Editing Histone (section)

{{short description|Protein family around which DNA winds to form nucleosomes}}
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[[Image:Nucleosome structure.png|thumb|300px|Schematic representation of the assembly of the core histones into the nucleosome]]
In [[biology]], '''histones''' are highly [[Base (chemistry)|basic]] [[protein]]s abundant in [[lysine]] and [[arginine]] residues that are found in [[eukaryotic]] [[cell nuclei]] and in most [[Archaea|Archaeal]] [[Phylum|phyla]]. They act as spools around which [[DNA]] winds to create structural units called [[nucleosome]]s.<ref>{{cite book| vauthors = Youngson RM | title= Collins Dictionary of Human Biology| year= 2006| publisher= HarperCollins| location= Glasgow| isbn = 978-0-00-722134-9 }}</ref><ref name="Nelson&Cox">{{cite book | vauthors = Cox M, Nelson DR, Lehninger AL | title = Lehninger Principles of Biochemistry | publisher = W.H. Freeman | location = San Francisco | year = 2005 | isbn = 978-0-7167-4339-2 | url-access = registration | url = https://archive.org/details/lehningerprincip00lehn_0 }}</ref> Nucleosomes in turn are wrapped into 30-[[nanometer]] fibers that form tightly packed [[chromatin]]. Histones prevent DNA from becoming tangled and protect it from [[DNA damage]].  In addition, histones play important roles in [[gene regulation]] and [[DNA replication]]. Without histones, unwound DNA in [[chromosome]]s would be very long. For example, each human cell has about 1.8 meters of DNA if completely stretched out; however, when wound about histones, this length is reduced to about 9 micrometers (0.09&nbsp;mm) of 30&nbsp;nm diameter chromatin fibers.<ref>{{cite journal |vauthors=Redon C, Pilch D, Rogakou E, Sedelnikova O, Newrock K, Bonner W |date=April 2002 |title=Histone H2A variants H2AX and H2AZ |url=https://www.sciencedirect.com/science/article/abs/pii/S0959437X02002824 |journal=Current Opinion in Genetics & Development |volume=12 |issue=2 |pages=162–9 |doi=10.1016/S0959-437X(02)00282-4 |pmid=11893489|url-access=subscription }}</ref>

There are five families of histones, which are designated H1/H5 (linker histones), H2, H3, and H4 (core histones). The nucleosome core is formed of two H2A-H2B [[protein dimer|dimers]] and a H3-H4 [[tetrameric protein|tetramer]]. The tight wrapping of DNA around histones, is to a large degree, a result of [[electrostatic]] attraction between the positively charged histones and negatively charged phosphate backbone of DNA.

Histones may be chemically modified through the action of enzymes to regulate gene transcription. The most common modifications are the [[methylation]] of arginine or lysine residues or the [[acetylation]] of lysine. Methylation can affect how other proteins such as [[transcription factor]]s interact with the nucleosomes. Lysine acetylation eliminates a positive charge on lysine thereby weakening the electrostatic attraction between histone and DNA, resulting in partial unwinding of the DNA, making it more accessible for gene expression.