Chromosome
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225515608
2008-07-14T02:36:57Z
R. S. Shaw
102175
/* External links */ cleanup link format
{{for|information about chromosomes in [[genetic algorithm]]s|chromosome (genetic algorithm)}}
{{for|a non-technical introduction to the topic|Introduction to genetics}}
[[Image:Chromosome-upright.png|frame|Diagram of a duplicated and condensed ([[metaphase]]) eukaryotic chromosome. (1) [[Chromatid]] - one of the two identical parts of the chromosome after [[S phase]]. (2) [[Centromere]] - the point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm.]]
'''Chromosomes''' are organized structures of [[DNA]] and [[protein]]s that are found in [[Cell (biology)|cells]]. A chromosome is a singular piece of DNA, which contains many [[gene]]s, [[regulatory sequence|regulatory elements]] and other [[genetic sequence|nucleotide sequences]]. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word ''chromosome'' comes from the [[Greek language|Greek]] {{polytonic|χρῶμα}} (''chroma'', color) and {{polytonic|σῶμα}} (''soma'', body) due to their property of being stained very strongly by some [[dyes]].
Chromosomes vary extensively between different [[organism]]s. The DNA molecule may be circular or linear, and can contain anything from tens of [[kilobase]] pairs to hundreds of [[megabase]] pairs. Typically [[eukaryote|eukaryotic]] cells (cells with nuclei) have large linear chromosomes and [[prokaryotic]] cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example [[mitochondria]] in most [[eukaryotes]] and [[chloroplasts]] in plants have their own small chromosomes.
In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called [[chromatin]]. This allows the very long DNA molecules to fit into the [[cell nucleus]]. The structure of chromosomes and chromatin varies through the [[cell cycle]]. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, while duplicated chromosomes (copied during [[S phase|synthesis phase]]) contain two copies joined by a [[centromere]]. Compaction of the duplicated chromosomes during [[mitosis]] and [[meiosis]] results in the classic four-arm structure (pictured to the right).
"Chromosome" is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a [[plasmid]] or a small chromosome. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest chromosomes are found in [[viruses]]: these DNA or RNA molecules are short linear or circular chromosomes that often lack any structural proteins.
==History==
This is a brief history of research in a complex field where each advance was hard won, and often hotly disputed at the time.
'''Visual discovery of chromosomes'''. Textbooks have often said that chromosomes were first observed in [[plant]] cells by a [[Switzerland|Swiss]] [[botanist]] named [[Karl Wilhelm von Nägeli]] in 1842.<ref>Nägeli C. 1842. ''Zur Entwickelungsgeschichte des Pollens bei den Phanerogamen''. Ovell & Füssli, Zürich.</ref> However, this opinion has been challenged, perhaps decisively, by [[Henry Harris (scientist)|Henry Harris]], who has freshly reviewed the primary literature.<ref>Harris H. 1999. ''The birth of the cell''. Yale University Press. p138</ref> In his opinion the claim of Nägeli to have seen spore mother cells divide is mistaken, as are some of his interpretations. Harris considers other candidates, especially [[Wilhelm Hofmeister]], whose publications in 1848-9 include plates which definitely show mitotic events.<ref>Hofmeister W. 1848. ''Bot. Zeit''. '''6''', cols 425, 649, 670.</ref><ref>Hofmeister W. 1849. ''Die Entstehung des Embryos der Phanerogamen''. Friedrich Hofmeister, Leipzig.</ref> Hofmeister was also the choice of [[Cyril Darlington]].
The work of other cytologists such as [[Walther Flemming]], [[Eduard Strasburger]], [[Otto Bütschli]], [[Oskar Hertwig]] and [[Carl Rabl]] should definitely be acknowledged. The use of [[basophilic]] [[aniline]] [[dye]]s was a new technique for effectively [[staining]] the [[chromatin]] material in the nucleus. Their behavior in animal ([[salamander]]) cells was later described in detail by [[Walther Flemming]], who in 1882 "provided a superb summary of the state of the field".<ref>Mayr E. 1982. ''The growth of biological thought''. Harvard. p677</ref><ref>Flemming W. ''Zellsubstanz Kern und Zelltheilung''. Vogel, Leipzig.</ref> The name ''chromosome'' was invented in 1888 by [[Heinrich Wilhelm Gottfried von Waldeyer-Hartz|Heinrich von Waldeyer]]. However, van Beneden's monograph of 1883 on the fertilised eggs of the parasitic roundworm ''[[Ascaris]] megalocephala'' was the outstanding work of this period.<ref>Van Beneden E. 1883. ''Arch. Biol.'' '''4'''.</ref> His conclusions are classic:
*Thus there is no fusion between the male chromatin and the female chromatin at any stage of division...
