Editing Centromere (section)

== Structure ==
The centromeric DNA is normally in a [[heterochromatin]] state, which is essential for the recruitment of the [[cohesin]] complex that mediates sister chromatid cohesion after DNA replication as well as coordinating sister chromatid separation during anaphase. In this chromatin, the normal [[histone]] H3 is replaced with a centromere-specific variant, CENP-A in humans.<ref>{{cite journal | vauthors = Chueh AC, Wong LH, Wong N, Choo KH | title = Variable and hierarchical size distribution of L1-retroelement-enriched CENP-A clusters within a functional human neocentromere | journal = Human Molecular Genetics | volume = 14 | issue = 1 | pages = 85–93 | date = January 2005 | pmid = 15537667 | doi = 10.1093/hmg/ddi008 | doi-access = free }}</ref> The presence of CENP-A is believed to be important for the assembly of the kinetochore on the centromere. CENP-C has been shown to localise almost exclusively to these regions of CENP-A associated chromatin. In human cells, the histones are found to be most enriched for [[H4K20me]]3 and [[H3K9me3]]<ref name="Rosenfeld_2009">{{cite journal | vauthors = Rosenfeld JA, Wang Z, Schones DE, Zhao K, DeSalle R, Zhang MQ | title = Determination of enriched histone modifications in non-genic portions of the human genome | journal = BMC Genomics | volume = 10 | pages = 143 | date = March 2009 | pmid = 19335899 | pmc = 2667539 | doi = 10.1186/1471-2164-10-143 | doi-access = free }}</ref> which are known heterochromatic modifications. In  Drosophila, Islands of retroelements are major components of the centromeres.<ref>{{cite journal | vauthors = Chang CH, Chavan A, Palladino J, Wei X, Martins NM, Santinello B, Chen CC, Erceg J, Beliveau BJ, Wu CT, Larracuente AM, Mellone BG | display-authors = 6 | title = Islands of retroelements are major components of Drosophila centromeres | journal = PLOS Biology | volume = 17 | issue = 5 | pages = e3000241 | date = May 2019 | pmid = 31086362 | pmc = 6516634 | doi = 10.1371/journal.pbio.3000241 | doi-access = free }}</ref>

In the yeast ''[[Schizosaccharomyces pombe]]'' (and probably in other eukaryotes), the formation of centromeric heterochromatin is connected to [[RNAi]].<ref>{{cite journal | vauthors = Volpe TA, Kidner C, Hall IM, Teng G, Grewal SI, Martienssen RA | title = Regulation of heterochromatic silencing and histone H3 lysine-9 methylation by RNAi | journal = Science | volume = 297 | issue = 5588 | pages = 1833–1837 | date = September 2002 | pmid = 12193640 | doi = 10.1126/science.1074973 | s2cid = 2613813 | bibcode = 2002Sci...297.1833V | doi-access = free }}</ref> In nematodes such as ''[[Caenorhabditis elegans]]'', some plants, and the insect orders Lepidoptera and Hemiptera, chromosomes are "holocentric", indicating that there is not a primary site of microtubule attachments or a primary constriction, and a "diffuse" kinetochore assembles along the entire length of the chromosome.