Histone: Difference between revisions

Histones are highly <scene name='Taylor_histone_sandbox/Conservation/1'>conserved proteins</scene> (more purple = more conserved) with <scene name='Taylor_histone_sandbox/Charge_distribution/1'>positive charge</scene> (blue is positive charge, red is negative charge).  Because of this positive charge, they interact electrostatically with the negatively charged phosphate groups in DNA.   

There are five major classes of histones: H1/H5, H2A, H2B, H3, and H4.<ref name="Bhasin_2006">{{cite journal | author = Bhasin M, Reinherz EL, Reche PA | title = Recognition and classification of histones using support vector machine | journal = J. Comput. Biol. | volume = 13 | issue = 1 | pages = 102–12 | year = 2006 | pmid = 16472024 | doi = 10.1089/cmb.2006.13.102 | url =  }}</ref><ref name="Voet, Voet, and Pratt">{{Cite book|surname1= Voet|given1= Donald |surname2= Voet|given2= Judith|surname3= Pratt|given3= Leon A.|  year=1988|title=Basic Genetics|publication-place=Boston|publisher=Jones and Bartlett Publishers|isbn=0-86720-090-1}}</ref> Histones <scene name='46/468228/2a/3'>H2A</scene>, <scene name='46/468228/2b/4'>H2B</scene>, <scene name='46/468228/3/3'>H3</scene>, and <scene name='46/468228/H4/1'>H4</scene> are known as the core histones, while histones H1 and H5 are known as the linker histones.

Histones are the chief protein components of <scene name='46/468228/Nucleosome/1'>chromatin</scene>, acting as spools around which DNA winds, and play a role in gene regulation. Without histones, the unwound DNA in chromosomes would be very long;  each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 micrometers (0.09&nbsp;mm) of chromatin, which, when duplicated and condensed during mitosis, result in about 120 micrometers of chromosomes.<ref name="pmid11893489">{{cite journal | author = Redon C, Pilch D, Rogakou E, Sedelnikova O, Newrock K, Bonner W | title = Histone H2A variants H2AX and H2AZ | journal = Curr. Opin. Genet. Dev. | volume = 12 | issue = 2 | pages = 162–9 | year = 2002 | month = April | pmid = 11893489 | doi = 10.1016/S0959-437X(02)00282-4 | url =  }}</ref> DNA is wrapped around nucleosomes with approximately 50 base pairs of DNA between subsequent nucleosomes (also referred to as linker DNA). The assembled histones and DNA is called chromatin. During mitosis and meiosis, the condensed chromosomes are assembled through interactions between nucleosomes and other regulatory proteins.

The nucleosome core is formed of two <scene name='46/468228/H2a_h2b_dimer/1'>H2A-H2B dimers</scene> and a <scene name='46/468228/H4_h3_tetramer/1'>H3-H4 tetramer</scene>, forming two nearly <scene name='46/468228/Nucleosome_dimer/1'>symmetrical halves</scene> by tertiary structure.<ref name=pmid9305837/>  147 base pairs of <scene name='46/468228/Dna_wrap_around_histone/1'>DNA wrap</scene> around this core particle 1.65 times in a left-handed super-helical turn.<ref name=pmid9305837>{{cite journal | author = Luger K, Mäder AW, Richmond RK, Sargent DF, Richmond TJ | title = Crystal structure of the nucleosome core particle at 2.8 A resolution | journal = Nature | volume = 389 | issue = 6648 | pages = 251–60 | year = 1997 | month = September | pmid = 9305837 | doi = 10.1038/38444 | url =  }} {{PDB|1AOI}}</ref>  The linker histone H1 binds the nucleosome and the entry and exit sites of the DNA, thus locking the DNA into place<ref name="isbn0-915274-84-1">{{cite book |author=Farkas, Daniel |title=DNA simplified: the hitchhiker's guide to DNA |publisher=AACC Press |location=Washington, D.C |year=1996 |isbn=0-915274-84-1 }}</ref> and allowing the formation of higher order structure.   

Histones are subject to post translational modification by enzymes primarily on their N-terminal tails, but also in their globular domains.  Such modifications include methylation, acetylation, phosphorylation, SUMOylation, ubiquitination, and ADP-ribosylation. This affects gene expression. The core of the histones H2A, H2B, and H3 can also be modified. Combinations of modifications are thought to constitute a code, the so-called "histone code".<ref name=pmid10638745>{{cite journal |author=Strahl BD, Allis CD |title=The language of covalent histone modifications |journal=Nature |volume=403 |issue=6765 |pages=41–5 |date=Jan 2000 |pmid=10638745 |doi=10.1038/47412}}</ref><ref name=pmid11498575>{{cite journal |author=Jenuwein T, Allis CD |title=Translating the histone code |journal=Science |volume=293 |issue=5532 |pages=1074–80 |date=Aug 2001 |pmid=11498575 |doi=10.1126/science.1063127}}</ref> Histone modifications act in diverse biological processes such as gene regulation, DNA repair, chromosome condensation (in mitosis, spermatogenesis, and meiosis).<ref>{{cite journal |author=Ning Song, Jie Liu, Shucai An, Tomoya Nishino, Yoshitaka Hishikawa and Takehiko Koji |year=2011 |title= Immunohistochemical Analysis of Histone H3 Modifications in Germ Cells during Mouse Spermatogenesis |journal=Acta Histochemica et Cytochemica |volume=44 |issue=4 |doi=10.1267/ahc.11027 |url=http://www.jstage.jst.go.jp/article/ahc/advpub/0/advpub_1107140114/_article |pages=183–90 |pmid=21927517 |pmc=3168764}}</ref>