*The elements of male origin and those of female origin are never fused together in a cleavage nucleus, and perhaps they remain distinct in all the nuclei derived from them. [tranl: Harris p162]
<blockquote>"It is not easy to identify who first discerned chromosomes during mitosis, but there is no doubt that those who first saw them had no idea of their significance... [but] with the work of [[Edouard-Gérard Balbiani|Balbiani]] and [[Edouard Van Beneden|van Beneden]] we move away from... the mechanism of cell division to a precise delineation of chromosomes and what they do during the division of the cell." <ref>Harris H. 1999. ''The birth of the cell''. Yale University Press. p141, 153</ref></blockquote>
Van Beneden's master work was closely followed by that of Carl Rabl, who reached similar conclusions. <ref>Rabl C. 1885. ''Morphol. Jahrb''. '''10''', 24.</ref> This more or less concludes the first period, in which chromosomes were visually sighted, and the morphological stages of mitosis were described. Coleman also gives a useful review of these discoveries.<ref>Coleman W. 1965. Cell, nucleus, and inheritance: an historical discovery. ''Proc. Am. Philos. Soc''. '''109''', 124-158.</ref>
'''Nucleus as the seat of heredity'''. The origin of this epoch-making idea lies in a few sentences tucked away in Ernst Haeckel's ''Generelle Morphologie'' of 1866.<ref>Haeckel E. 1866. ''Generelle Morphologie der Organismen: Allgemeine Gründzuge der organischen Formen-Wissenschaft''. 2 vols, Reimer, Berlin.</ref> The evidence for this insight gradually acumulated until, after twenty or so years, two of the greatest in a line of great German scientists spelt it out. [[August Weismann]] proposed that the [[germ line]] was separate from the [[somatic cell|soma]], and that the cell nucleus was the repository of the hereditary material, which he proposed was arranged along the chromosomes in a linear manner. Furthermore, he proposed that at fertilisation a new combination of chromosomes (and their hereditary material) would be formed. This was the explanation for the reduction division of [[meiosis]] (first described by van Beneden).
'''Chromosomes as vectors of heredity'''. In a series of outstanding experiments, [[Theodor Boveri]] gave the definitive demonstration that chromosomes were the vectors of heredity. His two principles were:
:The continuity of chromosomes
:The individuality of chromosomes.
It was the second of these principles which was so original. He was able to test the proposal put forward by [[Wilhelm Roux]], that each chromosome carries a different genetic load, and showed that Roux was right. Upon the rediscovery of Mendel, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. It is interesting to see that Boveri influenced two generations of American cytologists: [[Edmund Beecher Wilson]], [[Walter Sutton]] and [[Theophilus Painter]] were all influenced by Boveri (Wilson and Painter actually worked with him). In his famous textbook ''The Cell'', Wilson linked Boveri and Sutton together by the [[Boveri-Sutton Chromosome Theory|Boveri-Sutton theory]]. Mayr remarks that the theory was hotly contested by some famous geneticists: [[William Bateson]], [[Wilhelm Johannsen]], [[Richard Goldschmidt]] and [[T.H. Morgan]], all of a rather dogmatic turn of mind. Eventually complete proof came from chromosome maps – in Morgan's own lab! <ref>Mayr E. 1982. ''The growth of biological thought''. Harvard. p749</ref>
==Chromosomes in eukaryotes==
{{Mergefrom|Eukaryotic chromosome fine structure|date=December 2007}}
[[Eukaryotes]] ([[cell (biology)|cells]] with nuclei such as plants, yeast, 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 [[mitochondria]]l genome, and some eukaryotes may have additional small circular or linear [[cytoplasm]]ic chromosomes.
In the nuclear chromosomes of [[eukaryote]]s, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around [[histone]]s (structural [[protein]]s), forming a composite material called [[chromatin]].
===Chromatin===
{{Main|Chromatin}}
[[Image:Chromatin Structures.png|thumb|right|350px|'''Fig. 2:''' The major structures in DNA compaction; [[DNA]], the [[nucleosome]], the 10nm "beads-on-a-string" fibre, the 30nm fibre and the [[metaphase]] chromosome.]]
Chromatin is the complex of DNA and protein found in the [[eukaryote|eukaryotic]] nucleus which packages chromosomes. The structure of chromatin varies significantly between different stages of the [[cell cycle]], according to the requirements of the DNA.
====Interphase chromatin====
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., 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.
Individual chromosomes cannot be distinguished at this stage - they appear in the nucleus as a homogeneous tangled mix of DNA and protein.
====Metaphase chromatin and division====
{{seealso|mitosis|meiosis}}
<!-- too many pictures! [[Image:ChickenChromosomesBMC Genomics5-56Fig4.jpg|thumb|200px|right|Chicken chromosomes during [[metaphase]], with highlighted [[locus (genetics)|genetic loci]]]] -->
[[Image:HumanChromosomesChromomycinA3.jpg|thumb|200px|right|Human chromosomes during [[metaphase]].]]
