Editing Chromosome

{{short description|DNA molecule containing genetic material of a cell}}
{{about|the DNA molecule|the genetic algorithm|Chromosome (genetic algorithm)}}
{{pp-pc}}
{{Use dmy dates|date=November 2024}}
{{Chromosome}}
[[File:Condenced chromosome.jpg|thumb|212x212px|Condensed chromosome (purple rod) inside a bone marrow erythrokaryocyte undergoing mitosis]]
{{Genetics sidebar}}
[[File:Chromosome.svg|thumb|upright=0.9|Diagram of a replicated and condensed [[metaphase]] eukaryotic chromosome: {{Ordered list |list_style_type=decimal |[[Chromatid]] |[[Centromere]] |Short arm |Long arm }}]]

A '''chromosome''' is a [[Nuclear organization|package]] of [[DNA]] containing part or all of the [[Genome|genetic material]] of an [[organism]]. In most chromosomes, the very long thin DNA fibers are coated with [[nucleosome]]-forming packaging [[protein]]s; in [[eukaryotic]] cells, the most important of these proteins are the [[histone]]s. Aided by [[Chaperone (protein)|chaperone proteins]], the histones bind to and [[DNA condensation|condense]] the DNA molecule to maintain its integrity.<ref name="Hammond-2017">{{cite journal | vauthors = Hammond CM, Strømme CB, Huang H, Patel DJ, Groth A | title = Histone chaperone networks shaping chromatin function | journal = Nature Reviews. Molecular Cell Biology | volume = 18 | issue = 3 | pages = 141–158 | date = March 2017 | pmid = 28053344 | pmc = 5319910 | doi = 10.1038/nrm.2016.159 }}</ref><ref>{{cite book | last = Wilson | first = John | title = Molecular biology of the cell : a problems approach | publisher = Garland Science | location = New York | year = 2002 | isbn = 978-0-8153-3577-1 | url-access = registration | url = https://archive.org/details/molecularbiolog000wils }}</ref> These eukaryotic chromosomes display a complex [[Three-dimensional space|three-dimensional structure]] that has a significant role in [[transcriptional regulation]].<ref>{{Cite journal|last1=Bonev|first1=Boyan|last2=Cavalli|first2=Giacomo|date=14 October 2016|title=Organization and function of the 3D genome|journal=Nature Reviews Genetics|volume=17|issue=11|pages=661–678|doi=10.1038/nrg.2016.112|pmid=27739532|hdl=2027.42/151884|s2cid=31259189|hdl-access=free}}</ref>

Normally, chromosomes are visible under a [[light microscope]] only during the [[metaphase]] of [[cell division]], where all chromosomes are aligned in the center of the cell in their condensed form.<ref>{{cite book|last1=Alberts|first1=Bruce|last2=Bray|first2=Dennis|last3=Hopkin|first3=Karen|last4=Johnson|first4=Alexander|last5=Lewis|first5=Julian|last6=Raff|first6=Martin|last7=Roberts|first7=Keith|last8=Walter|first8=Peter | name-list-style = vanc |title=Essential Cell Biology|year=2014|publisher=Garland Science|location=New York, New York, US|isbn=978-0-8153-4454-4|pages=621–626|edition=Fourth}}</ref> Before this stage occurs, each chromosome is duplicated ([[S phase]]), and the two copies are joined by a [[centromere]]—resulting in either an X-shaped structure if the centromere is located equatorially, or a two-armed structure if the centromere is located distally; the joined copies are called '[[sister chromatids]]'. During [[metaphase]], the duplicated structure (called a 'metaphase chromosome') is highly condensed and thus easiest to distinguish and study.<ref name="Schleyden-1847">{{Cite book|url=http://vlp.mpiwg-berlin.mpg.de/library/data/lit28715?|title=Microscopical researches into the accordance in the structure and growth of animals and plants|last=Schleyden|first=M. J.|year=1847|publisher=Printed for the Sydenham Society}}</ref> In animal cells, chromosomes reach their highest compaction level in [[anaphase]] during [[chromosome segregation]].<ref>{{cite journal | vauthors = Antonin W, Neumann H | title = Chromosome condensation and decondensation during mitosis | journal = Current Opinion in Cell Biology | volume = 40 | pages = 15–22 | date = June 2016 | pmid = 26895139 | doi = 10.1016/j.ceb.2016.01.013 | doi-access = free | url = https://publications.goettingen-research-online.de/bitstream/2/40465/2/1-s2.0-S0955067416300059-main.pdf }}</ref>

Chromosomal [[Genetic recombination|recombination]] during [[meiosis]] and subsequent [[sexual reproduction]] plays a crucial role in [[genetic diversity]]. If these structures are manipulated incorrectly, through processes known as [[chromosomal instability]] and [[Chromosomal translocation|translocation]], the cell may undergo [[mitotic catastrophe]]. This will usually cause the cell to initiate [[apoptosis]], leading to its own [[Cell death|death]], but the process is occasionally hampered by cell mutations that result in the progression of [[cancer]].

The term 'chromosome' is sometimes used in a wider sense to refer to the individualized portions of [[chromatin]] in cells, which may or may not be visible under light microscopy. In a narrower sense, 'chromosome' can be used to refer to the individualized portions of chromatin during cell division, which are visible under light microscopy due to high condensation.

== Etymology ==

The word ''chromosome'' ({{IPAc-en|ˈ|k|r|oʊ|m|ə|ˌ|s|oʊ|m|,_|-|ˌ|z|oʊ|m}})<ref>{{cite encyclopedia |last=Jones |first=Daniel |author-link=Daniel Jones (phonetician) |title=[[English Pronouncing Dictionary]] |editor=Peter Roach |editor2=James Hartmann |editor3=Jane Setter |place=Cambridge |publisher=[[Cambridge University Press]] |orig-year=1917 |year=2003 |isbn=978-3-12-539683-8}}</ref><ref>{{cite Merriam-Webster|chromosome}}</ref> comes from the [[Ancient Greek]] words {{wikt-lang|grc|χρῶμα}} (''{{grc-transl|χρῶμα}}'', "colour") and {{wikt-lang|grc|σῶμα}} (''{{grc-transl|σῶμα}}'', "body"), describing the strong [[stain]]ing produced by particular [[dye]]s.<ref>{{cite book |title=Biological Stains: A Handbook on the Nature and Uses of the Dyes Employed in the Biological Laboratory |last=Coxx|first=H. J. |publisher=Commission on Standardization of Biological Stains |year=1925 |url=https://archive.org/stream/biologicalstains00conn/biologicalstains00conn_djvu.txt}}</ref> The term was coined by the German anatomist [[Heinrich Wilhelm Waldeyer]],<ref>{{cite journal | vauthors = Waldeyer-Hartz | year = 1888 | title = Über Karyokinese und ihre Beziehungen zu den Befruchtungsvorgängen | journal = Archiv für Mikroskopische Anatomie und Entwicklungsmechanik | volume = 32 | page = 27 }}</ref> referring to the term '[[chromatin]]', which was introduced by [[Walther Flemming]].

Some of the early [[Karyotype|karyological]] terms have become outdated.<ref>{{cite journal | last1 = Garbari | first1 = Fabio | last2 = Bedini | first2 = Gianni | last3 = Peruzzi | first3 = Lorenzo | name-list-style = vanc | year = 2012 | title = Chromosome numbers of the Italian flora. From the Caryologia foundation to present | journal = Caryologia – International Journal of Cytology, Cytosystematics and Cytogenetics | volume = 65 | issue = 1 | pages = 65–66 | doi = 10.1080/00087114.2012.678090 | s2cid = 83748967 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Peruzzi L, Garbari F, Bedini G | year = 2012 | title = New trends in plant cytogenetics and cytoembryology: Dedicated to the memory of Emilio Battaglia | journal = Plant Biosystems| volume = 146 | issue = 3 | pages = 674–675 | doi = 10.1080/11263504.2012.712553 | bibcode = 2012PBios.146..674P | s2cid = 83749502 | url=https://www.tandfonline.com/doi/abs/10.1080/11263504.2012.712553| url-access = subscription }}</ref> For example, 'chromatin' (Flemming 1880) and 'chromosom' (Waldeyer 1888) both ascribe color to a non-colored state.<ref>{{cite journal | last = Battaglia | first = Emilio | year = 2009 | title = Caryoneme alternative to chromosome and a new caryological nomenclature | journal = Caryologia – International Journal of Cytology, Cytosystematics | volume = 62 | issue = 4 | pages = 1–80 | url = http://www.caryologia.unifi.it/past_volumes/62_4supplement/62-4_supplement.pdf | access-date = 6 November 2017 }}</ref>

