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Human mitotic chromosomes consist predominantly of irregularly folded nucleosome fibres
without a 30-nm chromatin structure

 Press Release 

The EMBO Journal

Structural Biology Center, Biological Macromolecules Laboratory (Maeshima Lab.)
Human mitotic chromosomes consist predominantly of irregularly folded nucleosome fibres without a 30-nm chromatin structure.  Yoshinori Nishino, Mikhail Eltsov, Yasumasa Joti, Kazuki Ito, Hideaki Takata, Yukio Takahashi, Saera Hihara, Achilleas S Frangakis, Naoko Imamoto, Tetsuya Ishikawa and Kazuhiro Maeshima.
The EMBO Journal, Published online: 17 February 2012  DOI: 10.1038/emboj.2012.35

How a long strand of genomic DNA is compacted into a mitotic chromosome remains one of the basic questions in biology. The nucleosome fibre, in which DNA is wrapped around core histones, has long been assumed to be folded into a 30-nm chromatin fibre and further hierarchical regular structures to form mitotic chromosomes, although the actual existence of these regular structures is controversial. Here, we show that human mitotic HeLa chromosomes are mainly composed of irregularly folded nucleosome fibres rather than 30-nm chromatin fibres. Our comprehensive and quantitative study using cryoelectron microscopy and synchrotron X-ray scattering resolved the long-standing contradictions regarding the existence of 30-nm chromatin structures and detected no regular structure 411 nm. Our finding suggests that the mitotic chromosome consists of irregularly arranged nucleosome fibres, with a fractal nature, which permits a more dynamic and flexible genome organization than would be allowed by static regular structures.

Chromosomes consist essentially of irregularly folded nucleosome fibres. Condensins (blue) hold the nucleosome fibres (red) globally around the chromosome centre. Locally, the nucleosome fibre is folded in an irregular or disordered manner, forming loop structures that are collapsed towards the chromosome centre (blue).