorescence is more efficient when the dye is bound to euchromatin than when it is bound to heterochromatin. Although this observation is consistent with the hypothesis that the dull quinacrine fluorescence of mouse centromeres is due to quenching by guanine residues, two other factors should also be considered: the lower absolute amount of dye bound to the centromeres, and a concentration-dependent quenching of fluorescence.