== History of discovery ==
{{multiple image
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| image2 = Theodor Boveri.jpg
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| footer = [[Walter Sutton]] (left) and [[Theodor Boveri]] (right) independently developed the chromosome theory of inheritance in 1902.
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[[Otto Bütschli]] was the first scientist to recognize the structures now known as chromosomes.<ref>{{cite journal | vauthors = Fokin SI | year = 2013 | title = Otto Bütschli (1848–1920) Where we will genuflect? | url=https://www.zin.ru/journals/protistology/num8_1/fokin_protistology_8-1.pdf | journal = Protistology | volume = 8 | issue = 1 | pages = 22–35 | url-status = dead |archive-url = https://web.archive.org/web/20210421055737/https://www.zin.ru/journals/protistology/num8_1/fokin_protistology_8-1.pdf |archive-date = 21 April 2021}}</ref>

In a series of experiments beginning in the mid-1880s, [[Theodor Boveri]] gave definitive contributions to elucidating that chromosomes are the [[Vector (molecular biology)|vectors]] of [[heredity]], with two notions that became known as 'chromosome continuity' and 'chromosome individuality'.<ref>{{Cite journal|last=Maderspacher|first=Florian|year=2008|title=Theodor Boveri and the natural experiment|journal=Current Biology|volume=18|issue=7|pages=R279–R286|doi=10.1016/j.cub.2008.02.061|pmid=18397731|s2cid=15479331|doi-access=free|bibcode=2008CBio...18.R279M }}</ref>

[[Wilhelm Roux]] suggested that every chromosome carries a different [[Genetic load|genetic configuration]], and Boveri was able to test and confirm this hypothesis. Aided by the rediscovery at the start of the 1900s of [[Gregor Mendel]]'s earlier experimental work, Boveri identified the connection between the rules of inheritance and the behaviour of the chromosomes. Two generations of American [[cytologist]]s were influenced by Boveri: [[Edmund Beecher Wilson]], [[Nettie Stevens]], [[Walter Sutton]] and [[Theophilus Painter]] (Wilson, Stevens, and Painter actually worked with him).<ref>{{cite book | last = Carlson | first = Elof A. | title = Mendel's Legacy: The Origin of Classical Genetics | location = Cold Spring Harbor, NY | publisher = Cold Spring Harbor Laboratory Press | pages = 88 | year = 2004 | isbn = 978-087969675-7 | url = http://www.cshlpress.com/pdf/sample/mendel7.pdf }}</ref>

In his famous textbook, ''The Cell in Development and Heredity'', Wilson linked together the independent work of Boveri and Sutton (both around 1902) by naming the chromosome theory of inheritance the '[[Boveri–Sutton chromosome theory]]' (sometimes known as the 'Sutton–Boveri chromosome theory').<ref>Wilson, E.B. (1925). ''The Cell in Development and Heredity'', Ed. 3. Macmillan, New York. p. 923.</ref> [[Ernst Mayr]] remarks that the theory was hotly contested by some famous geneticists, including [[William Bateson]], [[Wilhelm Johannsen]], [[Richard Goldschmidt]] and [[T.H. Morgan]], all of a rather dogmatic mindset. Eventually, absolute proof came from chromosome maps in Morgan's own laboratory.<ref>Mayr, E. (1982). ''The growth of biological thought''. Harvard. p. 749. {{ISBN|9780674364462}}</ref>

The number of human chromosomes was published by Painter in 1923. By inspection through a microscope, he counted 24 pairs of chromosomes, giving 48 in total. His error was copied by others, and it was not until 1956 that the true number (46) was determined by Indonesian-born [[cytogeneticist]] [[Joe Hin Tjio]].<ref>{{Cite journal|last=Gartler|first=Stanley M.|date=1 August 2006|title=The chromosome number in humans: a brief history|journal=Nature Reviews Genetics|volume=7|issue=8 |pages=655–660|doi=10.1038/nrg1917|pmid=16847465 |s2cid=21365693 }}</ref>

== Prokaryotes ==
{{Main|Nucleoid}}

The [[prokaryote]]s&nbsp;– [[bacteria]] and [[archaea]]&nbsp;– typically have a single [[circular chromosome]].<ref>{{cite journal | vauthors = Thanbichler M, Shapiro L | title = Chromosome organization and segregation in bacteria | journal = Journal of Structural Biology | volume = 156 | issue = 2 | pages = 292–303 | date = November 2006 | pmid = 16860572 | doi = 10.1016/j.jsb.2006.05.007 }}</ref> The chromosomes of most bacteria (also called [[genophore]]s), can range in size from only 130,000 [[base pair]]s in the [[endosymbiotic]] bacteria ''[[Candidatus Hodgkinia cicadicola]]''<ref name="VanLeuven-2014">{{cite journal | vauthors = Van Leuven JT, Meister RC, Simon C, McCutcheon JP | title = Sympatric speciation in a bacterial endosymbiont results in two genomes with the functionality of one | journal = Cell | volume = 158 | issue = 6 | pages = 1270–1280 | date = September 2014 | pmid = 25175626 | doi = 10.1016/j.cell.2014.07.047 | s2cid = 11839535 | doi-access = free }}</ref> and ''[[Tremblaya princeps|Candidatus Tremblaya princeps]]'',<ref>{{cite journal | vauthors = McCutcheon JP, von Dohlen CD | title = An interdependent metabolic patchwork in the nested symbiosis of mealybugs | journal = Current Biology | volume = 21 | issue = 16 | pages = 1366–72 | date = August 2011 | pmid = 21835622 | pmc = 3169327 | doi = 10.1016/j.cub.2011.06.051 | bibcode = 2011CBio...21.1366M }}</ref> to more than 14,000,000 base pairs in the soil-dwelling bacterium ''[[Sorangium cellulosum]]''.<ref>{{cite journal | vauthors = Han K, Li ZF, Peng R, Zhu LP, Zhou T, Wang LG, Li SG, Zhang XB, Hu W, Wu ZH, Qin N, Li YZ | title = Extraordinary expansion of a Sorangium cellulosum genome from an alkaline milieu | journal = Scientific Reports | volume = 3 | pages = 2101 | year = 2013 | pmid = 23812535 | pmc = 3696898 | doi = 10.1038/srep02101 | bibcode = 2013NatSR...3.2101H }}</ref>

Some bacteria have more than one chromosome. For instance, [[Spirochaete]]s such as ''[[Borrelia burgdorferi]]'' (causing [[Lyme disease]]), contain a single ''linear'' chromosome.<ref>{{cite journal | vauthors = Hinnebusch J, Tilly K | title = Linear plasmids and chromosomes in bacteria | journal = Molecular Microbiology | volume = 10 | issue = 5 | pages = 917–22 | date = December 1993 | pmid = 7934868 | doi = 10.1111/j.1365-2958.1993.tb00963.x | s2cid = 23852021 | url = https://zenodo.org/record/1230611 }}</ref> ''[[Vibrio]]s'' typically carry two chromosomes of very different size. Genomes of the genus ''[[Burkholderia]]'' carry one, two, or three chromosomes.<ref>{{Cite journal |last1=Touchon |first1=Marie |last2=Rocha |first2=Eduardo P.C. |date=January 2016 |title=Coevolution of the Organization and Structure of Prokaryotic Genomes |journal=Cold Spring Harbor Perspectives in Biology |language=en |volume=8 |issue=1 |pages=a018168 |doi=10.1101/cshperspect.a018168 |issn=1943-0264 |pmc=4691797 |pmid=26729648}}</ref>

=== Structure in sequences ===
Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the [[origin of replication]]) from which replication starts, whereas some archaea contain multiple replication origins.<ref>{{cite journal | vauthors = Kelman LM, Kelman Z | title = Multiple origins of replication in archaea | journal = Trends in Microbiology | volume = 12 | issue = 9 | pages = 399–401 | date = September 2004 | pmid = 15337158 | doi = 10.1016/j.tim.2004.07.001 }}</ref> The genes in prokaryotes are often organized in [[operon]]s and do not usually contain [[intron]]s, unlike eukaryotes.