In the early stages of mitosis or meiosis (cell division), the chromatin strands become more and more condensed. They cease to function as accessible genetic material ([[Transcription (genetics)|transcription]] stops) and become a compact transportable form. This compact form makes the individual chromosomes visible, and they form the classic four arm structure, a pair of sister [[chromatids]] attached to each other at the [[centromere]]. The shorter arms are called ''p arms'' (from the [[French language|French]] ''petit'', small) and the longer arms are called ''q arms'' (''q'' follows ''p'' in the Latin alphabet). This is the only natural context in which individual chromosomes are visible with an optical [[microscope]].
During divisions long [[microtubule]]s attach to the centromere and the two opposite ends of the cell. The microtubules then pull the chromatids apart, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and can function again as chromatin. In spite of their appearance, chromosomes are structurally highly condensed which enables these giant DNA structures to be contained within a cell nucleus (Fig. 2).
The self assembled microtubules form the spindle, which attaches to chromosomes at specialized structures called kinetochores, 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.
==Chromosomes in prokaryotes==
The prokaryotes - [[bacteria]] and [[archaea]] - typically have a single circular chromosome, but many variations do exist.<ref>{{cite journal |author=Thanbichler M, Shapiro L |title=Chromosome organization and segregation in bacteria |journal=J. Struct. Biol. |volume=156 |issue=2 |pages=292–303 |year=2006 |pmid=16860572 |doi=10.1016/j.jsb.2006.05.007}}</ref> Most bacteria have a single circular chromosome that can range in size from only 160,000 [[base pair]]s in the [[endosymbiont|endosymbiotic]] bacteria ''[[Candidatus Carsonella ruddii]]'',<ref>{{cite journal |author=Nakabachi A, Yamashita A, Toh H, Ishikawa H, Dunbar H, Moran N, Hattori M |title=The 160-kilobase genome of the bacterial endosymbiont Carsonella |journal=Science |volume=314 |issue=5797 |pages=267 |year=2006 |pmid=17038615 |doi=10.1126/science.1134196}}</ref> to 12,200,000 base pairs in the soil-dwelling bacteria ''[[Sorangium cellulosum]]''.<ref>{{cite journal |author=Pradella S, Hans A, Spröer C, Reichenbach H, Gerth K, Beyer S |title=Characterisation, genome size and genetic manipulation of the myxobacterium Sorangium cellulosum So ce56 |journal=Arch Microbiol |volume=178 |issue=6 |pages=484–92 |year=2002 |pmid=12420170 | doi = 10.1007/s00203-002-0479-2 <!--Retrieved from PMID by DOI bot-->}}</ref> [[Spirochaete]]s of the [[genus]] ''Borrelia'' are a notable exception to this arrangement, with bacteria such as ''[[Borrelia burgdorferi]]'', the cause of [[Lyme disease]], containing a single linear chromosome.<ref>{{cite journal |author=Hinnebusch J, Tilly K |title=Linear plasmids and chromosomes in bacteria |journal=Mol Microbiol |volume=10 |issue=5 |pages=917–22 |year=1993|pmid = 7934868 |doi=10.1111/j.1365-2958.1993.tb00963.x}}</ref>
===Structure in sequences===
Prokaryotes chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a single point (the [[origin of replication]]) from which replication starts, while some archaea contain multiple replication origins.<ref>{{cite journal |author=Kelman LM, Kelman Z |title=Multiple origins of replication in archaea |journal=Trends Microbiol. |volume=12 |issue=9 |pages=399–401 |year=2004 |pmid=15337158 |doi=10.1016/j.tim.2004.07.001}}</ref> The genes in prokaryotes are often organised in [[operons]], and do not 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 |author=Thanbichler M, Wang SC, Shapiro L |title=The bacterial nucleoid: a highly organized and dynamic structure |journal=J. Cell. Biochem. |volume=96 |issue=3 |pages=506–21 |year=2005 |pmid=15988757 | doi = 10.1002/jcb.20519 <!--Retrieved from PMID by DOI bot-->}}</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 |author=Sandman K, Pereira SL, Reeve JN |title=Diversity of prokaryotic chromosomal proteins and the origin of the nucleosome |journal=Cell. Mol. Life Sci. |volume=54 |issue=12 |pages=1350–64 |year=1998 |pmid=9893710 |doi=10.1007/s000180050259}}</ref> In [[archaea]], the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.<ref>{{cite journal |author=Sandman K, Reeve JN |title=Structure and functional relationships of archaeal and eukaryal histones and nucleosomes | doi = 10.1007/s002039900122 <!--Retrieved from Yahoo! by DOI bot-->|journal=Arch. Microbiol. |volume=173 |issue=3 |pages=165–9 |year=2000 |pmid=10763747}}</ref><ref>{{cite journal |author=Pereira SL, Grayling RA, Lurz R, Reeve JN |title=Archaeal nucleosomes | doi = 10.1073/pnas.94.23.12633 <!--Retrieved from URL by DOI bot-->|journal=Proc. Natl. Acad. Sci. U.S.A. |volume=94 |issue=23 |pages=12633–7 |year=1997 |pmid=9356501 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=9356501}}</ref>
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 [[Mechanical properties of DNA#Supercoiled DNA|supercoiled]]. The DNA must first be released into its relaxed state for access for [[Transcription (genetics)|transcription]], regulation, and [[DNA replication|replication]].
