C-value: Difference between revisions

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{{short description|C-value is the amount, in picograms, of DNA contained within a haploid nucleus}}

{{about|the term '''C-value''' in cell biology|the tool used by [[architects]] and [[engineers]] to calculate lines-of-sight by spectators in theaters and stadiums|sightline}}

{{short description|C-value is the amount, in picograms, of DNA contained within a haploid nucleus}}

'''C-value''' is the amount, in [[picogram]]s, of [[DNA]] contained within a [[haploid]] [[Cell nucleus|nucleus]] (e.g. a [[gamete]]) or one half the amount in a [[diploid]] [[somatic cell]] of a [[eukaryotic]] organism. In some cases (notably among diploid organisms), the terms C-value and [[genome size]] are used interchangeably; however, in [[Polyploidy|polyploids]] the C-value may represent two or more [[genome]]s contained within the same nucleus. Greilhuber ''et al.''<ref name="Greilhuber2005">{{cite journal |vauthors=Greilhuber J, Doležel J, Lysák M, Bennett MD |year=2005 |title=The origin, evolution and proposed stabilization of the terms 'genome size' and 'C-value' to describe nuclear DNA contents |journal=Annals of Botany |volume=95 |doi=10.1093/aob/mci019 |pmid=15596473 |issue=1 |pages=255–60|pmc=4246724 }}</ref> have suggested some new layers of terminology and associated abbreviations to clarify this issue, but these somewhat complex additions are yet to be used by other authors.

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The '''C-value enigma''' or '''C-value paradox''' is the complex puzzle surrounding the extensive variation in nuclear [[genome size]] among [[eukaryotic]] species. At the center of the C-value enigma is the observation that genome size does not correlate with organismal complexity; for example, some single-celled [[protozoa|protists]] have genomes much larger than that of [[humans]].

Some prefer the term C-value enigma because it explicitly includes all of the questions that will need to be answered if a complete understanding of [[genome size]] [[evolution]] is to be achieved (Gregory 2005). Moreover, the term [[paradox]] implies a lack of understanding of one of the most basic features of eukaryotic genomes: namely that they are composed primarily of [[non-coding DNA]]. Some have claimed that the term paradox also has the unfortunate tendency to lead authors to seek simple one-dimensional solutions to what is, in actuality, a multi-faceted puzzle.<ref name="kew" /> For these reasons, in 2003 the term "C-value enigma" was endorsed in preference to "C-value paradox" at the Second Plant Genome Size Discussion Meeting and Workshop at the [[Royal Botanic Gardens, Kew]], [[United Kingdom|UK]],<ref name=kew>{{Cite web |url=http://www.rbgkew.org.uk/cval/pgsm/index.html# |title=Second Plant Genome Size Discussion Meeting and Workshop |access-date=2015-04-19 |archive-url=https://web.archive.org/web/20081201130244/http://www.rbgkew.org.uk/cval/pgsm/index.html# |archive-date=2008-12-01 |dead-url-status=yesdead |df= }}</ref>

and an increasing number of authors have begun adopting this term.

=== C-value paradox ===

In 1948, Roger and Colette Vendrely reported a "remarkable constancy in the nuclear DNA content of all the cells in all the individuals within a given animal species",<ref>{{cite journal |vauthors=Vendrely R, Vendrely C|year=1948 |title=La teneur du noyau cellulaire en acide désoxyribonucléique à travers les organes, les individus et les espèces animales: Techniques et premiers résultats |journal=Experientia |volume=4 |pages=434–436 |doi=10.1007/bf02144998 |pmid=18098821 |issue=11}}</ref> which they took as evidence that [[DNA]], rather than [[protein]], was the substance of which [[genes]] are composed. The term C-value reflects this observed constancy. However, it was soon found that C-values ([[genome size]]s) vary enormously among species and that this bears no relationship to the ''presumed'' number of genes (''as reflected by'' the [[complexity]] of the [[organism]]).<ref name=“Ancestor”"Ancestor">{{cite book |title=[[The Ancestor’s Tale]] |isbn=978-0544859937 |last1=[[Richard Dawkins|Dawkins]] |first1=Richard |last2=Wong |first2=Yan |year=2016 }}</ref> For example, the [[Somatic cells|cells]] of some [[salamanders]] may contain 40 times more DNA than those of humans.<ref name="Gregory, T.R. (2013). Animal Genome Size Database">{{cite web|title=Animal Genome Size Database|url=http://www.genomesize.com/statistics.php?stats=amphibs|accessdate=14 May 2013}}</ref> Given that C-values were assumed to be constant because genetic information is encoded by DNA, and yet bore no relationship to presumed gene number, this was understandably considered [[paradox]]ical; the term "C-value paradox" was used to describe this situation by C.A. Thomas, Jr. in 1971.

The discovery of [[non-coding DNA]] in the early 1970s resolved the main question of the C-value paradox: [[genome size]] does not reflect [[gene]] number in [[eukaryotes]] since most of their DNA is non-coding and therefore does not consist of genes. The [[human genome]], for example, comprises less than 2% protein-coding regions, with the remainder being various types of non-coding DNA (especially [[transposable elements]]).<ref>{{Cite journal