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C-value enigma

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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 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 protists have genomes much larger than that of humans.

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",[1] 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 sizes) vary enormously among species and that this bears no relationship to the presumed number of genes (as reflected by the complexity of the organism). For example, the cells of some salamanders may contain 40 times more DNA than those of humans. Given that C-values 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 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 only about 1.5% protein-coding genes, with the other 98.5% being various types of non-coding DNA (especially transposable elements) (International Human Genome Sequencing Consortium 2001).

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The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term "C-value" (Swift 1950) in reference to haploid nuclear DNA contents. The term was coined by Canadian biologist Dr. T. Ryan Gregory of the University of Guelph in 2000/2001. In general terms, the C-value enigma relates to the issue of variation in the amount of non-coding DNA found within the genomes 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 for chromosomes, nuclei, cells, and organisms?
  • Why do some species exhibit remarkably streamlined chromosomes, while others possess massive amounts of non-coding DNA?

See also

Notes

  1. ^ Vendrely R and Vendrely C (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.

References

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