==Number of chromosomes in various organisms==
{{main|List of number of chromosomes of various organisms}}
===Eukaryotes===
These tables give the total number of chromosomes (including sex chromosomes) in a cell nucleus. For example, human cells are [[diploid]] and have 22 different types of [[autosome]]s, each present as two copies, and two [[sex chromosomes]]. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosomes, such as [[Bread wheat]] which is ''hexaploid'' and has six copies of seven different chromosomes - 42 chromosomes in total.
{| style="float:right;"|}
|
{| class="wikitable" style="margin:1em 0 1em 1em"
|+ Chromosome numbers in some plants
|-
! Plant Species !! #
|-
| ''[[Arabidopsis thaliana]]'' (diploid)<ref>{{cite journal |author=Armstrong SJ, Jones GH |title=Meiotic cytology and chromosome behaviour in wild-type Arabidopsis thaliana |journal=J. Exp. Bot. |volume=54 |issue=380 |pages=1–10 |year=2003 |month=January |pmid=12456750 |url=http://jexbot.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=12456750 |doi=10.1093/jxb/54.380.1}}</ref> || 10
|-
| [[Rye]] (diploid)<ref>{{cite journal |author=Gill BS, Kimber G |title=The Giemsa C-banded karyotype of rye |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=71 |issue=4 |pages=1247–9 |year=1974 |month=April |pmid=4133848 |url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=388202&blobtype=pdf |doi=10.1073/pnas.71.4.1247}}</ref> || 14
|-
| [[Maize]] (diploid)<ref>{{cite journal |author=Kato A, Lamb JC, Birchler JA |title=Chromosome painting using repetitive DNA sequences as probes for somatic chromosome identification in maize |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=101 |issue=37 |pages=13554–9 |year=2004 |month=September |pmid=15342909 |doi=10.1073/pnas.0403659101 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15342909}}</ref> || 20
|-
| [[Einkorn wheat]] (diploid)<ref name=Dubcovsky>{{cite journal |author=Dubcovsky J, Luo MC, Zhong GY, ''et al'' |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 |year=1996 |pmid=8725244 |url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1207354&blobtype=pdf}}</ref> || 14
|-
| [[Durum wheat]] (tetraploid)<ref name=Dubcovsky/> || 28
|-
| [[Bread wheat]] (hexaploid)<ref name=Dubcovsky/> || 42
|-
| [[Potato]] (tetraploid)<ref name=Ellison1935>{{cite journal | author = Ellison, W. | year = 1935 | title = A study of the chromosome numbers and morphology in certain British varieties of the common cultivated potato (solanum buberosum L.) | journal = Genetica | volume = 17 | issue = 1 | pages = 1–26 | url = http://www.springerlink.com/index/V8437QN6316363UX.pdf | accessdate = 2008-05-11 | doi = 10.1007/BF01984179}}</ref> || 48
|-
| Cultivated tobacco (diploid)<ref>{{cite journal |author=Kenton A, Parokonny AS, Gleba YY, Bennett MD |title=Characterization of the Nicotiana tabacum L. genome by molecular cytogenetics |journal=Mol. Gen. Genet. |volume=240 |issue=2 |pages=159–69 |year=1993 |month=August |pmid=8355650 |doi=10.1007/BF00277053}}</ref> || 48
|-
| [[Ophioglossum|Adder's Tongue Fern]] (diploid)<ref>{{cite journal |author=Leitch IJ, Soltis DE, Soltis PS, Bennett MD |title=Evolution of DNA amounts across land plants (embryophyta) |journal=Ann. Bot. |volume=95 |issue=1 |pages=207–17 |year=2005 |pmid=15596468 |url=http://aob.oxfordjournals.org/cgi/content/full/95/1/207 | doi = 10.1093/aob/mci014 <!--Retrieved from url by DOI bot-->}}</ref> || approx 1,440
|}
|
{| class="wikitable" style="margin:1em 0 1em 1em"
|+ Chromosome numbers (2n) in some animals
|-
! Species !! # !! Species !! #
|-
| [[Drosophila melanogaster|Common fruit fly]] || 8
