Metaphase Chromosome Structure: The Role of Nonhistone Proteins

How is the enormous length of DNA in a mitotic chromosome packaged into such a compact structure? Recently it has become clear that at least one level of packaging, the coiling of the DNA into the basic nucleohistone fiber, is accomplished by the histones. The nucleohistone fiber of interphase and mitotic chromosomes has been studied extensively by electron microscopy. Thin-section techniques and surface spreading (see, e.g., DuPraw 1970; Ris 1975) reveal a knobby, "thick" fiber of about 200–300-Å diameter. This thick fiber appears to unfold at low ionic strength, or by removal of proteins, to a beads-on-a-string conformation (Woodcock 1973; Olins and Olins 1974) in which the "beads" are nucleosomes containing two each of the four histones (as an octamer) and 140–200 base pairs of DNA coiled around a histone core (Kornberg 1974; Oudet et al. 1975; and papers in this volume). To account for the thick chromatin fiber, Finch...