Higher-order structure of chromatin from resting cells. II. High-resolution computer analysis of native chromatin fibres and freeze-etching of nuclei from rat liver cells.

Non-destructive electron microscopy of native chromatin from rat liver nuclei reveals that the 30 nm fibre is formed of four 11 nm nucleofilaments, arranged in a coiled-coil (or rope-like) conformation. At low ionic strength, native fibres show an alternating pattern of compact and unwound regions. Freeze-etching experiments carried out on the same nuclei are compatible with the existence of periodic attachments of the fibres to the nuclear envelope near the pores in a regular, drapery-like fashion. For the first time, computer image analysis has been applied to electron micrographs of giant chromatin fibres and a few essential geometrical parameters characterizing the conformation of the higher-order structures have been determined. No significant difference has been found between calf thymus and rat liver chromatin.