Genome size

Genome size refers to the total amount of DNA contained within one copy of a genome. It is typically measured in terms of mass (in picograms, or trillionths [10^-12] of a gram [abbreviated pg], or less frequently in Daltons) or as the total number of nucleotide base pairs (typically in millions of base pairs, or megabases [abbreviated Mb or Mbp]). One picogram (pg) equals 978 megabases (Mb) (Dolezel et al. 2003). In diploid organisms, genome size is used interchangeably with the term C-value.

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Origin of the term

The term "genome size" is often erroneously attributed to Hinegardner (1976), even in discussions dealing specifically with terminology in this area of research (e.g., Greilhuber et al. 2005). Notably, Hinegardner (1976) used the term only once: in the title. The term actually seems to have first appeared in 1968 when Hinegardner wondered, in the last paragraph of his article, whether "cellular DNA content does, in fact, reflect genome size". In this context, "genome size" was being used in the sense of genotype to mean the number of genes. In a paper submitted only two months later (in February of 1969), Wolf et al. (1969) used the term "genome size" throughout and in its present usage; therefore these authors should probably be credited with originating the term in its modern sense. By the early 1970s, "genome size" was in common usage with its present definition, probably as a result of its inclusion in Ohno’s influential book Evolution by Gene Duplication, published in 1970.

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Variation in genome size

The genome sizes of thousands of eukaryotes have been analyzed over the past 50 years, and these data are available in online databases for animals, plants, and fungi (see external links). Nuclear genome size is typically measured in eukaryotes using either densitometric measurements of Feulgen-stained nuclei (previously using specialized densitometers, now more commonly using computerized image analysis; Hardie et al. 2002) or flow cytometry. In prokaryotes, pulsed-field gel electrophoresis and complete genome sequencing are the predominant methods of genome size determination. Nuclear genome sizes are well known to vary enormously among eukaryotic 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. In eukaryotes (but not prokaryotes), variation in genome size bears no relationship to the number of genes, 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 genome size 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 by one author to more accurately comprise a puzzle or an enigma (the C-value enigma). Genome size correlates 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, and/or extinction risk (for recent reviews, see Bennett and Leitch 2005; Gregory 2005).

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References

Bennett, M.D. and I.J. Leitch. 2005. Genome size evolution in plants. In The Evolution of the Genome (ed. T.R. Gregory), pp. 89-162. Elsevier, San Diego.

Doležel, J., J. Bartoš, H. Voglmayr, and J. Greilhuber. 2003. Nuclear DNA content and genome size of trout and human. Cytometry 51A: 127-128.

Gregory, T.R. 2005. Genome size evolution in animals. In The Evolution of the Genome (ed. T.R. Gregory), pp. 3-87. Elsevier, San Diego.

Greilhuber, J., J. Doležel, M. Lysák, and M.D. Bennett. 2005. The origin, evolution and proposed stabilization of the terms 'genome size' and 'C-value' to describe nuclear DNA contents. Annals of Botany 95: 255-260.

Hardie, D.C., T.R. Gregory, and P.D.N. Hebert. 2002. From pixels to picograms: a beginners' guide to genome quantification by Feulgen image analysis densitometry. Journal of Histochemistry and Cytochemistry 50: 735-749.

Hinegardner, R. 1968. Evolution of cellular DNA content in teleost fishes. American Naturalist 102: 517-523.

Hinegardner, R. 1976. Evolution of genome size. In Molecular Evolution (ed. F.J. Ayala), pp. 179-199. Sinauer Associates, Inc., Sunderland.

Ohno, S. 1970. Evolution by Gene Duplication. Springer-Verlag, New York.

Wolf, U., H. Ritter, N.B. Atkin, and S. Ohno. 1969. Polyploidization in the fish family Cyprinidae, Order Cypriniformes. I. DNA-content and chromosome sets in various species of Cyprinidae. Humangenetik 7: 240-244.

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See also

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External links