Chromosomal localisation of DNA sequences in condensed and dispersed human chromatin.

The DNAs purified from condensed and dispersed human chromatin were used as templates for the in vitro synthesis of ³H-labelled complementary RNAs (cRNAs). These cRNAs were hybridised in situ to preparations of fixed human metaphase chromosomes which had previously been stained with quinacrine and photographed with fluorescent (UV) light. Autoradiographs of the hybridised chromosomes were stained and photographed and the results analysed by comparison of the fluorescence photographs with the autoradiographs. This method allowed positive identification of every chromosomal site of hybridisation and quantitative analysis of grain distribution over a number of metaphase spreads. The cRNA transcribed from condensed chromatin DNA (cRNA_C) hybridised mainly to a limited number of sites close to or including centromeric heterochromatin (C-bands) and also to the brightly fluorescent regions of the Y chromosome. Many of these C-band regions are known to contain satellite DNAs, indicating that the repeated DNA in the condensed chromatin fraction consists largely, if not entirely, of satellite sequences. The cRNA transcribed from dispersed chromatin DNA (cRNA_D) does not contain satellite DNAs and hybridised more generally over the chromosome arms. However, the main sites of hybridisation with cRNA_D included the C-bands in the Y chromosome and autosomes, i.e. those regions which bound cRNA_C. This suggests that nonsatellite repeated DNA sequences may be associated with satellite DNAs in the chromosomes. No general correlation between the distribution of either kind of cRNA and the overall level of quinacrine fluorescence in chromosomes or chromosome arms was detectable, nor could the dispersed fraction be equated with cytological euchromatin, since it hybridised in many sites which appear heterochromatic. However, there was a suggestion that some non-fluorescing Q-bands bound cRNA_D preferentially. The differences which were found between the distribution of the cRNAs from the two chromatin fractions may be associated with differences in genetic activity.