=== DNA packaging ===
[[Prokaryote]]s do not possess nuclei. Instead, their DNA is organized into a structure called the [[nucleoid]].<ref>{{cite journal | vauthors = Thanbichler M, Wang SC, Shapiro L | title = The bacterial nucleoid: a highly organized and dynamic structure | journal = Journal of Cellular Biochemistry | volume = 96 | issue = 3 | pages = 506–21 | date = October 2005 | pmid = 15988757 | doi = 10.1002/jcb.20519 | s2cid = 25355087 | doi-access = free }}</ref><ref name="Le-2013">{{cite journal | vauthors = Le TB, Imakaev MV, Mirny LA, Laub MT | title = High-resolution mapping of the spatial organization of a bacterial chromosome | journal = Science | volume = 342 | issue = 6159 | pages = 731–4 | date = November 2013 | pmid = 24158908 | pmc = 3927313 | doi = 10.1126/science.1242059 | bibcode = 2013Sci...342..731L }}</ref> The nucleoid is a distinct structure and occupies a defined region of the bacterial cell. This structure is, however, dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome.<ref>{{cite journal | vauthors = Sandman K, Pereira SL, Reeve JN | title = Diversity of prokaryotic chromosomal proteins and the origin of the nucleosome | journal = Cellular and Molecular Life Sciences | volume = 54 | issue = 12 | pages = 1350–64 | date = December 1998 | pmid = 9893710 | doi = 10.1007/s000180050259 | s2cid = 21101836 | pmc = 11147202 }}</ref> In [[archaea]], the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.<ref>{{cite journal | vauthors = Sandman K, Reeve JN | title = Structure and functional relationships of archaeal and eukaryal histones and nucleosomes | journal = Archives of Microbiology | volume = 173 | issue = 3 | pages = 165–9 | date = March 2000 | pmid = 10763747 | doi = 10.1007/s002039900122 | bibcode = 2000ArMic.173..165S | s2cid = 28946064 }}</ref><ref>{{cite journal | vauthors = Pereira SL, Grayling RA, Lurz R, Reeve JN | title = Archaeal nucleosomes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 94 | issue = 23 | pages = 12633–7 | date = November 1997 | pmid = 9356501 | pmc = 25063 | doi = 10.1073/pnas.94.23.12633 | bibcode = 1997PNAS...9412633P | doi-access = free }}</ref>

Certain bacteria also contain [[plasmid]]s or other [[extrachromosomal DNA]]. These are circular structures in the [[cytoplasm]] that contain cellular DNA and play a role in [[horizontal gene transfer]].<ref name="Schleyden-1847" /> In prokaryotes and viruses,<ref name="Johnson-2000">{{cite journal | vauthors = Johnson JE, Chiu W | title = Structures of virus and virus-like particles | journal = Current Opinion in Structural Biology | volume = 10 | issue = 2 | pages = 229–35 | date = April 2000 | pmid = 10753814 | doi = 10.1016/S0959-440X(00)00073-7 }}</ref> the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by [[Histone-like nucleoid-structuring protein|histone-like]] proteins.

Bacterial chromosomes tend to be tethered to the [[plasma membrane]] of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of the membranes (and the attached DNA).

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally [[supercoiled]]. The DNA must first be released into its relaxed state for access for [[Transcription (genetics)|transcription]], regulation, and [[DNA replication|replication]].

== Eukaryotes ==
{{Main|Chromatin}}
{{See also|DNA condensation|Chromosome condensation|Nucleosome|Histone|Protamine}}
{{See also|Eukaryotic chromosome fine structure}}
[[File:Eukaryote DNA-en.svg|thumb|upright=1.15|Organization of DNA in a eukaryotic cell]]

Each eukaryotic chromosome consists of a long linear [[DNA|DNA molecule]] associated with [[protein]]s, forming a compact complex of proteins and DNA called ''[[chromatin]].'' Chromatin contains the vast majority of the DNA in an organism, but a [[Mitochondrial DNA|small amount]] inherited maternally can be found in the [[mitochondria]]. It is present in most [[cell (biology)|cells]], with a few exceptions, for example, [[red blood cell]]s.

[[Histone]]s are responsible for the first and most basic unit of chromosome organization, the [[nucleosome]].

[[Eukaryote]]s ([[cell (biology)|cells]] with nuclei such as those found in plants, fungi, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one [[centromere]], with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular [[mitochondrial genome]], and some eukaryotes may have additional small circular or linear [[cytoplasm]]ic chromosomes.

[[File:Chromatin Structures.png|thumb|center|upright=3.9|The major structures in DNA compaction: [[DNA]], the [[nucleosome]], the 10&nbsp;nm "beads-on-a-string" fibre, the 30&nbsp;nm fibre and the [[metaphase]] chromosome]]
In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around [[histone]]s (structural proteins), forming a composite material called chromatin.

=== Interphase chromatin ===
The packaging of DNA into nucleosomes causes a 10 nanometer fibre which may further condense up to 30&nbsp;nm fibres.<ref name="Cooper-2019" /> Most of the euchromatin in interphase nuclei appears to be in the form of 30-nm fibers.<ref name="Cooper-2019" /> Chromatin structure is the more decondensed state, i.e. the 10-nm conformation allows transcription.<ref name="Cooper-2019" />

[[File:Heterochromatin vs. euchromatin.svg|thumb|Heterochromatin vs. euchromatin|upright=1.8]]

During [[interphase]] (the period of the [[cell cycle]] where the cell is not dividing), two types of chromatin can be distinguished:
* [[Euchromatin]], which consists of DNA that is active, e.g., being expressed as protein.
* [[Heterochromatin]], which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:
** ''Constitutive heterochromatin'', which is never expressed. It is located around the centromere and usually contains [[repeated sequence (DNA)|repetitive sequences]].
** ''Facultative heterochromatin'', which is sometimes expressed.

=== Metaphase chromatin and division ===
{{See also|chromosome condensation|mitosis|meiosis}}
[[File:HumanChromosomesChromomycinA3.jpg|thumb|left|upright=0.9|Human chromosomes during [[metaphase]]]]
[[File:Stages of early mitosis in a vertebrate cell with micrographs of chromatids.svg|thumb|right|Stages of early mitosis in a vertebrate cell with micrographs of chromatids]]

In the early stages of [[mitosis]] or [[meiosis]] (cell division), the chromatin double helix becomes more and more condensed. They cease to function as accessible genetic material ([[Transcription (genetics)|transcription]] stops) and become a compact transportable form. The loops of thirty-nanometer chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells. The DNA is thus condensed about ten-thousand-fold.<ref name="Cooper-2019">{{Cite book|last1=Cooper|first1=G.M.|title=The Cell|publisher=[[Oxford University Press]]|year=2019|isbn=978-1605357072|edition=8}}</ref>

The [[chromosome scaffold]], which is made of proteins such as [[condensin]], [[TOP2A]] and [[KIF4A|KIF4]],<ref>{{Cite journal|last1=Poonperm|first1=Rawin|last2=Takata|first2=Hideaki|last3=Hamano|first3=Tohru|last4=Matsuda|first4=Atsushi|last5=Uchiyama|first5=Susumu|last6=Hiraoka|first6=Yasushi|last7=Fukui|first7=Kiichi|date=1 July 2015|title=Chromosome Scaffold is a Double-Stranded Assembly of Scaffold Proteins|journal=Scientific Reports|volume=5|issue=1|pages=11916|doi=10.1038/srep11916|pmid=26132639|pmc=4487240|bibcode=2015NatSR...511916P}}</ref> plays an important role in holding the chromatin into compact chromosomes. Loops of thirty-nanometer structure further condense with scaffold into higher order structures.<ref>{{Cite book|last1=Lodish|first1=U.H.|title=Molecular Cell Biology|last2=Lodish|first2=H.|last3=Berk|first3=A.|last4=Kaiser|first4=C.A.|last5=Kaiser|first5=C.|last6=Kaiser|first6=U.C.A.|last7=Krieger|first7=M.|last8=Scott|first8=M.P.|last9=Bretscher|first9=A.|year=2008|publisher=W. H. Freeman|isbn=978-0-7167-7601-7|last10=Ploegh|first10=H.|last11=others}}</ref>

This highly compact form makes the individual chromosomes visible, and they form the classic four-arm structure, a pair of sister [[chromatid]]s attached to each other at the [[centromere]]. The shorter arms are called ''[[p arm]]s'' (from the French ''petit'', small) and the longer arms are called ''[[q arm]]s'' (''q'' follows ''p'' in the Latin alphabet; q-g "grande"; alternatively it is sometimes said q is short for ''queue'' meaning tail in French<ref>"[http://www.nature.com/scitable/topicpage/chromosome-mapping-idiograms-302 Chromosome Mapping: Idiograms]" ''Nature Education'' – 13 August 2013</ref>). This is the only natural context in which individual chromosomes are visible with an optical [[microscope]].

Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops.<ref name="Naumova-2013">{{cite journal | vauthors = Naumova N, Imakaev M, Fudenberg G, Zhan Y, Lajoie BR, Mirny LA, Dekker J | title = Organization of the mitotic chromosome | journal = Science | volume = 342 | issue = 6161 | pages = 948–53 | date = November 2013 | pmid = 24200812 | pmc = 4040465 | doi = 10.1126/science.1236083 | bibcode = 2013Sci...342..948N }}</ref>

During mitosis, [[microtubule]]s grow from centrosomes located at opposite ends of the cell and also attach to the centromere at specialized structures called [[kinetochore]]s, one of which is present on each sister [[chromatid]]. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region. The microtubules then pull the chromatids apart toward the centrosomes, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and DNA can again be transcribed. In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus.

=== Human chromosomes ===
Chromosomes in humans can be divided into two types: [[autosome]]s (body chromosome(s)) and allosome ([[sex chromosome]](s)). Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. The autosomes contain the rest of the genetic hereditary information. All act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the [[mitochondrial genome]]. [[DNA sequencing|Sequencing]] of the [[human genome]] has provided a great deal of information about each of the chromosomes. Below is a table compiling statistics for the chromosomes, based on the [[Sanger Institute]]'s human genome information in the [[Vertebrate and Genome Annotation Project|Vertebrate Genome Annotation (VEGA) database]].<ref>[http://vega.sanger.ac.uk/Homo_sapiens/index.html Vega.sanger.ad.uk], all data in this table was derived from this database, 11 November 2008.</ref> Number of genes is an estimate, as it is in part based on [[gene prediction]]s. Total chromosome length is an estimate as well, based on the estimated size of unsequenced [[heterochromatin]] regions.

{| class="wikitable sortable" style="text-align:right"
|+
|-
! Chromosome !! [[Gene]]s<ref>{{Cite web|url=http://apr2013.archive.ensembl.org/Homo_sapiens/Location/Chromosome?r=1:1-1000000|title=Ensembl genome browser 71: Homo sapiens – Chromosome summary – Chromosome 1: 1–1,000,000|website=apr2013.archive.ensembl.org|access-date=11 April 2016}}</ref> !! Total [[nucleobase|base pairs]] !! % of bases 
|-
| [[Chromosome 1|1]] ||2000|| 247,199,719 ||8.0
|-
| [[Chromosome 2|2]] ||1300|| 242,751,149 ||7.9
|-
| [[Chromosome 3|3]] ||1000|| 199,446,827 ||6.5
|-
| [[Chromosome 4|4]] ||1000|| 191,263,063 ||6.2
|-
| [[Chromosome 5|5]] ||900|| 180,837,866 ||5.9
|-
| [[Chromosome 6|6]] ||1000|| 170,896,993 ||5.5
|-
| [[Chromosome 7|7]] ||900|| 158,821,424 ||5.2
|-
| [[Chromosome 8|8]] ||700|| 146,274,826 ||4.7
|-
| [[Chromosome 9|9]] ||800|| 140,442,298 ||4.6
|-
| [[Chromosome 10|10]] ||700|| 135,374,737 ||4.4
|-
| [[Chromosome 11|11]] ||1300|| 134,452,384 ||4.4
|-
| [[Chromosome 12|12]] ||1100|| 132,289,534 ||4.3
|-
| [[Chromosome 13|13]] ||300|| 114,127,980 ||3.7
|-
| [[Chromosome 14|14]] ||800|| 106,360,585 ||3.5
|-
| [[Chromosome 15|15]] ||600|| 100,338,915 ||3.3
|-
| [[Chromosome 16|16]] ||800|| 88,822,254 ||2.9
|-
| [[Chromosome 17|17]] ||1200|| 78,654,742 ||2.6
|-
| [[Chromosome 18|18]] ||200|| 76,117,153 ||2.5
|-
| [[Chromosome 19|19]] ||1500|| 63,806,651 ||2.1
|-
| [[Chromosome 20|20]] ||500|| 62,435,965 ||2.0
|-
| [[Chromosome 21|21]] ||200|| 46,944,323 ||1.5
|-
| [[Chromosome 22|22]] ||500|| 49,528,953 ||1.6
|-
| [[X chromosome|X (sex chromosome)]] ||800|| 154,913,754 ||5.0
|-
| [[Y chromosome|Y (sex chromosome)]] ||200<ref name="NCBI-1998">{{Cite book| title = Genes and Disease| chapter = Chromosome Map| publisher = National Center for Biotechnology Information| location = Bethesda, Maryland|url = https://www.ncbi.nlm.nih.gov/books/NBK22266/#A296| year = 1998}}</ref>|| 57,741,652 ||1.9
|- class="sortbottom"
! Total ||style="text-align:right"| 21,000 ||style="text-align:right"| 3,079,843,747 ||style="text-align:right"| 100.0 
|}

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>The colors of each row match those of the karyogram (see Karyotype section)</ref><ref>{{cite journal|author1=Erwinsyah, R.|author2=Riandi|author3=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
|- style="background:lavenderblush"
| '''A'''
| 1–3
| Large, metacentric or submetacentric
|- style="background:honeydew"
| '''B'''
| 4–5
| Large, submetacentric
|- style="background:lightyellow"
| '''C'''
| 6–12, X
| Medium-sized, submetacentric
|- style="background:linen"
| '''D'''
| 13–15
| Medium-sized, acrocentric, with [[Satellite chromosome|satellite]]
|- style="background:lightcyan"
| '''E'''
| 16–18
| Small, metacentric or submetacentric
|- style="background:lavender"
| '''F'''
| 19–20
| Very small, metacentric
|- style="background:lavenderblush"
| '''G'''
| 21–22, Y
| Very small, acrocentric (and 21, 22 with [[Satellite chromosome|satellite]])
|}

== Karyotype ==
{{Main|Karyotype}}
[[File:NHGRI human male karyotype.png|thumb|upright=0.9|Karyogram of a human male]]
[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyogram]] of a human, 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]]. Each row is vertically aligned at [[centromere]] level. It shows 22 [[homologous chromosome]]s, both the female (XX) and male (XY) versions of the [[sex chromosome]] (bottom right), as well as the [[Human mitochondrial genetics|mitochondrial genome]] (at bottom left). {{further|Karyotype}}]]

In general, the [[karyotype]] is the characteristic chromosome complement of a [[eukaryote]] [[species]].<ref>{{cite book |author=White, M. J. D. |title=The chromosomes |url=https://archive.org/details/chromosomes01whit |url-access=registration |publisher=Chapman and Hall, distributed by Halsted Press, New York |location=London |year=1973 |page=28 |isbn=978-0-412-11930-9 |edition=6th}}</ref> The preparation and study of karyotypes is part of [[cytogenetics]].

Although the [[DNA replication|replication]] and [[Transcription (genetics)|transcription]] of [[DNA]] is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are often highly variable. There may be variation between species in chromosome number and in detailed organization.
In some cases, there is significant variation within species. Often there is:
:1. variation between the two sexes
:2. variation between the [[germline]] and [[Somatic cell|soma]] (between [[gamete]]s and the rest of the body)
:3. variation between members of a population, due to [[Polymorphism (biology)|balanced genetic polymorphism]]
:4. [[Allopatric speciation|geographical variation]] between [[Race (classification of human beings)|races]]
:5. [[Mosaic (genetics)|mosaics]] or otherwise abnormal individuals.
Also, variation in karyotype may occur during development from the fertilized egg.

The technique of determining the karyotype is usually called ''karyotyping''. Cells can be locked part-way through division (in metaphase) [[in vitro]] (in a reaction vial) with [[colchicine]]. These cells are then stained, photographed, and arranged into a ''karyogram'', with the set of chromosomes arranged, autosomes in order of length, and sex chromosomes (here X/Y) at the end.

