ABSTRACT
Chromatin is composed chiefly of DNA, a lesser quantity of RNA, and in general two classes of nuclear protein, the histones and nonhistones. A number of nuclear protein fractions, some likely important in cellular differentiation, may be either enriched or diminished depending on the method of chromatin isolation. Nuclear contractile proteins may be involved in a general inhibition or inactivation of chromatin activity via their participation in chromatin condensation. The increase in nonhistone chromosomal proteins (NHCP) during embryogenesis has been confirmed with studies demonstrating a progressive enrichment of newly synthesized NHCP. The polytene chromosomes of Drosophila and some other insect species represent extremely useful systems for the correlation of nuclear events during embryogenesis and development. Stage-dependent alterations in nonhistones are seen throughout embryogenesis and differentiation, which is in accordance with the hypothesis that some of these molecules are gene regulators.
