The chromosome fiber: evidence for an ordered superstructure of nucleosomes

Chromosome fibers isolated from lymphocyte nuclei and prepared for electron microscopy by techniques designed to preserve their native structure have a distinctly knobby appearance, suggesting that DNA and protein are not distributed evenly along the fiber axis. Individual knobs (superbeads) are arranged in tandem and have an average diameter of about 200 A. Mild nuclease digestion of isolated nuclei releases apparent monomer superbeads that are composed of nucleohistone particles with the properties of nucleosomes. The kinetics of digestion indicate that the superbead is a discrete structural unit containing, on the average, about eight nucleosomes.