- C-value
The term C-value refers to the amount of
DNA contained within ahaploid nucleus (e.g., in a gamete or one half the amount in adiploid somatic cell ) of aeukaryotic organism. In some cases (notably among diploid organisms), the terms C-value andgenome size are used interchangeably, however in polyploids the C-value may represent two genomes contained within the same nucleus. Greilhuber et al. (2005) 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. C-values are reported inpicogram s.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) amongchromosome 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 (Bennett and Leitch 2005; Gregory 2005).
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 ofgenes contained in its genome, an observation that was deemed wholly counterintuitive before the discovery ofnon-coding DNA and which became known as theC-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 thetaxon , body size,metabolic rate , developmental rate, organ complexity, geographical distribution, and/orextinction risk (for recent reviews, see Bennett and Leitch 2005; Gregory 2005).Calculating C-values
†Source of table: Doležel "et al.", 2003cite journal |author=Doležel J, Bartoš J,Voglmayr H, Greilhuber J |title=Letter to the editor: Nuclear DNA Content and Genome Size of Trout and Human |journal=Cytometry |volume=51A |issue=2 |pages=127–128 |year=2003 |doi=10.1002/cyto.a.10013]By using the data in Table 1, relative weights of nucleotide pairs can be calculated as follows: AT = 615.3830 and GC = 616.3711. Provided the ratio of AT to GC pairs is 1:1, the mean relative weight of one nucleotide pair is 615.8771 (±1%).
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
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