| [[Guinea Pig]]<ref>{{cite journal |author=Umeko Semba, Yasuko Umeda, Yoko Shibuya, Hiroaki Okabe, Sumio Tanase and Tetsuro Yamamoto |title=Primary structures of guinea pig high- and low-molecular-weight kininogens | doi = 10.1016/j.intimp.2004.06.003 <!--Retrieved from URL by DOI bot-->|journal=International Immunopharmacology |volume=4 |issue=10-11 |pages=1391–1400 |year=2004 |url=http://www.sciencedirect.com/science/article/B6W7N-4CX6PRC-1/2/97487fdf611a04e3c88690da9e6d853b}}</ref>|| 64
|-
| [[Dove]]{{Fact|date=February 2007}} || 16<!-- taxon? -->
| [[Garden snail]]<ref>{{cite journal |author=Vitturi R, Libertini A, Sineo L, ''et al'' |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
|-
| [[Earthworm]] ''Octodrilus complanatus''<ref>{{cite journal |author=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 | doi = 10.1093/jhered/93.4.279 <!--Retrieved from URL by DOI bot-->|journal=J. Hered. |volume=93 |issue=4 |pages=279–82 |year=2002 |pmid=12407215 |url=http://jhered.oxfordjournals.org/cgi/content/full/93/4/279}}</ref> || 36
| [[Tibetan fox]] || 36
|-
| [[Domestic cat]]<ref>{{cite journal |author=Nie W, Wang J, O'Brien PC, ''et al'' |title=The genome phylogeny of domestic cat, red panda and five mustelid species revealed by comparative chromosome painting and G-banding |journal=Chromosome Res. |volume=10 |issue=3 |pages=209–22 |year=2002 |pmid=12067210 |doi=10.1023/A:1015292005631}}</ref> || 38
| [[Domestic pig]] || 38
|-
| [[Lab mouse]] || 40
| [[Lab rat]] || 42
|-
| [[Rabbit]]{{Fact|date=February 2007}} || 44<!-- taxon? -->
| [[Syrian hamster]] || 44
|-
| [[Hare]]{{Fact|date=February 2007}} || 46<!-- taxon? -->
| [[Human]]<ref name=Grouchy/> ||46
|-
| [[Gorilla]]s<!--both species-->, [[Chimpanzee]]s<!--both species--><ref name=Grouchy>{{cite journal |author=De Grouchy J |title=Chromosome phylogenies of man, great apes, and Old World monkeys |journal=Genetica |volume=73 |issue=1-2 |pages=37–52 |year=1987 |pmid=3333352}}</ref> || 48
| [[Domestic sheep]] || 54
|-
| [[Elephant]]s<ref>{{cite journal |author=Houck ML, Kumamoto AT, Gallagher DS, Benirschke K |title=Comparative cytogenetics of the African elephant (Loxodonta africana) and Asiatic elephant (Elephas maximus) |journal=Cytogenet. Cell Genet. |volume=93 |issue=3-4 |pages=249–52 |year=2001 |pmid=11528120 |doi=10.1159/000056992}}</ref> || 56<!-- taxon? -->
| [[Cattle|Cow]] || 60
|-
| [[Donkey]] || 62
| [[Horse]] || 64
|-
| [[Dog]]<ref>{{cite journal |author=Wayne RK, Ostrander EA |title=Origin, genetic diversity, and genome structure of the domestic dog |journal=Bioessays |volume=21 |issue=3 |pages=247–57 |year=1999 |pmid=10333734 | doi = 10.1002/(SICI)1521-1878(199903)21:3 <!--Retrieved from PMID by DOI bot-->}}</ref> || 78
| [[Kingfisher]]<ref>{{cite journal |author=Burt DW |title=Origin and evolution of avian microchromosomes |journal=Cytogenet. Genome Res. |volume=96 |issue=1-4 |pages=97–112 |year=2002 |pmid=12438785 |doi=10.1159/000063018}}</ref> || 132
|-
| [[Goldfish]]<ref>{{cite journal |author=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 |year=2002 |pmid=12116377 | doi = 10.1002/cyto.10100 <!--Retrieved from PMID by DOI bot-->}}</ref> || 100-104
| [[Bombyx mori|Silkworm]]<ref>{{cite journal |author=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 |year=2006 |pmid=16547103 |doi=10.1534/genetics.106.055541}}</ref> || 56
|}
|-
| colspan="2" |
{| class="wikitable" style="float:right; margin:1em 0 1em 1em"
|+ Chromosome numbers in other organisms
|-
! Species !! Large<br />Chromosomes !! Intermediate<br />Chromosomes !! Small<br />Chromosomes
|-
| ''[[Trypanosoma brucei]]'' || 11 || 6 || ~100
|-
| [[Chicken]]<ref>{{cite journal |author=Smith J, Burt DW |title=Parameters of the chicken genome (Gallus gallus) |journal=Anim. Genet. |volume=29 |issue=4 |pages=290–4 |year=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 (see the table). 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.
