Cryo-EM Study of the Chromatin Fiber Reveals a Double Helix Twisted by Tetranucleosomal Units

Double Helix, Doubled Chromatin consists of genomic DNA packaged onto nucleosomes—double donut-shaped complexes of histone proteins. Roughly 150 base pairs of DNA are wrapped around each nucleosome with variable lengths of linker DNA in-between. Using cryogenic electron microscopy, Song et al. (p. 376 ; see the Perspective by Travers ) determined the 11 angstrom–resolution structure of a 12-nucleosome string of DNA. The segment forms a 30-nanometer fiber structure, which is itself double-helical, like the DNA it packages.