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. Greilhuber et al. have suggested some new layers of terminology and associated abbreviations to clarify this issue, but these somewhat complex additions have yet to be used by other authors.
Origin of the term
Many authors have incorrectly assumed that the "C" in "C-value" refers to "characteristic", "content", or "complement". Even among authors who have attempted to trace the origin of the term, there had been some confusion because Hewson Swift did not define it explicitly when he coined it in 1950. In his original paper, Swift appeared to use the designation "1C value", "2C value", etc., in reference to "classes" of DNA content (e.g., Gregory 2001, 2002); however, Swift explained in personal correspondence to Prof. Michael D. Bennett in 1975 that "I am afraid the letter C stood for nothing more glamorous than 'constant', i.e., the amount of DNA that was characteristic of a particular genotype" (quoted in Bennett and Leitch 2005). This is in reference to the report in 1948 by Vendrely and Vendrely of a "remarkable constancy in the nuclear DNA content of all the cells in all the individuals within a given animal species" (translated from the original French). Swift's study of this topic related specifically to variation (or lack thereof) among chromosome sets in different cell types within individuals, but his notation evolved into "C-value" in reference to the haploid DNA content of individual species and retains this usage today.
Variation among species
C-values vary enormously among species. In animals they range more than 3,300-fold, and in land plants they differ by a factor of about 1,000. Protist genomes have been reported to vary more than 300,000-fold in size, but the high end of this range (Amoeba) has been called into question. Variation in C-values bears no relationship to the complexity of the organism or the number of genes contained in its genome, an observation that was deemed wholly counterintuitive before the discovery of non-coding DNA and which became known as the C-value paradox as a result. However, although there is no longer any paradoxical aspect to the discrepancy between C-value and gene number, this term remains in common usage. For reasons of conceptual clarification, the various puzzles that remain with regard to genome size variation instead have been suggested to more accurately comprise a complex but clearly defined puzzle known as the C-value enigma. C-values correlate with a range of features at the cell and organism levels, including cell size, cell division rate, and, depending on the taxon, body size, metabolic rate, developmental rate, organ complexity, geographical distribution, or extinction risk (for recent reviews, see Bennett and Leitch 2005; Gregory 2005).
|Nucleotide||Chemical formula||Relative molecular weight (Da)|
†Source of table: Doležel et al., 2003
By using the data in Table 1, relative weights of nucleotide pairs can be calculated as follows: AT = 615.383 and GC = 616.3711, bearing in mind that formation of one phosphodiester linkage involves a loss of one H2O molecule. Further, phosphates of nucleotides in the DNA chain are acidic so at physiologic pH the H+ ion is dissociated. Provided the ratio of AT to GC pairs is 1:1, the mean relative weight of one nucleotide pair is 615.8771.
The relative molecular weight may be converted to an absolute value by multiplying it by the atomic mass unit (1 u), which equals one-twelfth of a mass of 12C, i.e., 1.660539 × 10−27 kg. Consequently, the mean weight of one nucleotide pair would be 1.023× 10−9 pg, and 1 pg of DNA would represent 0.978 × 109 base pairs.
The formulas for converting the number of nucleotide pairs (or base pairs) to picograms of DNA and vice-versa are:
genome size (bp) = (0.978 x 109) x DNA content (pg) DNA content (pg) = genome size (bp) / (0.978 x 109) 1 pg = 978 Mb
The current estimates for human female and male diploid genome sizes are 6.406 × 109 bp and 6.294 × 109 bp, respectively (female diploid genome sizes are larger than males because they have two X chromosomes, whereas males have one X and one Y chromosome and the Y chromosome is much smaller than the X chromosome). By using the conversion formulas given above, diploid human female and male nuclei in G1 phase of the cell cycle should contain 6.550 and 6.436 pg of DNA, respectively.
The following is not correct. In contrast, the estimates for human female and male haploid genome sizes are 3.203 × 109 bp and 3.254 × 109 bp, respectively. The female haploid genome sizes are smaller than males because their two X chromosomes are diploids, whereas the X and Y chromosome of males are not diploids, just haploids. That is, these two X chromosomes of females are copies of each other, whereas the X and Y chromosome of males do not have a copy. Therefore, the female haploid genome sizes are smaller than males by 0.051 × 109 bp, which is the size of the Y chromosome. Thus, the haploid human female and male genome contain 3.275 and 3.327 pg of DNA, respectively.
- Greilhuber J, Doležel J, Lysák M, Bennett MD (2005). "The origin, evolution and proposed stabilization of the terms 'genome size' and 'C-value' to describe nuclear DNA contents". Annals of Botany 95 (1): 255–60. doi:10.1093/aob/mci019. PMID 15596473.
- Swift H (1950). "The constancy of deoxyribose nucleic acid in plant nuclei". Proceedings of the National Academy of Sciences of the USA 36 (11): 643–654. doi:10.1073/pnas.36.11.643. PMC 1063260. PMID 14808154.
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