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
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", 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
) 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 C-value paradox. It is no longer a mystery why genome size does not reflect gene number in eukaryotes: most eukaryotic (but not prokaryotic) 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. 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). 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The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term "C-value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 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?
- 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). 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.