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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.
The size of genomes in various species was known to vary considerably and there did not seem to be a correlation between genome size and the complexity of the species. Even closely related species could have very different genome sizes. This observation led to what came to be known as the C-value paradox. [16]
The reasons for the changes in genome size are still being worked out and this problem is called the C-value Enigma. [ 5 ] This led to the observation that the number of genes does not seem to correlate with perceived notions of complexity because the number of genes seems to be relatively constant, an issue termed the G-value Paradox . [ 6 ]
Genome size ranges (in base pairs) of various life forms. Genome size is the total amount of DNA contained within one copy of a single complete genome.It is typically measured in terms of mass in picograms (trillionths (10 −12) of a gram, abbreviated pg) or less frequently in daltons, or as the total number of nucleotide base pairs, usually in megabases (millions of base pairs, abbreviated ...
Onions and their relatives vary dramatically in their genome sizes, [10] without changing their ploidy, and this gives an exceptionally valuable window on the genomic expansion junk DNA. Since the onion (Allium cepa) is a diploid organism having a haploid genome size of 15.9 Gb, [10] it has 4.9x as much DNA as does a human genome (3.2 Gb).
The C-value paradox refers to the lack of correlation between organism 'complexity' and genome size. Explanations for the so-called paradox are two-fold. First, repetitive genetic elements can comprise large portions of the genome for many organisms, thereby inflating DNA content of the haploid genome.
Attempts to understand the extraordinary variation in genome size —animals vary 7,000 fold and land plants some 2,400-fold—has a long history in biology. [105] However, this variation is poorly correlated with gene number or any measure of organismal complexity, which led CA Thomas to coin the term C-value paradox in 1971. [106]
ΦX174 is regularly used as a positive control in DNA sequencing due to its relatively small genome size in comparison to other organisms, its relatively balanced nucleotide content — about 23% G, 22% C, 24% A, and 31% T, i.e., 45% G+C and 55% A+T, see the accession NC_001422.1 [10] for its 5,386 nucleotide long sequence.