[[Image:PLoSBiol3.5.Fig1bNucleus46Chromosomes.jpg|thumb|200px|left|The 24 human chromosome territories during prometaphase in [[fibroblast]] cells.]]
[[Asexual reproduction|Asexually reproducing]] species have one set of chromosomes, which is the same in all body cells.
[[sexual reproduction|Sexually reproducing]] species have [[somatic cell]]s (body cells), which are [[ploidy#diploid|diploid]] [2n] having two sets of chromosomes, one from the mother and one from the father. [[Gamete]]s, reproductive cells, are [[ploidy#haploid and monoploidy|haploid]] [n]: they have one set of chromosomes. Gametes are produced by [[meiosis]] of a diploid [[germ line]] cell. During meiosis, 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 ([[fertilization]]), a new diploid organism is formed.
Some animal and plant species are [[ploidy#polyploid|polyploid]] [Xn]: they have more than two sets of [[homologous chromosome]]s. Agriculturally important plants 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 [[pasta]] and [[bread]] wheats are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes compared to the 14 (diploid) chromosomes in the wild wheat.<ref>Sakamura, T. (1918), ''Kurze Mitteilung uber die Chromosomenzahlen und die Verwandtschaftsverhaltnisse der Triticum-Arten''. Bot. Mag., 32: 151-154.</ref>
===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]]'', a [[symbiosis|symbiont]] of [[aphid]]s has multiple copies of its chromosome, ranging from 10–400 copies per cell.<ref>{{cite journal |author=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 Biochem. Mol. Biol. |volume=30 |issue=3 |pages=253–8 |year=2000 |month=March |pmid=10732993 |doi=10.1016/S0965-1748(99)00125-3}}</ref> However, in some large bacteria, such as ''[[Epulopiscium fishelsoni]]'' up to 100,000 copies of the chromosome can be present.<ref>{{cite journal |author=Mendell JE, Clements KD, Choat JH, Angert ER |title=Extreme polyploidy in a large bacterium |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=18 |pages=6730–4 |year=2008 |month=May |pmid=18445653 |doi=10.1073/pnas.0707522105 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=18445653}}</ref> Plasmids and plasmid-like small chromosomes are, like in eukaryotes, very variable in copy number. The number of plasmids in the cell is almost entirely determined by the rate of division of the plasmid - fast division causes high copy number, and vice versa.
==Karyotype==
{{main|Karyotype}}
[[Image:NHGRI human male karyotype.png|thumb|right|200px|'''Figure 3''': Karyogram of a human male]]
In general, the '''karyotype''' is the characteristic chromosome complement of a [[eukaryote]] [[species]].<ref>White M.J.D. 1973. ''The chromosomes''. 6th ed, Chapman & Hall, London. p28</ref> The preparation and study of karyotypes is part of [[cytogenetics]].
Although the [[replication]] and [[transcription]] of [[DNA]] is highly standardized in [[eukaryotes]], ''the same cannot be said for their karotypes'', 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 variation 1. between the two sexes. 2. between the [[germ-line]] and [[soma]] (between [[gametes]] and the rest of the body). 3. 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. Finally, variation in karyotype may occur during development from the fertilised 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 XY) at the end: Fig. 3.
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]]."--->
==== Historical note ====
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]] mechanism.<ref>von Winiwarter H. 1912. Études sur la spermatogenese humaine. ''Arch. biologie'' '''27''', 93, 147-9.</ref> [[Theophilus Painter|Painter]] in 1922 was not certain whether the diploid number of man was 46 or 48, at first favouring 46.<ref>Painter T.S. 1922. The spermatogenesis of man. ''Anat. Res.'' '''23''', 129.</ref> He revised his opinion later from 46 to 48, and he correctly insisted on man having an [[XY sex-determination system|XX/XY]] system.<ref>Painter T.S. 1923. Studies in mammalian spermatogenesis II. The spermatogenesis of man. ''J. Exp. Zoology'' '''37''', 291-336.</ref>
New techniques were needed to definitively solve the problem:
:1. Using cells in culture
:2. Pretreating cells in a [[Tonicity#Hypotonicity|hypotonic solution]], which swells them and spreads the chromosomes
:3. Arresting [[mitosis]] in [[metaphase]] by a solution of [[colchicine]]
:4. Squashing the preparation on the slide forcing the chromosomes into a single plane
:5. Cutting up a photomicrograph and arranging the result into an indisputable karyogram.
