C-value: Difference between revisions

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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|s2cid=22272730 }}</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-codingrepetitive DNA]] in the earlylate 1970s1960s resolved the main question of the C-value paradox: [[genome size]] does not reflect [[gene]] number in [[eukaryotes]] since most of theirthe excess DNA isin non-codingmany andspecies thereforeappears doesto notbe consist of[[Junk genesDNA]]. The [[human genome]], for example, comprisescontains lessabout than 210% protein-codingfunctional regions,elements withand the remainderremaining being90% variousis typesthought ofto non-codingbe DNAjunk. (especiallySpecies [[transposablewith elements]]).<ref>{{Citelarger journalgenomes are thought to contain a higher proportion of junk DNA.

| last1 = Elgar | first1 = G.

| last2 = Vavouri | first2 = T.

| doi = 10.1016/j.tig.2008.04.005

| title = Tuning in to the signals: Noncoding sequence conservation in vertebrate genomes

| journal = Trends in Genetics

| volume = 24

| issue = 7

| pages = 344–352

| year = 2008

| pmid = 18514361

| pmc =

}}</ref>

=== C-value enigma ===