Molecular structure of human chromosomes / edited by Jorge J. Yunis.

Date:
1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Molecular structure of human chromosomes / edited by Jorge J. Yunis. Source: Wellcome Collection.

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    5. Chromosomes and Chromatin Structure 155 28.94:1. In other words, there are approximately 29 lengths of DNA in every length of chromatin fiber. The simplest way to accommodate so much DNA in a 200 A fiber appears to be a supercoil, and several models of such an organization have been proposed (Bahr, 1970). Such models consider only two global quantities, DNA and fiber length. How does the new insight into the nucleosomal substructure of chroma¬ tin agree with the observation that chromatin is organized into a fibrous structure with an average diameter of 200 A? We can approach this question with a modest experiment, namely, gluing spheres, having a diameter of 9 mm, at intervals of 20 mm to a jute cord 2 mm thick. Those familiar with published measurements on nucleosomes will recognize that a very simple model is being constructed. If a flexible plastic tube or hose with an inner diameter of 20 mm is placed on end on a flat surface, one can easily pack 15 spheres, attached and connected with cord, into a 40 mm length of tube. The spheres are mostly layered in sets of three to a plane approximately perpendicular to the axis of the tube. Assuming, as do Pardon et al. (1975), that 140 base pairs are integrated into a nucleosome and 60 base pairs constitute the internucleosomal DNA, we have 10,200 A of DNA double helix in 400 A (40 ¡лт) fiber length. This represents a packing ratio of 2 5.5, in fair agreement with the previously esti¬ mated packing ratio of 28.3. Such a simple model leaves more than about half of the volume of the tube free for the missing liistone HI and nonhistone proteins. We should not fail to emphasize that there was much irregularity in the position of the spheres and the course of the thread (DNA molecule). In fact, the outside of the tube resembled the familiar irregular shape of a chromatin fibril. At present the nature of nucleosomes is under intensive investigation. Noll (1974), Shaw et al. (1974), and SoUner-Webb and Felsenfeld (1975) estimate 600-680 A as the length of DNA associated with a nucleosome. If, in the above model, 680 A (or 200 base pairs) is used in the calculations instead of 140 base pairs, the packing ratio for DNA in a fiber would exceed 33. The size and shape of the nucleosome, the length of internucleosomal DNA, and the degree of mutual penetration of nucleosomes in tight packing represent other variables that affect the packing ratio. The concept of nucleosomes being held together and perhaps controlled by specific proteins, among them histone HI, non¬ histones, and contractile proteins, is attractive for several reasons. First, the observed thickening of chromatin fibrils upon exposure to a mitotic poison is much easier to understand with nucleosomes than with a relatively rigid coiled coil configuration. Second, it explains images of 100 A fibers emerging from 200 A fibers (100 is the approximate diameter of nucleosomes). Further, the ob¬ served intermediate and thinner fibers can be imagined, since DNA stretches devoid of nucleosomes must be seen as part of the picture. Why is the packing ratio independent of proteins picked up by chromatin