C-value: Difference between revisions

Revision as of 18:37, 2 January 2021

Revision as of 22:09, 13 January 2021

Line 1:

{{aboutShort 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.

Line 9:

== Variation among species ==

C-values vary enormously among species. In animals they range more than 3,300-fold, and in land plants they differ by a factor of about 1,000.<ref name="Bennett2005"/><ref name="Gregory2005">{{cite book |author=Gregory TRT.R. |year=2005 |chapter=Genome size evolution in animals |title=The Evolution of the Genome |editor=T.R. Gregory |pages=3–87 |publisher=Elsevier |___location=San Diego|title-link=The Evolution of the Genome }}</ref> [[Protist]] genomes have been reported to vary more than 300,000-fold in size, but the high end of this range ([[Amoeba (genus)|''Amoeba'']]) has been called into question. Variation in C-values bears no relationship to the complexity of the organism or the number of [[genes]] contained in its genome; for example, some single-celled [[protozoa|protists]] have genomes much larger than that of [[humans]]. This observation was deemed counterintuitive before the discovery of [[non-coding DNA]]. It became known as the C-value paradox as a result. However, although there is no longer any [[paradox]]ical aspect to the discrepancy between C-value and gene number, this term remains in common usage. For reasons of conceptual clarification, the various puzzles that remain with regard to genome size variation instead have been suggested to more accurately comprise a complex but clearly defined puzzle known as the C-value enigma. C-values correlate with a range of features at the [[Cell (biology)|cell]] and organism levels, including [[cell size]], [[cell division]] rate, and, depending on the [[taxon]], body size, [[metabolic rate]], developmental rate, [[Organ (anatomy)|organ]] complexity, geographical distribution, or [[extinction]] risk (for recent reviews, see Bennett and Leitch 2005;<ref name="Bennett2005"/> Gregory 2005<ref name="Gregory2005"/>).

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]].

Line 17:

=== 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">{{cite book |title=The Ancestor's Tale |isbn=978-0544859937 |last1=Dawkins |first1=Richard |last2=Wong |first2=Yan |year=2016 |title-link=The Ancestor's Tale |author1-link=Richard Dawkins }}</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

Line 118:

== References ==<!-- ZoolSci23:1017. -->

{{reflistReflist|2}}

== External links ==