- C-value enigma
The C-value enigma or C-value paradox is a term used to describe the complex puzzle surrounding the extensive variation in nuclear
genome size amongeukaryotic 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 protists have genomes much larger than that ofhumans .C-value paradox history
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"Fact|date=December 2007, which they took as evidence that
DNA , rather thanprotein , was the substance of whichgenes are composed. The termC-value reflects this observed constancy. However, it was soon found thatC-value s (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 theorganism ). For example, the cells of somesalamanders may contain 40 times more DNA than those of humans. Given thatC-value s were assumed to be constant because DNA is the stuff of genes, and yet bore no relationship to presumed gene number, this was understandably considered paradoxical; 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 C-value paradox. It is no longer a mystery whygenome size does not reflectgene number ineukaryotes : most eukaryotic (but notprokaryotic ) DNA is non-coding and therefore does not consist of genes, and as such total DNA content is not determined by gene number in eukaryotes. Thehuman genome , for example, comprises only about 1.5% protein-coding genes, with the other 98.5% being various types of non-coding DNA (especiallytransposable elements ) (International Human Genome Sequencing Consortium 2001). It is unclear why some species have a remarkably higher amount of non-coding sequences than others of the same level of complexity. Non-coding DNA may have many functions yet to be discovered. Though now it is known that only a fraction of the genome consists of genes, the paradox remains unsolved.C-value enigma origin
The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term "
C-value " (Swift 1950) in reference tohaploid nuclearDNA contents. The term was coined by Canadian biologist Dr. T. Ryan Gregory of theUniversity of Guelph in 2000/2001. In general terms, the C-value enigma relates to the issue of variation in the amount ofnon-coding DNA found within thegenome s of different eukaryotes.The C-value enigma, unlike the older C-value paradox, is explicitly defined as a series of independent but equally important component questions, including:
# What types of non-coding DNA are found in different eukaryotic genomes, and in what proportions?
# From where does this non-coding DNA come, and how is it spread and/or lost from genomes over time?
# What effects, or perhaps even functions, does this non-coding DNA have forchromosomes , nuclei, cells, andorganisms ?
# Why do some species exhibit remarkably streamlined chromosomes, while others possess massive amounts of non-coding DNA?References
* Gregory TR (2005). Genome size evolution in animals. In "
The Evolution of the Genome " (ed. T.R. Gregory), pp. 3-87. Elsevier, San Diego.
* Gregory TR (2001). Coincidence, coevolution, or causation? DNA content, cell size, and the C-value enigma. "Biological Reviews", 76:65-101.
* Gregory TR (2000). Nucleotypic effects without nuclei: genome size and erythrocyte size in mammals. "Genome", 43:895-901.
* Thomas CA (1971). The genetic organization of chromosomes. "Annual Review of Genetics" 5:237-256.
* Swift H (1950). [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=14808154 The constancy of desoxyribose nucleic acid in plant nuclei] . "Proc Natl Acad Sci USA", 36:643-654.
*Vendrely, R. and C. Vendrely. 1948. La teneur du noyau cellulaire en acide désoxyribonucléique à travers les organes, les individus et les espèces animales : Techniques et premiers résultats. "Experientia" 4: 434-436.See also
*
Animal Genome Size Database
*Cell nucleus
*Comparative genomics
*C-value
*Genome
*Genome size
*Human genome
*Junk DNA
*Noncoding DNA
*Plant DNA C-values Database
*Selfish DNA
*Transposable elements External links
* [http://www.genomesize.com Animal Genome Size Database]
* [http://www.rbgkew.org.uk/cval/homepage.html Plant DNA C-values Database]
* [http://www.zbi.ee/fungal-genomesize/index.php Fungal Genome Size Database]
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