Editing Centromere (section)

==Centromere types==

=== Acentric ===
An acentric chromosome is fragment of a chromosome that lacks a centromere. Since centromeres are the attachment point for spindle fibers in cell division, acentric fragments are not evenly distributed to daughter cells during cell division. As a result, a daughter cell will lack the acentric fragment and deleterious consequences could occur.

Chromosome-breaking events can also generate acentric chromosomes or acentric fragments.   

===Dicentric===

A [[dicentric chromosome]] is an abnormal chromosome with two centromeres, which can be unstable through cell divisions. It can form through translocation between or fusion of two chromosome segments, each with a centromere. Some rearrangements produce both dicentric chromosomes and acentric fragments which can not attach to spindles at mitosis.<ref name=":01">{{Cite book|title = Thompson & Thompson Genetics in Medicine| vauthors = Nussbaum R, McInnes R, Willard H, Hamosh A |first4 = Ada|publisher = Saunders|year = 2007|isbn = 978-1-4160-3080-5|location = Philadelphia(PA)|pages = 72}}</ref> The formation of dicentric chromosomes has been attributed to genetic processes, such as [[Robertsonian translocation]]<ref name=":0">{{cite book|title=Thompson & Thompson Genetics in Medicine | edition = 7th |pages=62}}</ref> and [[Chromosomal inversion|paracentric inversion.]]<ref name=":5">{{cite book|title = Genetics From Genes to Genomes | edition = 4th | vauthors = Hartwell L, Hood L, Goldberg M, Reynolds A, Lee S |publisher = McGraw-Hill|year = 2011|isbn = 9780073525266|location = New York}}</ref> Dicentric chromosomes can have a variety of fates, including mitotic stability.<ref name=":1">{{cite journal | vauthors = Lynch SA, Ashcroft KA, Zwolinski S, Clarke C, Burn J | title = Kabuki syndrome-like features in monozygotic twin boys with a pseudodicentric chromosome 13 | journal = Journal of Medical Genetics | volume = 32 | issue = 3 | pages = 227–230 | date = March 1995 | pmid = 7783176 | pmc = 1050324 | doi = 10.1136/jmg.32.3.227 }}</ref> In some cases, their stability comes from inactivation of one of the two centromeres to make a functionally monocentric chromosome capable of normal transmission to daughter cells during cell division.<ref>{{cite journal | url=https://doi.org/10.1007/s10577-012-9302-3 | doi=10.1007/s10577-012-9302-3 | title=Dicentric chromosomes: Unique models to study centromere function and inactivation | date=2012 | last1=Stimpson | first1=Kaitlin M. | last2=Matheny | first2=Justyne E. | last3=Sullivan | first3=Beth A. | journal=Chromosome Research | volume=20 | issue=5 | pages=595–605 | pmc=3557915 }}</ref>

For example, human [[chromosome 2]], which is believed to be the result of a Robertsonian translocation at some point in the evolution between the great apes and ''Homo'', has a second, vestigial, centromere near the middle of its long arm.<ref>{{cite journal
|author=Avarello
|title=Evidence for an ancestral alphoid domain on the long arm of human chromosome 2
|journal=Human Genetics
|year=1992
|pages=247–9
|volume=89
|pmid=1587535  |doi=10.1007/BF00217134
|last2=Pedicini
|first2=A
|last3=Caiulo
|first3=A
|last4=Zuffardi
|first4=O
|last5=Fraccaro
|first5=M
|issue=2
|s2cid=1441285
|display-authors=1
}}</ref>

===Monocentric===

The [[monocentric]] chromosome is a chromosome that has only one centromere in a chromosome and forms a narrow constriction.

Monocentric centromeres are the most common structure on highly repetitive DNA in plants and animals.<ref>{{cite journal | vauthors = Barra V, Fachinetti D | title = The dark side of centromeres: types, causes and consequences of structural abnormalities implicating centromeric DNA | journal = Nature Communications | volume = 9 | issue = 1 | pages = 4340 | date = October 2018 | pmid = 30337534 | pmc = 6194107 | doi = 10.1038/s41467-018-06545-y | bibcode = 2018NatCo...9.4340B }}</ref>

