Editing Nucleosome (section)

====Histone tail domains====
The histone tail extensions constitute up to 30% by mass of histones, but are not visible in the crystal structures of nucleosomes due to their high intrinsic flexibility, and have been thought to be largely unstructured.<ref name="pmid12666178">{{cite journal | vauthors = Zheng C, Hayes JJ | title = Structures and interactions of the core histone tail domains | journal = Biopolymers | volume = 68 | issue = 4 | pages = 539–546 | date = April 2003 | pmid = 12666178 | doi = 10.1002/bip.10303 }}</ref> The N-terminal tails of histones H3 and H2B pass through a channel formed by the minor grooves of the two DNA strands, protruding from the DNA every 20 bp. The [[N terminus|N-terminal]] tail of histone H4, on the other hand, has a region of highly basic amino acids (16–25), which, in the crystal structure, forms an interaction with the highly acidic surface region of a H2A-H2B dimer of another nucleosome, being potentially relevant for the higher-order structure of nucleosomes. This interaction is thought to occur under physiological conditions also, and suggests that [[acetylation]] of the H4 tail distorts the higher-order structure of chromatin.{{citation needed|date=February 2024}}