C-value

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 (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 genetic information is encoded by DNA, 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.

C-Value Paradox

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

What is the C-value paradox?You might expect more complex organisms to have progressively larger genomes, but eukaryotic genome size fails to correlate well with apparent complexity, and instead varies wildly over more than a 100,000-fold range. Single-celled amoebae have some of the largest genomes, up to 100-fold larger than the human genome. This variation suggested that genomes can contain a substantial fraction of DNA other than for genes and their regulatory sequences. C.A. Thomas Jr dubbed it the ‘C-value paradox’ in 1971.

The C-value paradox is related to another puzzling observation, called ‘mutational load’: the human genome seems too large, given the observed human mutation rate. If the entire human genome were functional (in the sense of being under selective pressure), we would have too many deleterious mutations per generation. By 1970, rough calculations had suggested to several authors that maybe only 1–20% of the human genome could be genic, with the rest evolving neutrally or nearly so.

So why not call it the ‘genome size paradox’? What is a ‘C-value’ anyway? ‘C-value’ means the ‘constant’ (or ‘characteristic’) value of haploid DNA content per nucleus, typically measured in picograms (1 picogram is roughly 1 gigabase). Around 1950, the observation that different cell types in the same organism generally have the same C-value was part of the evidence supporting the idea that DNA was responsible for heredity.