Fine structure of 33 258 H-treated chromosomes.

Metaphase chromosomes of mouse strain L cells show strikingly uncondensed pericentric heterochromatic regions after treatment of living cells with the benzimidazol-derivative 33 258 Hoechst. In electron micrographs of total preparations after G-band staining the chromosomes are seen to be made up of irregularly folded fibrils of 200-400 A in diameter. In the uncondensed regions only very few fibrils laid loose loops are present, making it probable that only one fibril forms one chromatid.