Like many sexually reproducing species, humans have special [[XY sex-determination system|gonosomes]] (sex chromosomes, in contrast to [[autosome]]s). These are XX in females and XY in males. <!--- Irrelevant in this section:"In females, one of the two X chromosomes is inactive and can be seen under a microscope as [[Barr body|Barr bodies]]."--->

=== History and analysis techniques ===
{{See also|Argument from authority#Use in science}}
Investigation into the human karyotype took many years to settle the most basic question: ''How many chromosomes does a normal [[diploid]] human cell contain?'' In 1912, [[Hans von Winiwarter]] reported 47 chromosomes in [[spermatogonia]] and 48 in [[oogonia]], concluding an [[XO sex-determination system|XX/XO]] [[Sex-determination system|sex determination mechanism]].<ref>{{cite journal |author=von Winiwarter H |title=Études sur la spermatogenèse humaine |journal=Archives de Biologie |volume=27 |issue=93 |pages=147–9 |year=1912}}</ref> In 1922, [[Theophilus Painter|Painter]] was not certain whether the diploid number of man is 46 or 48, at first favouring 46.<ref>{{cite journal |author=Painter TS |title=The spermatogenesis of man |journal=Anat. Res.|volume=23 |page=129 |year=1922}}</ref> He revised his opinion later from 46 to 48, and he correctly insisted on humans having an [[XY sex-determination system|XX/XY]] system.<ref>{{cite journal|last1=Painter|first1=Theophilus S.|title=Studies in mammalian spermatogenesis. II. The spermatogenesis of man|journal=Journal of Experimental Zoology|date=April 1923|volume=37|issue=3|pages=291–336|doi=10.1002/jez.1400370303|bibcode=1923JEZ....37..291P }}</ref>

New techniques were needed to definitively solve the problem:
# Using cells in culture
# Arresting [[mitosis]] in [[metaphase]] by a solution of [[colchicine]]
# Pretreating cells in a [[Hypotonicity|hypotonic solution]] {{nowrap|0.075 M KCl}}, which swells them and spreads the chromosomes
# Squashing the preparation on the slide forcing the chromosomes into a single plane
# Cutting up a photomicrograph and arranging the result into an indisputable karyogram.

It took until 1954 before the human diploid number was confirmed as 46.<ref>{{cite journal |doi=10.1111/j.1601-5223.1956.tb03010.x | vauthors = Tjio JH, Levan A | title=The chromosome number of man |journal=Hereditas |volume=42 |pages=723–4 |year=1956 |issue=1–2| pmid = 345813 |hdl=10261/15776 |doi-access=free }}</ref><ref>{{cite journal | vauthors = Ford CE, Hamerton JL | title = The chromosomes of man | journal = Nature | volume = 178 | issue = 4541 | pages = 1020–3 | date = November 1956 | pmid = 13378517 | doi = 10.1038/1781020a0 | bibcode = 1956Natur.178.1020F | s2cid = 4155320 }}</ref> Considering the techniques of Winiwarter and Painter, their results were quite remarkable.<ref>Hsu T.C. (1979) ''Human and mammalian cytogenetics: a historical perspective''. Springer-Verlag, N.Y. {{ISBN|9780387903644}} p. 10: "It's amazing that he [Painter] even came close!"</ref> [[Pan (genus)|Chimpanzees]], the closest living relatives to modern humans, have 48 chromosomes as do the other [[great apes]]: in humans two chromosomes fused to form [[chromosome 2]].

== Aberrations ==
{{Main|Chromosome abnormality}}
[[File:Chromosome 21.png|thumb|In Down syndrome, there are three copies of chromosome 21.]]

Chromosomal aberrations are disruptions in the normal chromosomal content of a cell. They can cause genetic conditions in humans, such as [[Down syndrome]],<ref>{{Citation |title=Chromosomal Abnormalities |date=8 July 2009 |url=https://www.ncbi.nlm.nih.gov/books/NBK115545/ |work=Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals |access-date=27 September 2023 |publisher=Genetic Alliance |language=en}}</ref> although most aberrations have little to no effect. Some chromosome abnormalities do not cause disease in carriers, such as [[Chromosomal translocation|translocations]], or [[chromosomal inversion]]s, although they may lead to a higher chance of bearing a child with a chromosome disorder.{{citation needed|date=April 2024}} Abnormal numbers of chromosomes or chromosome sets, called [[aneuploidy]], may be lethal or may give rise to genetic disorders.<ref>{{cite journal | vauthors = Santaguida S, Amon A | title = Short- and long-term effects of chromosome mis-segregation and aneuploidy | journal = Nature Reviews. Molecular Cell Biology | volume = 16 | issue = 8 | pages = 473–85 | date = August 2015 | pmid = 26204159 | doi = 10.1038/nrm4025 | hdl = 1721.1/117201 | s2cid = 205495880 | url = http://dspace.mit.edu/bitstream/1721.1/117201/1/Amon1.pdf }}</ref> [[Genetic counseling]] is offered for families that may carry a chromosome rearrangement.

The gain or loss of DNA from chromosomes can lead to a variety of [[genetic disorder]]s.<ref>{{Cite web |title=Genetic Disorders |url=https://medlineplus.gov/geneticdisorders.html |access-date=27 April 2022 |website=medlineplus.gov}}</ref> Human examples include:
* [[Cri du chat]], caused by the [[Genetic deletion|deletion]] of part of the short arm of chromosome 5. "Cri du chat" means "cry of the cat" in French; the condition was so-named because affected babies make high-pitched cries that sound like those of a cat. Affected individuals have wide-set eyes, a small head and jaw, moderate to severe mental health problems, and are very short.
* [[DiGeorge syndrome]], also known as 22q11.2 deletion syndrome. Symptoms are mild learning disabilities in children, with adults having an increased risk of [[schizophrenia]]. Infections are also common in children because of problems with the immune system's T cell-mediated response due to an absence of hypoplastic thymus.<ref>{{Cite web |title=DiGeorge Syndrome |url=https://www.ncbi.nlm.nih.gov/books/NBK549798 |access-date=8 August 2023 |website=www.ncbi.nlm.nih.gov}}</ref>
* [[Down syndrome]], the most common trisomy, usually caused by an extra copy of chromosome 21 ([[trisomy 21]]). Characteristics include decreased muscle tone, stockier build, asymmetrical skull, slanting eyes, and mild to moderate developmental disability.<ref>{{cite book|last=Miller|first=Kenneth R. | name-list-style = vanc | title=Biology|url=https://archive.org/details/biology0000mill|url-access=limited|publisher=Prentice Hall|location=Upper Saddle River, New Jersey|year=2000|edition=5th |pages=[https://archive.org/details/biology0000mill/page/194 194]–5|chapter=Chapter 9-3|isbn=978-0-13-436265-6}}</ref>
* [[Edwards syndrome]], or trisomy-18, the second most common trisomy.<ref>{{cite web|title=What is Trisomy 18?|url=http://www.trisomy18.org/what-is-trisomy-18/|website=Trisomy 18 Foundation|access-date=4 February 2017|archive-date=30 January 2017|archive-url=https://web.archive.org/web/20170130142121/http://www.trisomy18.org/what-is-trisomy-18/|url-status=dead}}</ref> Symptoms include motor retardation, developmental disability, and numerous congenital anomalies causing serious health problems. Ninety percent of those affected die in infancy. They have characteristic clenched hands and overlapping fingers.
* [[Isodicentric 15]], also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15).
* [[Jacobsen syndrome]], which is very rare. It is also called the 11q terminal deletion disorder.<ref>{{Cite web|url=https://chromosome11.org/en/disorders/11q-long-arm/terminal-deletion/jacobsen-syndrome/ |title=Terminal deletion |website=European Chromosome 11 Network |access-date=20 February 2023}}</ref> Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called [[Paris-Trousseau syndrome]].
* [[Klinefelter syndrome]] (XXY). Men with Klinefelter syndrome are usually sterile, and tend to be taller than their peers, with longer arms and legs. Boys with the syndrome are often shy and quiet, and have a higher incidence of [[speech delay]] and [[dyslexia]]. Without testosterone treatment, some may develop [[gynecomastia]] during puberty.
* [[Patau Syndrome]], also called D-Syndrome or trisomy-13. Symptoms are somewhat similar to those of trisomy-18, without the characteristic folded hand.
* [[Small supernumerary marker chromosome]]. This means there is an extra, abnormal chromosome. Features depend on the origin of the extra genetic material. [[Cat-eye syndrome]] and [[isodicentric chromosome 15 syndrome]] (or Idic15) are both caused by a supernumerary marker chromosome, as is [[Pallister–Killian syndrome]].
* [[Triple-X syndrome]] (XXX). XXX girls tend to be tall and thin, and have a higher incidence of dyslexia.
* [[Turner syndrome]] (X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. Females with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development, and a "caved-in" appearance to the chest.
* [[Wolf–Hirschhorn syndrome]], caused by partial deletion of the short arm of chromosome 4. It is characterized by growth retardation, delayed motor skills development, "Greek Helmet" facial features, and mild to profound mental health problems.
* [[XYY syndrome]]. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.

