PerspectiveStructural Biology

The 30-nm Fiber Redux

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Science  25 Apr 2014:
Vol. 344, Issue 6182, pp. 370-372
DOI: 10.1126/science.1253852

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The DNA of eukaryotic cells is packaged onto nucleosomes—complexes composed of histone proteins—that together form chromatin, enabling tight packing of the genome within the cell nucleus. In the first folded structure of chromatin to be characterized, nucleosomes were coiled into a ∼30-nm-diameter helix, with the “linker” histone located in the interior of the fiber (1). The precise molecular organization of this 30-nm fiber has long been extensively debated. Initially, structural studies on fibers assembled on natural DNA sequences were hampered by variation in the length of the linker DNA between nucleosomes. However, more recently, the construction of regularly spaced tandem DNA repeats for precise nucleosome positioning (2) has revolutionized analysis. On page 376 of this issue, Song et al. (3) determine by cryo–electron microscopy the 11 Å–resolution structure of 30-nm fibers assembled from arrays of 12 nucleosomes.