It took until the mid 1950s until it became generally accepted that the human karyotype included only 46 chromosomes. Considering the techniques of Winiwarter and Painter, their results were quite remarkable.<ref>Tjio J.H & Levan A. 1956. The chromosome number of man. ''Hereditas'' '''42''', 1-6.</ref><ref>Hsu T.C. ''Human and mammalian cytogenetics: a historical perspective''. Springer-Verlag, N.Y. p10: "It's amazing that he [Painter] even came close!"</ref> Chimpanzees (the closest living relatives to modern humans) have 48 chromosomes.
==Chromosomal aberrations==
{{main|Chromosome abnormalities|aneuploidy}}
[[Image:Single Chromosome Mutations.png|thumb|right|The three major single chromosome mutations; deletion (1), duplication (2) and inversion (3).]]
[[Image:Two Chromosome Mutations.png|thumb|right|The two major two-chromosome mutations; insertion (1) and translocation (2).]]
[[Image:Chromosome 21.gif|In Down syndrome, there are three copies of chromosome 21|thumb]]
Chromosomal aberrations are disruptions in the normal chromosomal content of a cell, and are a major cause of genetic conditions in humans, such as [[Down syndrome]]. Some chromosome abnormalities do not cause disease in carriers, such as [[translocations]], or [[chromosomal inversions]], although they may lead to a higher chance of having a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets, [[aneuploidy]], may be lethal or give rise to genetic disorders. [[Genetic counseling]] is offered for families that may carry a chromosome rearrangement.
The gain or loss of chromosome material can lead to a variety of [[genetic disorders]]. Human examples include:
* [[Cri du chat]], which is 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, and the condition was so-named because affected babies make high-pitched cries that sound like a cat. Affected individuals have wide-set eyes, a small head and jaw and are moderately to severely mentally retarded and very short.
* [[Wolf-Hirschhorn syndrome]], which is caused by partial deletion of the short arm of chromosome 4. It is characterized by severe growth retardation and severe to profound mental retardation.
* [[Down's syndrome]], usually is 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 mental retardation.<ref>{{cite book|last=Miller|first=Kenneth R.|title=Biology|publisher=Prentice Hall|Location=Upper Saddle River, New Jersey|date=2000|edition=5|pages=194-195|chapter=9-3|isbn=0-13-436265-9|accessdate=2008-03-31|language=English|=Add citation}}</ref>
* [[Edwards syndrome]], which is the second most common trisomy after Down syndrome. It is a trisomy of chromosome 18. Symptoms include mental and motor retardation and numerous congenital anomalies causing serious health problems. Ninety percent die in infancy; however, those who live past their first birthday usually are quite healthy thereafter. They have a characteristic hand appearance with clenched hands and overlapping fingers.
* [[Patau Syndrome]], also called D-Syndrome or trisomy-13. Symptoms are somewhat similar to those of trisomy-18, but they do not have the characteristic hand shape.
* [[Idic15]], abbreviation for Isodicentric 15 on chromosome 15; also called the following names due to various researches, but they all mean the same; IDIC(15), Inverted dupliction 15, extra Marker, Inv dup 15, partial tetrasomy 15
* [[Jacobsen syndrome]], also called the terminal 11q deletion disorder.<ref>http://www.11q.org</ref> This is a very rare disorder. Those affected have normal intelligence or mild mental retardation, with poor expressive language skills. Most have a bleeding disorder called [[Paris-Trousseau syndrome]].
* [[Klinefelter's syndrome]] (XXY). Men with Klinefelter syndrome are usually sterile, and tend to have longer arms and legs and to be taller than their peers. Boys with the syndrome are often shy and quiet, and have a higher incidence of speech delay and [[dyslexia]]. During puberty, without testosterone treatment, some of them may develop [[gynecomastia]].
* [[Turner syndrome]] (X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. People with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development and a "caved-in" appearance to the chest.
* [[XYY syndrome]]. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are somewhat more likely to have learning difficulties.
* [[Triple-X syndrome]] (XXX). XXX girls tend to be tall and thin. They have a higher incidence of dyslexia.
* [[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 15|isodicentric chromosome 15 syndrome]] (or Idic15) are both caused by a supernumerary marker chromosome, as is [[Pallister-Killian syndrome]].
Chromosomal mutations produce changes in whole chromosomes (more than one gene) or in the number of chromosomes present.
* Deletion - loss of part of a chromosome
* Duplication - extra copies of a part of a chromosome
* Inversion - reverse the direction of a part of a chromosome
* Translocation - part of a chromosome breaks off and attaches to another chromosome
Most mutations are neutral - have little or no effect
A detailed graphical display of all human chromosomes and the diseases annotated at the correct spot may be found at<ref>http://www.ornl.gov/hgmis/posters/chromosome/</ref>.