=== Holocentric ===
{{Main|Holocentric chromosome}}
Unlike monocentric chromosomes, holocentric chromosomes have no distinct primary constriction when viewed at mitosis. Instead, spindle fibers attach along almost the entire (Greek: holo-) length of the chromosome. In holocentric chromosomes centromeric proteins, such as [[CENPA]] (CenH3) are spread over the whole chromosome.<ref name="mono">{{cite journal | vauthors = Neumann P, Navrátilová A, Schroeder-Reiter E, Koblížková A, Steinbauerová V, Chocholová E, Novák P, Wanner G, Macas J | display-authors = 6 | title = Stretching the rules: monocentric chromosomes with multiple centromere domains | journal = PLOS Genetics | volume = 8 | issue = 6 | pages = e1002777 | year = 2012 | pmid = 22737088 | pmc = 3380829 | doi = 10.1371/journal.pgen.1002777 | doi-access = free }}</ref> The nematode, [[Caenorhabditis elegans]], is a well-known example of an organism with holocentric chromosomes,<ref>{{cite journal | vauthors = Dernburg AF | title = Here, there, and everywhere: kinetochore function on holocentric chromosomes | journal = The Journal of Cell Biology | volume = 153 | issue = 6 | pages = F33–F38 | date = June 2001 | pmid = 11402076 | pmc = 2192025 | doi = 10.1083/jcb.153.6.F33 }}</ref> but this type of centromere can be found in various species, plants, and animals, across eukaryotes. Holocentromeres are actually composed of multiple distributed centromere units that form a line-like structure along the chromosomes during mitosis.<ref>{{cite journal | vauthors = Marques A, Ribeiro T, Neumann P, Macas J, Novák P, Schubert V, Pellino M, Fuchs J, Ma W, Kuhlmann M, Brandt R, Vanzela AL, Beseda T, Šimková H, Pedrosa-Harand A, Houben A | display-authors = 6 | title = Holocentromeres in Rhynchospora are associated with genome-wide centromere-specific repeat arrays interspersed among euchromatin | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 44 | pages = 13633–13638 | date = November 2015 | pmid = 26489653 | pmc = 4640781 | doi = 10.1073/pnas.1512255112 | bibcode = 2015PNAS..11213633M | doi-access = free }}</ref> Alternative or nonconventional strategies are deployed at meiosis to achieve the homologous chromosome pairing and segregation needed to produce viable gametes or gametophytes for sexual reproduction.

Different types of holocentromeres exist in different species, namely with or without centromeric repetitive DNA sequences and with or without [[CENPA|CenH3]]. Holocentricity has evolved at least 13 times independently in various green algae, protozoans, invertebrates, and different plant families.<ref>{{cite journal | vauthors = Melters DP, Paliulis LV, Korf IF, Chan SW | title = Holocentric chromosomes: convergent evolution, meiotic adaptations, and genomic analysis | journal = Chromosome Research | volume = 20 | issue = 5 | pages = 579–593 | date = July 2012 | pmid = 22766638 | doi = 10.1007/s10577-012-9292-1 | s2cid = 3351527 | doi-access = free }}</ref> Contrary to monocentric species where acentric fragments usually become lost during cell division, the breakage of holocentric chromosomes creates fragments with normal spindle fiber attachment sites.<ref>{{Cite journal | author1 = Hughes-Schrader S| author-link1 =Sally Hughes-Schrader| author2 =Ris H |date=August 1941 |title=The diffuse spindle attachment of coccids, verified by the mitotic behavior of induced chromosome fragments |url=https://onlinelibrary.wiley.com/doi/10.1002/jez.1400870306 |journal=Journal of Experimental Zoology |language=en |volume=87 |issue=3 |pages=429–456 |doi=10.1002/jez.1400870306 |issn=0022-104X}}</ref> Because of this, organisms with holocentric chromosomes can more rapidly evolve karyotype variation, able to heal fragmented chromosomes through subsequent addition of telomere caps at the sites of breakage.<ref>{{cite journal | vauthors = Jankowska M, Fuchs J, Klocke E, Fojtová M, Polanská P, Fajkus J, Schubert V, Houben A | display-authors = 6 | title = Holokinetic centromeres and efficient telomere healing enable rapid karyotype evolution | journal = Chromosoma | volume = 124 | issue = 4 | pages = 519–528 | date = December 2015 | pmid = 26062516 | doi = 10.1007/s00412-015-0524-y | s2cid = 2530401 }}</ref>

