Involvement of higher order chromatin structures in metaphase chromosome organization.

Using electron microscopy we show that the metaphase chromatin fibers of Chinese hamster ovary cells form the same ionic strength-dependent higher order structures as the corresponding interphase chromatin fibers. We present evidence that such intact chromatin fibers are a prerequisite for the maintenance of the characteristic shape of metaphase chromosomes. The evidence is based on the finding that treatment of chromosomes with 0.5 M NaCl, a condition which is known to remove histone H1 and which destroys the higher order structure of chromatin fibers, also leads to a disintegration of the metaphase chromosome structure, whereas treatment with 0.3 M (or less) NaCl has no effect on the integrity of the chromosomes and their chromatin fibers. These data support a model in which the metaphase chromosome is maintained by a tight assembly of the 25-30 nm thick chromatin fibers containing all the histones.