==Human chromosomes==
Human cells have 23 pairs of large linear nuclear 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 Genome Annotation (VEGA) database.<ref>http://vega.sanger.ac.uk/Homo_sapiens/index.html All data in this table was derived from this database, July 7 2007.</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 !! [[Genes]] !! Total [[Nucleobase|bases]] !! Sequenced bases<ref>Sequenced percentages are based on fraction of euchromatin portion, as the [[Human Genome Project]] goals called for determination of only the [[euchromatin|euchromatic]] portion of the genome. [[Telomere]]s, [[centromere]]s, and other [[heterochromatin|heterochromatic]] regions have been left undetermined, as have a small number of unclonable gaps. See http://www.ncbi.nlm.nih.gov/genome/seq/ for more information on the Human Genome Project.</ref>
|-
| [[Chromosome 1 (human)|1]] || 3,148 || 247,200,000 || 224,999,719
|-
| [[Chromosome 2 (human)|2]] || 902 || 242,750,000 || 237,712,649
|-
| [[Chromosome 3 (human)|3]] || 1,436 || 199,450,000 || 194,704,827
|-
| [[Chromosome 4 (human)|4]] || 453 || 191,260,000 || 187,297,063
|-
| [[Chromosome 5 (human)|5]] || 609 || 180,840,000 || 177,702,766
|-
| [[Chromosome 6 (human)|6]] || 1,585 || 170,900,000 || 167,273,992
|-
| [[Chromosome 7 (human)|7]] || 1,824 || 158,820,000 || 154,952,424
|-
| [[Chromosome 8 (human)|8]] || 781 || 146,270,000 || 142,612,826
|-
| [[Chromosome 9 (human)|9]] || 1,229 || 140,440,000 || 120,312,298
|-
| [[Chromosome 10 (human)|10]] || 1,312 || 135,370,000 || 131,624,737
|-
| [[Chromosome 11 (human)|11]] || 405 || 134,450,000 || 131,130,853
|-
| [[Chromosome 12 (human)|12]] || 1,330 || 132,290,000 || 130,303,534
|-
| [[Chromosome 13 (human)|13]] || 623 || 114,130,000 || 95,559,980
|-
| [[Chromosome 14 (human)|14]] || 886 || 106,360,000 || 88,290,585
|-
| [[Chromosome 15 (human)|15]] || 676 || 100,340,000 || 81,341,915
|-
| [[Chromosome 16 (human)|16]] || 898 || 88,820,000 || 78,884,754
|-
| [[Chromosome 17 (human)|17]] || 1,367 || 78,650,000 || 77,800,220
|-
| [[Chromosome 18 (human)|18]] || 365 || 76,120,000 || 74,656,155
|-
| [[Chromosome 19 (human)|19]] || 1,553 || 63,810,000 || 55,785,651
|-
| [[Chromosome 20 (human)|20]] || 816 || 62,440,000 || 59,505,254
|-
| [[Chromosome 21 (human)|21]] || 446 || 46,940,000 || 34,171,998
|-
| [[Chromosome 22 (human)|22]] || 595 || 49,530,000 || 34,893,953
|-
| [[X chromosome|X (sex chromosome)]] || 1,093 || 154,910,000 || 151,058,754
|-
| [[Y chromosome|Y (sex chromosome)]] || 125 || 57,740,000 || 22,429,293
|}
==See also==
* [[Locus (genetics)|Locus]] (explains gene location nomenclature)
* [[Sex-determination system]]
** [[XY sex-determination system]]
*** [[X chromosome]]
**** [[X-inactivation]]
*** [[Y chromosome]]
**** [[Y-chromosomal Adam]]
**** [[Y-chromosomal Aaron]]
* [[Genetic genealogy]]
** [[Genealogical DNA test]]
* [[Deletion (genetics)|Genetic deletion]]
* [[List of number of chromosomes of various organisms]]
==External links==
* [http://atlasgeneticsoncology.org/Educ/PolyMecaEng.html Chromosome Abnormalities at AtlasGeneticsOncology]
* [http://gslc.genetics.utah.edu/units/disorders/karyotype/ What Can Our Chromosomes Tell Us?], from the University of Utah's Genetic Science Learning Center
* [http://gslc.genetics.utah.edu/units/disorders/karyotype/karyotype.cfm 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]
* [http://www.chromosomehelpstation.com/eurochromnet.htm Eurochromnet], European network for Rare Chromosome Disorders on the Internet
* http://www.ensembl.org [[Ensembl]] project, presenting chromosomes, their [[gene]]s and [[synteny|syntenic]] loci graphically via the web
* [https://www3.nationalgeographic.com/genographic/index.html Genographic Project]
* [http://ghr.nlm.nih.gov/ghr/chromosomes Home reference on Chromosomes] from the U.S. National Library of Medicine
==References==
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{{chromo}}
[[Category:Chromosomes| ]]
[[Category:Nuclear substructures]]
[[Category:Cytogenetics]]
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