===Polycentric===
{{Main|Polycentric chromosome}}
Polycentric chromosomes have several kinetochore clusters, i.e. centromes. The term overlaps partially with "holocentric", but "polycentric" is clearly preferred when discussing defectively formed monocentric chromosomes. There is some actual ambiguity as well, as there is no clear line dividing up the transition from kinetochores covering the whole chromosome to distinct clusters. In other words, the difference between "the whole chromosome is a centrome" and "the chromosome has no centrome" is hazy and usage varies. Beyond "polycentricity" being used more about defects, there is no clear preference in other topics such as evolutionary origin or kinetochore distribution and detailed structure (e.g. as seen in [[Fluorescent tag|tagging]] or [[Sequence assembly|genome assembly]] analysis).<ref>{{cite journal | vauthors = Godward MB | date= April 1954 |title=The 'Diffuse' Centromere or Polycentric Chromosomes in Spirogyra |journal=Annals of Botany |volume=18 |issue=2 |pages=143–144 |doi=10.1093/oxfordjournals.aob.a083387 }}</ref><ref name="holocentric_micro">{{cite journal |last1=Kuo |first1=YT |last2=Camara |first2=AS |last3=Schubert |first3=V |title=Holocentromeres can consist of merely a few megabase-sized satellite arrays |journal=Nature Communications |date=2023-06-13 |volume=14 |doi=10.1038/s41467-023-38922-7 |url=https://www.nature.com/articles/s41467-023-38922-7|pmc=10264360 }}</ref><ref name="holocentric_evo">{{cite journal |last1=Senaratne |first1=Aruni P. |last2=Cortes-Silva |first2=Nuria |last3=Drinnenberg |first3=Ines A. |title=Evolution of holocentric chromosomes: Drivers, diversity, and deterrents |journal=Seminars in Cell & Developmental Biology |date=July 2022 |volume=127 |pages=90–99 |doi=10.1016/j.semcdb.2022.01.003 |url=https://www.sciencedirect.com/science/article/pii/S1084952122000052 |access-date=2024-09-11}}</ref><ref name="polycentric_2">{{cite journal |last1=Ma B, Wang H, Liu J, Chen L, Xia X, Wei W, Yang Z, Yuan J, Luo Y, He N. |title=The gap-free genome of mulberry elucidates the architecture and evolution of polycentric chromosomes |journal=Horticulture Research |date=2023-05-31 |volume=10 |issue=7 |doi=10.1093/hr/uhad111 |url=https://pubmed.ncbi.nlm.nih.gov/37786730/ |access-date=2024-09-11|pmc=10541557 }}</ref>

Even clearly distinct clusters of kinetochore proteins do not necessarily produce more than one constriction: "Metapolycentric" chromosomes feature one elongated constriction of the chromosome, joining a longer segment which is still visibly shorter than the chromatids.<ref name="centromere_composition">{{cite journal |last1=Ishii |first1=Midori |last2=Akiyoshi |first2=Bungo |title=Plasticity in centromere organization and kinetochore composition: Lessons from diversity |journal=Current Opinion in Cell Biology |date=February 2022 |volume=74 |issue=Cell Nucleus 2022 |pages=47–54 |doi=10.1016/j.ceb.2021.12.007 |url=https://www.sciencedirect.com/science/article/pii/S0955067422000011#fig3 |access-date=2024-09-11|hdl=20.500.11820/4aab4486-f439-4d7e-863a-b08e0da52a6b |hdl-access=free }}</ref> Metapolycentric chromosomes may be a step in the emergence and suppression of centromere drive, a type of [[meiotic drive]] that disrupts parity by monocentric centromeres growing additional kinetochore proteins to gain an advantage during meiosis.<ref name="centromere_drive">{{cite journal |last1=Zedek |first1=Frantisek |last2=Bures |first2=Petr |title=Absence of positive selection on CenH3 in Luzula suggests that holokinetic chromosomes may suppress centromere drive |journal=Annals of Botany |date=December 2016 |volume=118 |pages=1347–1352 |doi=10.1093/aob/mcw186 |url=https://www.researchgate.net/publication/306091873_Absence_of_positive_selection_on_CenH3_in_Luzula_suggests_that_holokinetic_chromosomes_may_suppress_centromere_drive#read |access-date=2024-09-11|pmc=5155603 }}</ref>

