Chromatin superbeads

Although the higher order structure of chromatin is still not well understood, two models have been suggested and supported by experimental evidence (for references, see Felsenfeld, 1978 Chambon, 1978). One is the solenoid model of Finch and Klug (1976), obtained by supercoiling the chromatin thread, and the other model is the superbead model (Renz et al., 1977). Each may represent a different... 

With the demise of the uniform fiber model in 1974, it became necessary to devise other models to account for the early electron micrographs of chromatin fibers and the X-ray diffraction studies (see Ref. [1], Chapter 1). Two models appeared in 1976, and were the major contenders for consideration in 1978. The superbead model of Franke et al. [36] envisioned the chromatin fiber as a compaction of multi-nucleosome superbeads . The solenoid model of Finch and Klug [37] postulated a regular helical array of nucleosomes, with approximately six nucleosomes per turn and a pitch of 10 nm. Although a number of competing helical models appeared in the 1980s (see Ref. [1], Chapter 7) the solenoid model remains a serious contender to this day. Structural details of this model, such as the precise disposition of linker DNA, are still lacking. 

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. 

The twisted ribbon model of Worcel et al. and the superbead model of Zentgraf and Franke can be ruled out on the basis of X-ray diffraction studies of oriented chromatin fibres The former model predicts the 0.175,0.27 and 0.37 nm ... 

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