Nonhistone proteins and chromosome structure.

Abstract

One of the major unanswered questions about chromosome structure concerns the manner in which the chromatin fiber is arranged to form the chromosome bands, chromomeres, and chromatid. It is now clear to all but the most recalcitrant holdout that chromosomes are uninemic structures containing a single DNA molecule that somehow makes its way from one telomere to the other. The advent of chromosome banding indicates that straightforward models of multiple folding of the chromatin fiber (DuPraw 1965; Prescott 1970; Comings 1972) are not adequate. The fact that the chromosome bands of mitotic chromosomes correlate precisely with the chromomeres of meiotic pachytene chromosomes (Okada and Comings 1974; Luciani et al. 1975) indicates that these condensations are a fundamental substructure of the chromosome. Examination of banding patterns in partially condensed prometaphase chromosomes (Yunis 1976) and prematurely condensed interphase chromosomes (Unakul et al. 1973) show that each mitotic chromosome band is composed of multiple smaller sub-bands with a total of 3000 per genome. The question is, how far can this subdivision of bands be carried? An intriguing answer is that it can be carried to the same end point that is already well known from the giant polytene chromosomes of Drosophila salivary glands. The chromomeres of such polytene chromosomes appear to be approximately equivalent to one gene. In Drosophila melanogaster there are approximately 5000 chromomeres.