  We present a theoretical analysis of the structural and mechanical properties
of the 30-nm chromatin fiber. Our study is based on the two-angle model
introduced by Woodcock et al. (Woodcock, C. L., S. A. Grigoryev, R. A.
Horowitz, and N. Whitaker. 1993. PNAS 90:9021-9025) that describes the
chromatin fiber geometry in terms of the entry-exit angle of the nucleosomal
DNA and the rotational setting of the neighboring nucleosomes with respect to
each other. We explore analytically the different structures that arise from
this building principle, and demonstrate that the geometry with the highest
density is close to the one found in native chromatin fibers under
physiological conditions. On the basis of this model we calculate mechanical
properties of the fiber under stretching. We obtain expressions for the
stress-strain characteristics which show good agreement with the results of
recent stretching experiments (Cui, Y., and C. Bustamante. 2000. PNAS
97:127-132) and computer simulations (Katritch, V., C. Bustamante, and W. K.
Olson. 2000. J. Mol. Biol. 295:29-40), and which provide simple physical
insights into correlations between the structural and elastic properties of
chromatin.
