Nucleosome schematic model

Fig. 5. Schematic model of the nucleosome, with histone HI shown as stabilizing the fold of the DNA molecule around the core histones [based on results of Sperling and Sperling (1978)]. The nucleosome dimensions are derived from X-ray (Finch et al., 1977) and neutron (Baldwin et al., 1975 Pardon et al., 1977 Suauet al., 1977) scattering experiments. The histone core dimensions are derived from electron microscopic and X-ray studies (Sperling and Amos, 1977 Wachtel and Sperling, 1979 Sperling and Wachtel, 1979). The regions of the DNA molecule indicated by dashed lines indicate those base pairs which are not present in nucleosome core particles.

Fig. 6. (a) A schematic model of the helical, double-stranded, unstaggered, H4 fiber (Sperling and Amos, 1977). The asymmetric unit is an axial dimer and there are six such dimers per strand per repeat. The repeat distance is 330 A. The two different types of axial bonds—within and between dimers—are denoted by a thick and thin line, respectively. The tetrameric grouping is indicated, (b) A model of (a) upon which is superimposed a schematic representation of a nucleosome core particle... 

Figure 1. Schematic representation of remodelling mechanisms. (Adapted form Langst and Becker, 2004.) The schemes show nucleosomes from the top. (a) The twist diffusion model - Twisting of DNA moves it over the histone surface in one base pair increments. This changes the position of the DNA with respect to the histone, as shown by the open and closed circles, (b) The Loop recapture model - Extranucleosomal DNA is pulled into the nucleosomes to replace a DNA segment which consequently loops out. This loop is then propragated over the histone surface like ripples of a wave. The star,, indicates how this leads to a change in the position of DNA relative to the nucleosome. (See Colour Plate 4.)...

Fig. 2. Schematic view of the location of GHl in the nucleosome. (a) Symmetric model of Allan et al. [9] (b) Asymmetric model of Zhou et al. [17] (c) Model of Pruss et al. [23].

Figure 5-21 Nucleosomes. (A) Electron micrographs of individual nucleosomes reconstituted from 256-bp DNA fragments and separated proteins. From Hamiche et al.213 Courtesy of Ariel Prunell. (B) Model of a nucleosome core. The 1.75-tum (145-bp) DNA superhelix winds around the histone octomer which consists of two subunits apiece of histones H2A, H2B, H3, and H4. In addition, two elongated molecules of proteins HMG-14 or HMG-17 are indicated (see also Chapter 27). (C) Schematic radial projection of the doublehelical DNA showing areas protected from cleavage by hydroxyl radicals (see Fig. 5-50) by the bound proteins. The shaded areas are those protected by HMGs. The zigzag lines near the dyad axis indicate the most prominent regions of protection. (B) and (C) are from Alfonso et al.2U...

Studies of the condensed chromatin fibre structure and the condensation mechanism have resulted in basically two classes of models models based on a helical arrangement of nucleosomes along the fibre and those based on a linear array of globular nucleosome clusters (superbeads) along the fibre. The first class includes the solenoid, twisted ribbon and crossed linker models whereas the latter are the superbead models and related layered structures. Schematic representations of some models are shown in Fig. 10. 

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