Fine structure of unstained human chromosome fibres dried with no fixative as observed by X-ray contact microscopy.

X-ray contact microscopy was used to examine unstained human chromosomes dried without any fixative. This is the first report of the observation of single human chromosome fibres at high resolution without any modification(s) such as staining or fixation. The 'beads-on-a-string' structure was observed in stretched portions of chromosome fibres. The diameters of the thin filaments and small particles were 7-15 nm (mean +/- SD of 12.2 +/- 2.1 nm) and 16-69 nm (31.4 +/- 13.2 nm), respectively. These sizes correspond to those of fibres composed of nucleosomes (10-15 nm in diameter) and particle units (15-50 nm in diameter) called superbeads (unit structures composed of a multiple association of nucleosomes). These results are in good agreement with the current nucleosome and superbead model for chromosome fibre structure.