G+C ratio

In molecular biology and genetics, GC-content (or guanine-cytosine content) is the percentage of nitrogenous bases on a DNA or RNA molecule that are either guanine or cytosine (from a possibility of four different ones, also including adenine and thymine in DNA and adenine and uracil in RNA). This may refer to a certain fragment of DNA or RNA, or that of the whole genome. When it refers to a fragment of the genetic material, it may denote the GC-content of section of a gene (domain), single gene, group of genes (or gene clusters), or even a non-coding region. G (guanine) and C (cytosine) undergo a specific hydrogen bonding, whereas A (adenine) bonds specifically with T (thymine, in DNA) or U (uracil, in RNA).

The GC pair is bound by three hydrogen bonds, while AT and AU pairs are bound by two hydrogen bonds. To emphasize this difference in the number of hydrogen bonds, the base pairings can be represented as respectively G≡C versus A=T and A=U. DNA with low GC-content is less stable than DNA with high GC-content; however, the hydrogen bonds themselves do not have a particularly significant impact on stabilization, the stabilization is due mainly to interactions of base stacking. In spite of the higher thermostability conferred to the genetic material, it is envisaged that cells with DNA of high GC-content undergo autolysis, thereby reducing the longevity of the cell per se. Due to the thermostability given to the genetic materials in high GC organisms, it was commonly believed that the GC content played a necessary role in adaptation temperatures, a hypothesis that was refuted in 2001. However, it has been shown that there is a strong correlation between the prokaryotic optimal growth at higher temperatures and the GC content of structured RNAs (such as ribosomal RNA, transfer RNA, and many other non-coding RNAs). The AU base pairs are less stable than the GC base pairs because GC bonds have 3 hydrogen bonds and AU only has 2 hydrogen bonds, which makes high-GC-content RNA structures more resistant to the effects of high temperatures. More recently, one of the earliest large-scale systematic gene-centric association analysis recently demonstrated the correlation between GC content and temperature for certain genomic regions while not for others.

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