Editing Histone (section)

== Structure ==

[[File:Steps in nucleosome assembly.svg|thumb|upright=2|Steps in nucleosome assembly]]

The [[nucleosome]] core is formed of two H2A-H2B [[protein dimer|dimers]] and a H3-H4 tetramer, forming two nearly [[symmetry|symmetrical]] halves by [[tertiary structure]] ([[Point groups in three dimensions|C2]] symmetry; one [[macromolecule]] is the mirror image of the other).<ref name=pmid9305837/> The H2A-H2B dimers and H3-H4 tetramer also show pseudodyad symmetry. The 4 'core' histones (H2A, H2B, H3 and H4) are relatively similar in structure and are highly conserved through [[evolution]], all featuring a '[[helix turn helix]] turn helix' motif (DNA-binding protein motif that recognize specific DNA sequence).  They also share the feature of long 'tails' on one end of the [[amino acid]] structure - this being the location of post-translational modification (see below).<ref name=HSH/>

Archaeal histone only contains a H3-H4 like dimeric structure made out of a single type of unit. Such dimeric structures can stack into a tall superhelix ("hypernucleosome") onto which DNA coils in a manner similar to nucleosome spools.<ref>{{cite journal | vauthors = Mattiroli F, Bhattacharyya S, Dyer PN, White AE, Sandman K, Burkhart BW, Byrne KR, Lee T, Ahn NG, Santangelo TJ, Reeve JN, Luger K | title = Structure of histone-based chromatin in Archaea | journal = Science | volume = 357 | issue = 6351 | pages = 609–612 | date = August 2017 | pmid = 28798133 | pmc = 5747315 | doi = 10.1126/science.aaj1849 | bibcode = 2017Sci...357..609M }}</ref> Only some archaeal histones have tails.<ref name=pmid30212449/>

The distance between the spools around which eukaryotic cells wind their DNA has been determined to range from 59 to 70 Å.<ref>{{cite journal | vauthors = Ward R, Bowman A, El-Mkami H, Owen-Hughes T, Norman DG | title = Long distance PELDOR measurements on the histone core particle | journal = Journal of the American Chemical Society | volume = 131 | issue = 4 | pages = 1348–9 | date = February 2009 | pmid = 19138067 | pmc = 3501648 | doi = 10.1021/ja807918f }}</ref>

In all, histones make five types of interactions with DNA:
* [[Salt bridge (protein and supramolecular)|Salt bridges]] and hydrogen bonds between side chains of basic amino acids (especially [[lysine]] and [[arginine]]) and phosphate oxygens on DNA
* Helix-dipoles form [[alpha helix|alpha-helixes]] in H2B, H3, and H4 cause a net positive charge to accumulate at the point of interaction with negatively charged [[phosphate]] groups on DNA
* [[Hydrogen bonds]] between the DNA backbone and the [[Amide#Amide linkage .28peptide bond.29|amide]] group on the main chain of histone proteins
* Nonpolar interactions between the histone and [[deoxyribose]] sugars on DNA
* Non-specific minor groove insertions of the H3 and H2B N-terminal tails into two minor grooves each on the DNA molecule

The highly basic nature of histones, aside from facilitating DNA-histone interactions, contributes to their water solubility.

Histones are subject to post translational modification by enzymes primarily on their N-terminal tails, but also in their globular domains.<ref>{{cite journal | vauthors = Mersfelder EL, Parthun MR | title = The tale beyond the tail: histone core domain modifications and the regulation of chromatin structure | journal = Nucleic Acids Research | volume = 34 | issue = 9 | pages = 2653–62 | date = 19 May 2006 | pmid = 16714444 | pmc = 1464108 | doi = 10.1093/nar/gkl338 }}</ref><ref>{{cite journal |vauthors=Tropberger P, Schneider R |date=June 2013 |title=Scratching the (lateral) surface of chromatin regulation by histone modifications |url=https://www.nature.com/articles/nsmb.2581 |journal=Nature Structural & Molecular Biology |volume=20 |issue=6 |pages=657–61 |doi=10.1038/nsmb.2581 |pmid=23739170 |s2cid=2956823|url-access=subscription }}</ref> Such modifications include [[methylation]], [[citrullination]], [[acetylation]], [[phosphorylation]], [[SUMO protein|SUMOylation]], [[ubiquitin]]ation, and [[ADP-ribosylation]]. This affects their function of gene regulation.

In general, [[gene]]s that are active have less bound histone, while inactive genes are highly associated with histones during [[interphase]].<ref>{{Cite book|title=Fundamental Molecular Biology | edition = Second | vauthors = Allison LA |publisher=John Wiley & Sons |year=2012 |isbn=9781118059814 |location=United States of America |pages=102 }}</ref>  It also appears that the structure of histones has been [[evolution]]arily conserved, as any deleterious [[mutations]] would be severely maladaptive. All histones have a highly positively charged N-terminus with many [[lysine]] and [[arginine]] residues.