Editing Centromere (section)

== Position ==
[[File:Centromere Placement.svg|thumb|'''Classifications of Chromosomes'''<br />
{| class="wikitable sortable" style="text-align: left;"
|-
| '''I''' || Telocentric || Centromere placement very close to the top, p arms barely visible if visible at all.
|-
| '''II''' || Acrocentric || q arms are still much longer than the p arms, but the p arms are longer than those in telocentric.
|-
| '''III''' || Submetacentric || p and q arms are very close in length but not equal.
|-
| '''IV''' || Metacentric || p and q arms are equal in length.
|-
|}
'''A''': Short arm (p arm)<br />'''B''': Centromere<br />'''C''': Long arm (q arm)<br />'''D''': Sister Chromatids
]]
<!-- Subsections of this section are linked in [[Lund's Amphibious Rat]]-->
In humans, centromere positions define the chromosomal [[karyotype]], in which each chromosome has two arms, ''p'' (the shorter of the two) and ''q'' (the longer). The short arm 'p' is reportedly named for the French word "petit" meaning 'small'.<ref>{{cite web|url= http://thednaexchange.com/2011/05/02/p-q-solved-being-the-true-story-of-how-the-chromosome-got-its-name/|title= p + q = Solved, Being the True Story of How the Chromosome Got Its Name|date = 2011-05-03}}</ref> The position of the centromere relative to any particular linear chromosome is used to classify chromosomes as  metacentric, submetacentric, acrocentric, telocentric, or holocentric.<ref>{{Citation| work = Nikolay's Genetics Lessons|title=What different types of chromosomes exist?|date=2013-10-12|url=https://www.youtube.com/watch?v=0bfpOhbKEAk| archive-url=https://ghostarchive.org/varchive/youtube/20211211/0bfpOhbKEAk| archive-date=2021-12-11 | url-status=live|access-date=2017-05-28 | publisher = YouTube }}{{cbignore}}</ref><ref name="Levan A. 1964">{{cite journal | vauthors = Levan A, Fredga K, Sandberg AA | title = Nomenclature for centromeric position on chromosomes. | journal = Hereditas | date = December 1964 | volume = 52 | issue = 2 | pages = 201–220 | doi = 10.1111/j.1601-5223.1964.tb01953.x | doi-access =  }}</ref>

{| class="wikitable " style="font-size:95%;"
|+ Categorization of chromosomes according to the relative arms length<ref name="Levan A. 1964"/>
|-
! Centromere position
! Arms length ratio
! Sign
! Description
|-
|'''Medial ''sensu stricto'' '''
|1.0 – 1.6
|'''M'''
|'''[[#Metacentric|Metacentric]]'''
|-
|'''Medial region'''
|1.7
|'''m'''
|'''[[#Metacentric|Metacentric]]'''
|-
|'''Submedial'''
|3.0
|'''sm'''
|'''[[Submetacentric]]'''
|-
|'''Subterminal'''
|3.1 – 6.9
|'''st'''
|'''[[telocentric|Subtelocentric]]'''
|-
|'''Terminal region'''
|7.0
|'''t'''
|'''[[Acrocentric]]'''
|-
|'''Terminal ''sensu stricto'' '''
|'''''∞'''''
|'''T'''
|'''[[Telocentric]]'''
|-
|'''Notes'''
|'''''–'''''
|'''[[#Metacentric|Metacentric]]''': '''M'''+'''m'''
|'''[[Telocentric|Atelocentric]]''': '''M'''+'''m'''+'''sm'''+'''st'''+'''t'''
|-
|}

=== Metacentric ===
Metacentric means that the centromere is positioned midway between the chromosome ends, resulting in the arms being approximately equal in length. When the centromeres are metacentric, the chromosomes appear to be "x-shaped."

=== Submetacentric ===
Submetacentric means that the centromere is positioned below the middle, with one chromosome arm shorter than the other, often resulting in an L shape.

=== 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.

=== Telocentric ===
Telocentric chromosomes have a centromere at one end of the chromosome and therefore exhibit only one arm at the cytological (microscopic) level. They are not present in humans but can form through cellular chromosomal errors. Telocentric chromosomes occur naturally in many species, such as the [[house mouse]], in which all chromosomes except the Y are telocentric.

=== Subtelocentric ===
Subtelocentric chromosomes' centromeres are located between the middle and the end of the chromosomes, but reside closer to the end of the chromosomes.