Biochemistry

Unwinding Chromatin

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Science  21 Jan 2000:
Vol. 287, Issue 5452, pp. 393
DOI: 10.1126/science.287.5452.393d

In eukaryotic cells, DNA is packaged into a 10-nm filament of nucleosomes, which are particles containing two turns of DNA wrapped around a core of histone proteins, and the nucleosomes are condensed into a 30-nm chromatin fiber. Even higher-level organization enables cells to encapsulate a 5-cm chromosome into a 5-_m nucleus. Cui and Bustamante have begun to make measurements of the mechanical parameters by stretching single chromatin fibers using laser tweezers. From a model incorporating their force-extension data, they suggest that the attractive energy between nucleosomes may be about 3.4 kBT and therefore that interconversion between the filament and fiber may necessitate only a gentle tweak of the dynamics. How the cell might regulate these internucleosomal interactions, and thus gain access to genes that need to be turned on, is through highly specific and reversible covalent modifications of the histone tails that protrude from the core particles, as summarized by Strahl and Allis.—GJC

Proc. Natl. Acad. Sci. U.S.A.97, 127 (2000); Nature403, 41 (2000).

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