===Sperm aneuploidy===

Exposure of males to certain lifestyle, environmental and/or occupational hazards may increase the risk of aneuploid spermatozoa.<ref name="Templado-2013">{{cite journal | vauthors = Templado C, Uroz L, Estop A | title = New insights on the origin and relevance of aneuploidy in human spermatozoa | journal = Molecular Human Reproduction | volume = 19 | issue = 10 | pages = 634–43 | date = October 2013 | pmid = 23720770 | doi = 10.1093/molehr/gat039 | doi-access = }}</ref> In particular, risk of aneuploidy is increased by tobacco smoking,<ref name="Shi-2001">{{cite journal | vauthors = Shi Q, Ko E, Barclay L, Hoang T, Rademaker A, Martin R | title = Cigarette smoking and aneuploidy in human sperm | journal = Molecular Reproduction and Development | volume = 59 | issue = 4 | pages = 417–21 | date = August 2001 | pmid = 11468778 | doi = 10.1002/mrd.1048 | s2cid = 35230655 }}</ref><ref name="Rubes-1998">{{cite journal | vauthors = Rubes J, Lowe X, Moore D, Perreault S, Slott V, Evenson D, Selevan SG, Wyrobek AJ | title = Smoking cigarettes is associated with increased sperm disomy in teenage men | journal = Fertility and Sterility | volume = 70 | issue = 4 | pages = 715–23 | date = October 1998 | pmid = 9797104 | doi = 10.1016/S0015-0282(98)00261-1 | doi-access = free }}</ref> and occupational exposure to benzene,<ref name="Xing-2010">{{cite journal | vauthors = Xing C, Marchetti F, Li G, Weldon RH, Kurtovich E, Young S, Schmid TE, Zhang L, Rappaport S, Waidyanatha S, Wyrobek AJ, Eskenazi B | title = Benzene exposure near the U.S. permissible limit is associated with sperm aneuploidy | journal = Environmental Health Perspectives | volume = 118 | issue = 6 | pages = 833–9 | date = June 2010 | pmid = 20418200 | pmc = 2898861 | doi = 10.1289/ehp.0901531 | bibcode = 2010EnvHP.118..833X }}</ref> [[insecticide]]s,<ref name="Xia-2004">{{cite journal | vauthors = Xia Y, Bian Q, Xu L, Cheng S, Song L, Liu J, Wu W, Wang S, Wang X | title = Genotoxic effects on human spermatozoa among pesticide factory workers exposed to fenvalerate | journal = Toxicology | volume = 203 | issue = 1–3 | pages = 49–60 | date = October 2004 | pmid = 15363581 | doi = 10.1016/j.tox.2004.05.018 | bibcode = 2004Toxgy.203...49X | s2cid = 36073841 }}</ref><ref name="Xia-2005">{{cite journal | vauthors = Xia Y, Cheng S, Bian Q, Xu L, Collins MD, Chang HC, Song L, Liu J, Wang S, Wang X | title = Genotoxic effects on spermatozoa of carbaryl-exposed workers | journal = Toxicological Sciences | volume = 85 | issue = 1 | pages = 615–23 | date = May 2005 | pmid = 15615886 | doi = 10.1093/toxsci/kfi066 | doi-access = free }}</ref> and perfluorinated compounds.<ref name="Governini-2015">{{cite journal | vauthors = Governini L, Guerranti C, De Leo V, Boschi L, Luddi A, Gori M, Orvieto R, Piomboni P | title = Chromosomal aneuploidies and DNA fragmentation of human spermatozoa from patients exposed to perfluorinated compounds | journal = Andrologia | volume = 47 | issue = 9 | pages = 1012–9 | date = November 2015 | pmid = 25382683 | doi = 10.1111/and.12371 | hdl = 11365/982323 | s2cid = 13484513 | doi-access = free }}</ref> Increased aneuploidy is often associated with increased DNA damage in spermatozoa.

== Number in various organisms ==
{{Main|List of organisms by chromosome count}}

=== In eukaryotes ===
The number of chromosomes in [[eukaryote]]s is highly variable. It is possible for chromosomes to fuse or break and thus evolve into novel karyotypes. Chromosomes can also be fused artificially. For example, when the 16 chromosomes of [[Saccharomyces cerevisiae|yeast]] were fused into one giant chromosome, it was found that the cells were still viable with only somewhat reduced growth rates.<ref>{{Cite journal|last1=Shao|first1=Yangyang|last2=Lu|first2=Ning|last3=Wu|first3=Zhenfang|last4=Cai|first4=Chen|last5=Wang|first5=Shanshan|last6=Zhang|first6=Ling-Li|last7=Zhou|first7=Fan|last8=Xiao|first8=Shijun|last9=Liu|first9=Lin|last10=Zeng|first10=Xiaofei|last11=Zheng|first11=Huajun|date=August 2018|title=Creating a functional single-chromosome yeast|url=https://www.nature.com/articles/s41586-018-0382-x|journal=Nature|language=en|volume=560|issue=7718|pages=331–335|doi=10.1038/s41586-018-0382-x|pmid=30069045|bibcode=2018Natur.560..331S|s2cid=51894920|issn=1476-4687|url-access=subscription}}</ref>

