Жасуша биологиясындағы C мәні

C value is the amount, in picograms, of DNA contained within a haploid nucleus
the term C value in cell biology

C value is 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 are yet to be used by other authors.

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.

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; for example, some single celled protists have genomes much larger than that of humans. This observation was deemed counterintuitive before the discovery of non coding DNA. It 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;
and an increasing number of authors have begun adopting this term.

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. 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 (especially transposable elements).