DNA folding: Structural and mechanical properties of the two-angle model for chromatin

We present a theoretical analysis of the structural and mechanical properti es of the 30-nm chromatin fiber. Our study is based on the two-angle model introduced by Woodcock et at. (Woodcock, C. L., S. A. Grigoryev, R. A. Horo witz, and N. Whitaker. 1993. Proc. Natl. Acad. Sci. USA. 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 nucleoso mes with respect to each other. We analytically explore the different struc tures that arise from this building principle, and demonstrate that the geo metry with the highest density is close to the one found in native chromati n fibers under physiological conditions. On the basis of this model we calc ulate mechanical properties of the fiber under stretching. We obtain expres sions for the stress-strain characteristics that show good agreement with t he results of recent stretching experiments (Cui, Y., and C. Bustamante. 20 00. Proc. Natl. Acad. Sci USA. 97:127-132) and computer simulations (Katrit ch, V., C. Bustamante, and W. K. Olson. 2000. J. Mol. Biol. 295:29-40), and which provide simple physical insights into correlations between the struc tural and elastic properties of chromatin.