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C-value enigma

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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 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.[2] 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 less than 2% protein-coding regions, with the remainder being various types of non-coding DNA (especially transposable elements).[3]


Now scientists have discovered a vital clue to unraveling these riddles. The human genome is packed with at least four million gene switches that reside in bits of DNA that once were dismissed as “junk” but that turn out to play critical roles in controlling how cells, organs and other tissues behave. The discovery, considered a major medical and scientific breakthrough, has enormous implications for human health because many complex diseases appear to be caused by tiny changes in hundreds of gene switches.

The findings, which are the fruit of an immense federal project involving 440 scientists from 32 laboratories around the world, will have immediate applications for understanding how alterations in the non-gene parts of DNA contribute to human diseases, which may in turn lead to new drugs. They can also help explain how the environment can affect disease risk. In the case of identical twins, small changes in environmental exposure can slightly alter gene switches, with the result that one twin gets a disease and the other does not.

As scientists delved into the “junk” — parts of the DNA that are not actual genes containing instructions for proteins — they discovered a complex system that controls genes. At least 80 percent of this DNA is active and needed. http://www.nytimes.com/2012/09/06/science/far-from-junk-dna-dark-matter-proves-crucial-to-health.html?pagewanted=all&_r=0

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?

Puzzle versus paradox

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.[4] 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, UK,[4] and an increasing number of authors have begun adopting this term.

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.
  2. ^ "Animal Genome Size Database". Retrieved 14 May 2013.
  3. ^ Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 18514361, please use {{cite journal}} with |pmid=18514361 instead.
  4. ^ a b Second Plant Genome Size Discussion Meeting and Workshop

References

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