=== Human chromosomes ===
[[File:Human karyotype with bands and sub-bands.png|thumb|Human [[karyotype|karyogram]], with each row vertically aligned at centromere level, and with annotated [[Locus (genetics)|bands and sub-bands]]. It is a graphical representation of the idealized human [[diploid]] karyotype. It shows dark and white regions on [[G banding]]. It shows both the female (XX) and male (XY) versions of the [[sex chromosome]]. {{further|Karyotype}}]]
{| class="wikitable sortable" style="text-align: center;"
|+ Table of human chromosomes with data on centromeres and sizes.
|-
! Chromosome !! Centromere <br /> position ([[Base pair|Mbp]]) !! Category !! Chromosome <br /> Size (Mbp) !! Centromere <br /> size (Mbp)
|-
| [[Chromosome 1 (human)|1]] || 125.0 || metacentric || 247.2 || 7.4
|-
| [[Chromosome 2 (human)|2]] || 93.3 || submetacentric || 242.8 || 6.3
|-
| [[Chromosome 3 (human)|3]] || 91.0 || metacentric || 199.4 || 6.0
|-
| [[Chromosome 4 (human)|4]] || 50.4 || submetacentric || 191.3 || —
|-
| [[Chromosome 5 (human)|5]] || 48.4 || submetacentric || 180.8 || —
|-
| [[Chromosome 6 (human)|6]] || 61.0 || submetacentric || 170.9 || —
|-
| [[Chromosome 7 (human)|7]] || 59.9 || submetacentric || 158.8 || —
|-
| [[Chromosome 8 (human)|8]] || 45.6 || submetacentric || 146.3 || —
|-
| [[Chromosome 9 (human)|9]] || 49.0 || submetacentric || 140.4 || —
|-
| [[Chromosome 10 (human)|10]] || 40.2 || submetacentric || 135.4 || —
|-
| [[Chromosome 11 (human)|11]] || 53.7 || submetacentric || 134.5 || —
|-
| [[Chromosome 12 (human)|12]] || 35.8 || submetacentric || 132.3 || —
|-
| [[Chromosome 13 (human)|13]] || 17.9 || acrocentric || 114.1 || —
|-
| [[Chromosome 14 (human)|14]] || 17.6 || acrocentric || 106.3 || —
|-
| [[Chromosome 15 (human)|15]] || 19.0 || acrocentric || 100.3 || —
|-
| [[Chromosome 16 (human)|16]] || 36.6 || metacentric || 88.8 || —
|-
| [[Chromosome 17 (human)|17]] || 24.0 || submetacentric || 78.7 || —
|-
| [[Chromosome 18 (human)|18]] || 17.2 || submetacentric || 76.1 || —
|-
| [[Chromosome 19 (human)|19]] || 26.5 || metacentric || 63.8 || —
|-
| [[Chromosome 20 (human)|20]] || 27.5 || metacentric || 62.4 || —
|-
| [[Chromosome 21 (human)|21]] || 13.2 || acrocentric || 46.9 || —
|-
| [[Chromosome 22 (human)|22]] || 14.7 || acrocentric || 49.5 || —
|-
| [[X chromosome|X]] || 60.6 || submetacentric || 154.9 || —
|-
| [[Y chromosome|Y]] || 12.5 || acrocentric || 57.7 || —
|-
|}
Based on the micrographic characteristics of size, position of the centromere and sometimes the presence of a [[chromosomal satellite]], the human chromosomes are classified into the following groups:<ref>{{cite journal|author=Erwinsyah, R., Riandi, & Nurjhani, M.|year=2017|title=Relevance of human chromosome analysis activities against mutation concept in genetics course. IOP Conference Series.|journal=Materials Science and Engineering|doi=10.1088/1757-899x/180/1/012285|s2cid=90739754 |doi-access=free}}</ref>
{|class=wikitable
! Group
! Chromosomes
! Features
|-
| Group A
| Chromosome 1–3
| Large, metacentric and submetacentric
|-
| Group B
| Chromosome 4–5
| Large, submetacentric
|-
| Group C
| Chromosome 6–12, X
| Medium-sized, submetacentric
|-
| Group D
| Chromosome 13–15
| Medium-sized, acrocentric, with [[satellite chromosome|satellite]]
|-
| Group E
| Chromosome 16–18
| Small, metacentric and submetacentric
|-
| Group F
| Chromosome 19–20
| Very small, metacentric
|-
| Group G
| Chromosome 21–22, Y
| Very small, acrocentric, with [[satellite chromosome|satellite]]
|}