The tables below give the total number of chromosomes (including sex chromosomes) in a cell nucleus for various eukaryotes. Most are [[diploid]], such as [[Human#Genetics|humans]] who have 22 different types of [[autosome]]s—each present as two homologous pairs—and two [[sex chromosome]]s, giving 46 chromosomes in total. Some other organisms have more than two copies of their chromosome types, for example [[bread wheat]] which is ''hexaploid'', having six copies of seven different chromosome types for a total of 42 chromosomes. 
{| border="0"
| STYLE="vertical-align: top"|
{| class="wikitable sortable" style="float:left; margin:1em 0 1em 1em"
|+ Chromosome numbers in some plants
|-
! Plant species !! #
|-
| [[Thale cress]] (diploid)<ref>{{cite journal | vauthors = Armstrong SJ, Jones GH | title = Meiotic cytology and chromosome behaviour in wild-type Arabidopsis thaliana | journal = Journal of Experimental Botany | volume = 54 | issue = 380 | pages = 1–10 | date = January 2003 | pmid = 12456750 | doi = 10.1093/jxb/54.380.1 | doi-access = free }}</ref> || 10
|-
| [[Rye]] (diploid)<ref>{{cite journal | vauthors = Gill BS, Kimber G | title = The Giemsa C-banded karyotype of rye | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 71 | issue = 4 | pages = 1247–9 | date = April 1974 | pmid = 4133848 | pmc = 388202 | doi = 10.1073/pnas.71.4.1247 | bibcode = 1974PNAS...71.1247G | doi-access = free }}</ref> || 14
|-
| [[Einkorn wheat]] (diploid)<ref name="Dubcovsky-1996">{{cite journal | vauthors = Dubcovsky J, Luo MC, Zhong GY, Bransteitter R, Desai A, Kilian A, Kleinhofs A, Dvorák J | title = Genetic map of diploid wheat, Triticum monococcum L., and its comparison with maps of Hordeum vulgare L | journal = Genetics | volume = 143 | issue = 2 | pages = 983–99 | date = June 1996 | doi = 10.1093/genetics/143.2.983 | pmid = 8725244 | pmc = 1207354 }}</ref> || 14
|-
| [[Maize]] (diploid or palaeotetraploid)<ref>{{cite journal | vauthors = Kato A, Lamb JC, Birchler JA | title = Chromosome painting using repetitive DNA sequences as probes for somatic chromosome identification in maize | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 37 | pages = 13554–9 | date = September 2004 | pmid = 15342909 | pmc = 518793 | doi = 10.1073/pnas.0403659101 | bibcode = 2004PNAS..10113554K | doi-access = free }}</ref> || 20
|-
| [[Durum wheat]] (tetraploid)<ref name="Dubcovsky-1996"/> || 28 
|-
| [[Bread wheat]] (hexaploid)<ref name="Dubcovsky-1996"/> || 42
|-
| [[Cultivated tobacco]] (tetraploid)<ref>{{cite journal | vauthors = Kenton A, Parokonny AS, Gleba YY, Bennett MD | title = Characterization of the Nicotiana tabacum L. genome by molecular cytogenetics | journal = Molecular & General Genetics | volume = 240 | issue = 2 | pages = 159–69 | date = August 1993 | pmid = 8355650 | doi = 10.1007/BF00277053 | s2cid = 6953185 }}</ref> || 48
|-
| [[Adder's tongue fern]] (polyploid)<ref>{{cite journal | vauthors = Leitch IJ, Soltis DE, Soltis PS, Bennett MD | title = Evolution of DNA amounts across land plants (embryophyta) | journal = Annals of Botany | volume = 95 | issue = 1 | pages = 207–17 | date = January 2005 | pmid = 15596468 | doi = 10.1093/aob/mci014 | author-link3 = Pamela S. Soltis | pmc = 4246719 }}</ref> || approx. 1,200
|}
| STYLE="vertical-align: top"|
{| class="wikitable sortable" style="float:left; margin:1em 0 1em 1em"
|+ Chromosome numbers (2n) in some animals
|-
! Species !! #
|-
| [[Indian muntjac]] || 6♀, 7♂
|-
| [[Common fruit fly]] || 8
|-
| [[Pill millipede]]<ref>{{cite journal |author1=Ambarish, C.N. |author2=Sridhar, K.R.|title=Cytological and karyological observations on two endemic giant pill-millipedes ''Arthrosphaera'' (Pocock 1895) (Diplopoda: Sphaerotheriida) of the Western Ghats of India | doi = 10.1080/00087114.2014.891700 |journal=Caryologia |volume=67 |issue=1|year=2014|pages=49–56|s2cid=219554731 }}</ref> || 30
|-
| [[Earthworm]]<ref>{{cite journal | vauthors = Vitturi R, Colomba MS, Pirrone AM, Mandrioli M | title = rDNA (18S–28S and 5S) colocalization and linkage between ribosomal genes and (TTAGGG)(n) telomeric sequence in the earthworm, ''Octodrilus complanatus'' (Annelida: Oligochaeta: Lumbricidae), revealed by single- and double-color FISH | journal = The Journal of Heredity | volume = 93 | issue = 4 | pages = 279–82 | year = 2002 | pmid = 12407215 | doi = 10.1093/jhered/93.4.279 | doi-access = free }}</ref> || 36
|-
| [[Tibetan fox]] || 36
|-
| [[Domestic cat]]<ref>{{cite journal | vauthors = Nie W, Wang J, O'Brien PC, Fu B, Ying T, Ferguson-Smith MA, Yang F | title = The genome phylogeny of domestic cat, red panda and five mustelid species revealed by comparative chromosome painting and G-banding | journal = Chromosome Research | volume = 10 | issue = 3 | pages = 209–22 | year = 2002 | pmid = 12067210 | doi = 10.1023/A:1015292005631 | s2cid = 9660694 }}</ref> || 38
|-
| [[Domestic pig]] || 38
|-
| [[Laboratory mouse]]<ref name="Romanenko-2006">{{cite journal | vauthors = Romanenko SA, Perelman PL, Serdukova NA, Trifonov VA, Biltueva LS, Wang J, Li T, Nie W, O'Brien PC, Volobouev VT, Stanyon R, Ferguson-Smith MA, Yang F, Graphodatsky AS | title = Reciprocal chromosome painting between three laboratory rodent species | journal = Mammalian Genome | volume = 17 | issue = 12 | pages = 1183–92 | date = December 2006 | pmid = 17143584 | doi = 10.1007/s00335-006-0081-z | s2cid = 41546146 }}</ref><ref name="Painter-1928">{{cite journal | vauthors = Painter TS | title = A Comparison of the Chromosomes of the Rat and Mouse with Reference to the Question of Chromosome Homology in Mammals | journal = Genetics | volume = 13 | issue = 2 | pages = 180–9 | date = March 1928 | doi = 10.1093/genetics/13.2.180 | pmid = 17246549 | pmc = 1200977 }}</ref> || 40
|-
| [[Laboratory rat]]<ref name="Painter-1928"/> || 42
|-
| [[Rabbit]]<ref>{{cite journal | vauthors = Hayes H, Rogel-Gaillard C, Zijlstra C, De Haan NA, Urien C, Bourgeaux N, Bertaud M, Bosma AA | title = Establishment of an R-banded rabbit karyotype nomenclature by FISH localization of 23 chromosome-specific genes on both G- and R-banded chromosomes | journal = Cytogenetic and Genome Research | volume = 98 | issue = 2–3 | pages = 199–205 | year = 2002 | pmid = 12698004 | doi = 10.1159/000069807 | s2cid = 29849096 }}</ref> || 44
|-
| [[Syrian hamster]]<ref name="Romanenko-2006"/> || 44
|-
| [[Guppy]]<ref>{{cite web |url=http://fancyguppy.webs.com/genetics.htm |title=The Genetics of the Popular Aquarium Pet – Guppy Fish |access-date=6 December 2009 |archive-date=31 May 2023 |archive-url=https://web.archive.org/web/20230531003513/https://fancyguppy.webs.com/genetics.htm |url-status=dead }}</ref> || 46
|-
| Human<ref name="De Grouchy-1987"/> ||46
|-
| [[Hare]]<ref>{{cite journal | vauthors = Robinson TJ, Yang F, Harrison WR | title = Chromosome painting refines the history of genome evolution in hares and rabbits (order Lagomorpha) | journal = Cytogenetic and Genome Research | volume = 96 | issue = 1–4 | pages = 223–7 | year = 2002 | pmid = 12438803 | doi = 10.1159/000063034 | s2cid = 19327437 }}</ref><ref>{{citation  |url=https://books.google.com/books?id=Q994k86i0zYC|title=Rabbits, Hares and Pikas. Status Survey and Conservation Action Plan |section= section 4.W4 |pages= 61–94|isbn=9782831700199 | last1 = Chapman | first1 = Joseph A |last2=Flux |first2=John E. C | name-list-style = vanc | year=1990 }}</ref>|| 48
|-
| [[Gorilla]]<ref name="De Grouchy-1987">{{cite journal | vauthors = De Grouchy J | title = Chromosome phylogenies of man, great apes, and Old World monkeys | journal = Genetica | volume = 73 | issue = 1–2 | pages = 37–52 | date = August 1987 | pmid = 3333352 | doi = 10.1007/bf00057436 | s2cid = 1098866 }}</ref> || 48
|-
| [[Chimpanzee]]
|48
|-
| [[Domestic sheep]] || 54
|-
| [[Garden snail]]<ref>{{cite journal | vauthors = Vitturi R, Libertini A, Sineo L, Sparacio I, Lannino A, Gregorini A, Colomba M | title = Cytogenetics of the land snails Cantareus aspersus and C. mazzullii (Mollusca: Gastropoda: Pulmonata) | journal = Micron | volume = 36 | issue = 4 | pages = 351–7 | year = 2005 | pmid = 15857774 | doi = 10.1016/j.micron.2004.12.010 }}</ref> || 54
|-
| [[Silkworm]]<ref>{{cite journal | vauthors = Yasukochi Y, Ashakumary LA, Baba K, Yoshido A, Sahara K | title = A second-generation integrated map of the silkworm reveals synteny and conserved gene order between lepidopteran insects | journal = Genetics | volume = 173 | issue = 3 | pages = 1319–28 | date = July 2006 | pmid = 16547103 | pmc = 1526672 | doi = 10.1534/genetics.106.055541 }}</ref> || 56
|-
| [[Elephant]]<ref>{{cite journal | vauthors = Houck ML, Kumamoto AT, Gallagher DS, Benirschke K | title = Comparative cytogenetics of the African elephant (Loxodonta africana) and Asiatic elephant (Elephas maximus) | journal = Cytogenetics and Cell Genetics | volume = 93 | issue = 3–4 | pages = 249–52 | year = 2001 | pmid = 11528120 | doi = 10.1159/000056992 | s2cid = 23529399 }}</ref> || 56<!-- taxon? -->
|-
| [[Cow]] || 60
|-
| [[Donkey]] || 62
|-
| [[Guinea pig]]<ref>{{cite journal | vauthors = Semba U, Umeda Y, Shibuya Y, Okabe H, Tanase S, Yamamoto T | title = Primary structures of guinea pig high- and low-molecular-weight kininogens | journal = International Immunopharmacology | volume = 4 | issue = 10–11 | pages = 1391–400 | date = October 2004 | pmid = 15313436 | doi = 10.1016/j.intimp.2004.06.003 }}</ref>|| 64
|-
| [[Horse]] || 64
|-
| [[Dog]]<ref>{{cite journal | vauthors = Wayne RK, Ostrander EA | title = Origin, genetic diversity, and genome structure of the domestic dog | journal = BioEssays | volume = 21 | issue = 3 | pages = 247–57 | date = March 1999 | pmid = 10333734 | doi = 10.1002/(SICI)1521-1878(199903)21:3<247::AID-BIES9>3.0.CO;2-Z | s2cid = 5547543 }}</ref> || 78
|-
| [[Hedgehog]] || 90
|-
| [[Goldfish]]<ref>{{cite journal | vauthors = Ciudad J, Cid E, Velasco A, Lara JM, Aijón J, Orfao A | title = Flow cytometry measurement of the DNA contents of G0/G1 diploid cells from three different teleost fish species | journal = Cytometry | volume = 48 | issue = 1 | pages = 20–5 | date = May 2002 | pmid = 12116377 | doi = 10.1002/cyto.10100 | doi-access = }}</ref> || 100–104
|-
| [[Kingfisher]]<ref>{{cite journal | vauthors = Burt DW | title = Origin and evolution of avian microchromosomes | journal = Cytogenetic and Genome Research | volume = 96 | issue = 1–4 | pages = 97–112 | year = 2002 | pmid = 12438785 | doi = 10.1159/000063018 | s2cid = 26017998 }}</ref> || 132
|}
| STYLE="vertical-align: top"|
{| class="wikitable sortable" style="float:left; margin:1em 0 1em 1em"
|+ Chromosome numbers in other organisms
|-
! Species !! Large<br />chromosomes !! Intermediate<br />chromosomes !! [[Microchromosome]]s
|-
| ''[[Trypanosoma brucei]]'' || 11 || 6 || ≈100
|-
| [[Domestic pigeon]]<ref>{{cite journal |doi=10.1266/jjg.44.163 |title=A Comparative Karyotype Study in Fourteen Species of Birds |year=1969 |last1=Itoh |first1=Masahiro |last2=Ikeuchi |first2=Tatsuro |last3=Shimba |first3=Hachiro |last4=Mori |first4=Michiko |last5=Sasaki |first5=Motomichi |last6=Makino |first6=Sajiro | name-list-style = vanc |journal=The Japanese Journal of Genetics |volume=44 |issue=3 |pages=163–170|url=https://www.jstage.jst.go.jp/article/ggs1921/44/3/44_3_163/_pdf |doi-access=free }}</ref> || 18 || – || 59–63
|-
| Chicken<ref>{{cite journal | vauthors = Smith J, Burt DW | title = Parameters of the chicken genome (Gallus gallus) | journal = Animal Genetics | volume = 29 | issue = 4 | pages = 290–4 | date = August 1998 | pmid = 9745667 | doi = 10.1046/j.1365-2052.1998.00334.x }}</ref> || 8 || 2 sex chromosomes || 60
|}
|}

Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes. Other eukaryotic chromosomes, i.e., mitochondrial and plasmid-like small chromosomes, are much more variable in number, and there may be thousands of copies per cell.

[[File:PLoSBiol3.5.Fig1bNucleus46Chromosomes.jpg|thumb|upright=0.8|left|The 23 human [[chromosome territories]] during [[prometaphase]] in [[fibroblast]] cells]]

[[Asexually reproducing]] species have one set of chromosomes that are the same in all body cells. However, asexual species can be either haploid or diploid.

[[Sexually reproducing]] species have [[somatic cell]]s (body cells) that are [[diploid]] [2n], having two sets of chromosomes (23 pairs in humans), one set from the mother and one from the father. [[Gamete]]s (reproductive cells) are [[haploid]] [n], having one set of chromosomes. Gametes are produced by [[meiosis]] of a diploid [[germline]] cell, during which the matching chromosomes of father and mother can exchange small parts of themselves ([[Chromosomal crossover|crossover]]) and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge during [[fertilization]], a new diploid organism is formed.

Some animal and plant species are [[polyploid]] [Xn], having more than two sets of [[homologous chromosome]]s. Important crops such as tobacco or wheat are often polyploid, compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some [[cultivar]]s as well as the wild progenitors. The more common types of pasta and bread wheat are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in wild wheat.<ref>{{cite journal |last1=Sakamura |first1=Tetsu |year=1918 |title= Kurze Mitteilung über die Chromosomenzahlen und die Verwandtschaftsverhältnisse der Triticum-Arten |journal= Shokubutsugaku Zasshi |volume=32 |issue=379 |pages=150–3 |doi=10.15281/jplantres1887.32.379_150|url=https://www.jstage.jst.go.jp/article/jplantres1887/32/379/32_379_150/_pdf |doi-access=free }}</ref>

=== In prokaryotes ===
[[Prokaryote]] species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies.<ref>Charlebois R.L. (ed) 1999. ''Organization of the prokaryote genome''. ASM Press, Washington DC.</ref> For example, ''[[Buchnera (proteobacteria)|Buchnera]]'', a [[symbiont]] of [[aphid]]s has multiple copies of its chromosome, ranging from 10 to 400 copies per cell.<ref>{{cite journal | vauthors = Komaki K, Ishikawa H | title = Genomic copy number of intracellular bacterial symbionts of aphids varies in response to developmental stage and morph of their host | journal = Insect Biochemistry and Molecular Biology | volume = 30 | issue = 3 | pages = 253–8 | date = March 2000 | pmid = 10732993 | doi = 10.1016/S0965-1748(99)00125-3 | bibcode = 2000IBMB...30..253K }}</ref> However, in some large bacteria, such as ''[[Epulopiscium fishelsoni]]'' up to 100,000 copies of the chromosome can be present.<ref>{{cite journal | vauthors = Mendell JE, Clements KD, Choat JH, Angert ER | title = Extreme polyploidy in a large bacterium | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 18 | pages = 6730–4 | date = May 2008 | pmid = 18445653 | pmc = 2373351 | doi = 10.1073/pnas.0707522105 | bibcode = 2008PNAS..105.6730M | doi-access = free }}</ref> Plasmids and plasmid-like small chromosomes are, as in eukaryotes, highly variable in copy number. The number of plasmids in the cell is almost entirely determined by the rate of division of the plasmid&nbsp;– fast division causes high copy number.

== See also ==
{{div col}}
* [[Chromomere]]
* [[Cohesin]]
* [[Epigenetics]]
* [[Genetic genealogy]]
* [[Lampbrush chromosome]]
* [[Locus (genetics)]] – explains gene location nomenclature
* [[Minichromosome]]
* [[Neochromosome]]
* [[Nondisjunction]]
* [[Parasitic chromosome]]
* [[Polytene chromosome]]
* [[Secondary chromosome]]
* [[Sex-determination system]]
** [[Maternal influence on sex determination]]
** [[Temperature-dependent sex determination]]
{{div col end}}

== Notes and references ==
{{Reflist|32em}}

== External links ==
{{commons category|Chromosomes}}
* [https://web.archive.org/web/20090531182950/http://hopes.stanford.edu/basics/dna/b0.html An Introduction to DNA and Chromosomes] from [[HOPES]]: Huntington's Outreach Project for Education at Stanford
* [http://atlasgeneticsoncology.org/Educ/PolyMecaEng.html Chromosome Abnormalities at AtlasGeneticsOncology]
* [http://www.chromosomewalk.ch/en On-line exhibition on chromosomes and genome (SIB)]
* [https://web.archive.org/web/20081120000502/http://learn.genetics.utah.edu/content/begin/traits/predictdisorder/ What Can Our Chromosomes Tell Us?], from the University of Utah's Genetic Science Learning Center
* [http://learn.genetics.utah.edu/content/chromosomes/karyotype/ Try making a karyotype yourself], from the University of Utah's Genetic Science Learning Center
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html Kimballs Chromosome pages]
* [http://www.genomenewsnetwork.org/categories/index/genome/chromosomes.php Chromosome News from Genome News Network]
* [https://web.archive.org/web/20041210120428/http://www.chromosomehelpstation.com/eurochromnet.htm Eurochromnet], European network for Rare Chromosome Disorders on the Internet
* [http://www.ensembl.org/ Ensembl.org], [[Ensembl]] project, presenting chromosomes, their [[gene]]s and [[syntenic]] loci graphically via the web
* [https://www3.nationalgeographic.com/genographic/index.html Genographic Project] {{Webarchive|url=https://web.archive.org/web/20070712035011/https://www3.nationalgeographic.com/genographic/index.html |date=12 July 2007 }}
* [http://ghr.nlm.nih.gov/ghr/chromosomes Home reference on Chromosomes] from the U.S. National Library of Medicine
* [https://web.archive.org/web/20150321091921/http://www.dna-rainbow.org/ Visualisation of human chromosomes] and comparison to other species
* [http://www.rarechromo.org/ Unique – The Rare Chromosome Disorder Support Group] Support for people with rare chromosome disorders

{{Chromosome genetics}}
{{Self-replicating organic structures}}
{{Authority control}}

[[Category:Chromosomes| ]]
[[Category:Nuclear substructures]]
[[Category:Cytogenetics]]