Editing Cellular differentiation (section)

=====Histone acetylation and methylation=====
DNA-nucleosome interactions are characterized by two states: either tightly bound by nucleosomes and transcriptionally inactive, called [[heterochromatin]], or loosely bound and usually, but not always, transcriptionally active, called [[euchromatin]]. The epigenetic processes of histone methylation and acetylation, and their inverses demethylation and deacetylation primarily account for these changes. The effects of acetylation and deacetylation are more predictable. An acetyl group is either added to or removed from the positively charged Lysine residues in histones by enzymes called [[histone acetyltransferase]]s or [[histone deactylase]]s, respectively. The acetyl group prevents Lysine's association with the negatively charged DNA backbone. Methylation is not as straightforward, as neither methylation nor demethylation consistently correlate with either gene activation or repression. However, certain methylations have been repeatedly shown to either activate or repress genes. The trimethylation of lysine 4 on histone 3 (H3K4Me3) is associated with gene activation, whereas trimethylation of lysine 27 on histone 3 represses genes.<ref name="pmid12667454">{{cite journal | vauthors = Krogan NJ, Dover J, Wood A, Schneider J, Heidt J, Boateng MA, Dean K, Ryan OW, Golshani A, Johnston M, Greenblatt JF, Shilatifard A | title = The Paf1 complex is required for histone H3 methylation by COMPASS and Dot1p: linking transcriptional elongation to histone methylation | journal = Molecular Cell | volume = 11 | issue = 3 | pages = 721–729 | date = March 2003 | pmid = 12667454 | doi = 10.1016/S1097-2765(03)00091-1 | doi-access = free }}</ref><ref name="pmid12667453">{{cite journal | vauthors = Ng HH, Robert F, Young RA, Struhl K | title = Targeted recruitment of Set1 histone methylase by elongating Pol II provides a localized mark and memory of recent transcriptional activity | journal = Molecular Cell | volume = 11 | issue = 3 | pages = 709–719 | date = March 2003 | pmid = 12667453 | doi = 10.1016/S1097-2765(03)00092-3 | doi-access = free }}</ref><ref name="pmid15680324">{{cite journal | vauthors = Bernstein BE, Kamal M, Lindblad-Toh K, Bekiranov S, Bailey DK, Huebert DJ, McMahon S, Karlsson EK, Kulbokas EJ, Gingeras TR, Schreiber SL, Lander ES | title = Genomic maps and comparative analysis of histone modifications in human and mouse | journal = Cell | volume = 120 | issue = 2 | pages = 169–181 | date = January 2005 | pmid = 15680324 | doi = 10.1016/j.cell.2005.01.001 | s2cid = 7193829 | doi-access = free }}</ref>