Editing Centromere (section)

=== Acrocentric ===
An acrocentric chromosome's centromere is situated so that one of the chromosome arms is much shorter than the other. The "acro-" in acrocentric refers to the Greek word for "peak." The [[human genome]] has six acrocentric chromosomes, including five autosomal chromosomes ([[Chromosome 13 (human)|13]], [[Chromosome 14 (human)|14]], [[Chromosome 15 (human)|15]], [[Chromosome 21 (human)|21]], [[Chromosome 22 (human)|22]]) and the [[Y chromosome]].

Short acrocentric p-arms contain little genetic material and can be translocated without significant harm, as in a balanced [[Robertsonian translocation]]. In addition to some protein coding genes, human acrocentric p-arms also contain [[Nucleolus organizer region]]s (NORs), from which [[ribosomal RNA]] is transcribed. However, a proportion of acrocentric p-arms in cell lines and tissues from normal human donors do not contain detectable NORs.<ref>{{cite journal | vauthors = van Sluis M, van Vuuren C, Mangan H, McStay B | title = NORs on human acrocentric chromosome p-arms are active by default and can associate with nucleoli independently of rDNA | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 117 | issue = 19 | pages = 10368–10377 | date = May 2020 | pmid = 32332163 | pmc = 7229746 | doi = 10.1073/pnas.2001812117 | bibcode = 2020PNAS..11710368V | doi-access = free }}</ref> The [[Horse|domestic horse]] genome includes one metacentric chromosome that is [[Homologous chromosome|homologous]] to two acrocentric chromosomes in the [[Conspecificity|conspecific]] but undomesticated [[Przewalski's horse]]. This may reflect either fixation of a balanced Robertsonian translocation in domestic horses or, conversely, fixation of the fission of one metacentric chromosome into two acrocentric chromosomes in Przewalski's horses. A similar situation exists between the human and great ape genomes, with a reduction of two acrocentric chromosomes in the great apes to one metacentric chromosome in humans (see [[Karyotype#Aneuploidy|aneuploidy]] and the [[human chromosome 2]]).

Many diseases from the result of unbalanced translocations more frequently involve acrocentric chromosomes than other non-acrocentric chromosomes. Acrocentric chromosomes are usually located in and around the [[nucleolus]]. As a result, these chromosomes tend to be less densely packed than chromosomes in the nuclear periphery. Consistently, chromosomal regions that are less densely packed are also more prone to chromosomal translocations in cancers.