Editing Nucleosome (section)

===Higher order structure===
[[Image:Chromatin Structures.png|right|thumb|600px|The current chromatin compaction model]]
The organization of the DNA that is achieved by the nucleosome cannot fully explain the packaging of DNA observed in the cell nucleus. Further compaction of [[chromatin]] into the cell nucleus is necessary, but it is not yet well understood.  The current understanding<ref name=pmid15680970/> is that repeating nucleosomes with intervening "linker" DNA form a ''10-nm-fiber'', described as "beads on a string", and have a packing ratio of about five to ten.<ref name="pmid12540921"/> A chain of nucleosomes can be arranged in a ''30&nbsp;nm fiber'', a compacted structure with a packing ratio of ~50<ref name="pmid12540921"/> and whose formation is dependent on the presence of the [[Histone H1|H1 histone]].

A crystal structure of a tetranucleosome has been presented and used to build up a proposed structure of the 30&nbsp;nm fiber as a two-start helix.<ref>{{cite journal | vauthors = Schalch T, Duda S, Sargent DF, Richmond TJ | title = X-ray structure of a tetranucleosome and its implications for the chromatin fibre | journal = Nature | volume = 436 | issue = 7047 | pages = 138–141 | date = July 2005 | pmid = 16001076 | doi = 10.1038/nature03686 | s2cid = 4387396 | bibcode = 2005Natur.436..138S }}</ref>
There is still a certain amount of contention regarding this model, as it is incompatible with recent [[electron microscopy]] data.<ref name="pmid16617109">{{cite journal | vauthors = Robinson PJ, Fairall L, Huynh VA, Rhodes D | title = EM measurements define the dimensions of the "30-nm" chromatin fiber: evidence for a compact, interdigitated structure | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 17 | pages = 6506–6511 | date = April 2006 | pmid = 16617109 | pmc = 1436021 | doi = 10.1073/pnas.0601212103 | doi-access = free | bibcode = 2006PNAS..103.6506R }}</ref>  Beyond this, the structure of chromatin is poorly understood, but it is classically suggested that the 30&nbsp;nm fiber is arranged into loops along a central protein scaffold to form transcriptionally active [[euchromatin]]. Further compaction leads to transcriptionally inactive [[heterochromatin]].