Higher-order structure of chromatin from resting cells. I. Electron microscopy of chromatin from calf thymus

Extremely large domains of the genome of resting cells (calf thymus) have been visualized in the electron microscope by combining mild extraction procedures with a non-artifactual method of mounting the sample (the phospholipid monolayer technique). The observed chromatin strands, free from distortion, reach contour lengths up to 60 micrometers. After lysis of the nuclei, four classes of fibres may be identified on the basis of their diameters (30, 24, 18 and 11 nm, respectively). The morphology of giant chromatin strands is strikingly regular; long trains of equally sized, arc-shaped segments are observed, their length being, in many cases, multiples of a fixed value. The inflection points delimiting contiguous segments are often associated with laminar fragments of the nuclear envelope or, less frequently, linked to fibrillar elements. It appears that higher-order structures of chromatin in resting cells conform, to a large extent, to a so called 'drapery-like' mode, according to which a continuous strand runs between contiguous anchorage sites placed on the nuclear envelope. Because of the presence of regularly spaced inflection points, this organization is much more ordered than expected. Spontaneous unwinding of the fibres at low ionic strength, limited nuclease digestion, and relaxation in the presence of ethidium bromide, have been used as probes of the conformation. All these experiments rule out its identification with a single-strand helix. The final ordered state is attained by folding the basic 11 nm strand and by winding up this configuration on itself. This leads to a coiled-coil or 'rope-like' model. The 11 nm strand is 'punctuated' by sharp kinks. Roughly, it may be assimilated to a chain of semirigid, freely joined elements. As a consequence, local flexibility is greatly enhanced, so allowing the assembly mode described.