C-VALUE PARADOX/C-VALUE ENIGMA
Each species has a characteristic content of DNA, which is constant in all the individuals of that species and has
thus been called the C-value. The term C-value refers to the amount, in picograms, of DNA contained within a haploid nucleus (e.g., a gamete) or one half the amount in a diploid somatic cell of a eukaryotic organism. In some cases (notably among diploid organisms), the terms C-value and genome size are used interchangeably, however in polyploids the C-value may represent two or more genomes contained within the same nucleus.
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. Eukaryotes vary greatly in DNA content but lways contain much more DNA than prokaryotes. Lower eukaryotes have less DNA such as nematode C. elegans
which has only 20 times more DNA than E.coli or the fruit fly Drosophila melanogaster, which has 40 times more
Vertebrates have greater DNA content (about 3 pg). One of the highest DNA contents is that of SalaSalamr amphiuma, which has (84pg) of DNA. Man has about 3 pg of DNA per haploid genome, or 3 x 10 base pairs i.e., the human genome could accommodate about 3 million average sized proteins if all the DNA were coding. But then, if this were true, salamanders would have 30 times more genes than man. 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 considered paradoxical; the term 'C-value paradox" was used to describe this situation by C.A. Thomas, Jr. in 1971 C-values correlate with a range of features at the cell and organism levels, including cell size, cell division rate
and also depend on the taxon, body size, metabolic rate, developmental rate, organ complexity, geographical distribution or extinction ris The discovery of non-coding DNA in early 1970s resolved the main issue of the C-value paradox: It made clear that 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). The term "C-value enigma" was endorsed in preference to"C-value paradox.
