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In molecular biology and genetics, GC-content (or guanine-cytosine content) is the percentage of nitrogenous bases on a DNA molecule that are either guanine or cytosine (from a possibility of four different ones, also including adenine and thymine). This may refer to a specific 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 part 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).

The GC pair is bound by three hydrogen bonds, while AT pairs are bound by two hydrogen bonds. To emphasize this difference in the number of hydrogen bonds, the base pairings can be represented as G≡C versus A=T. DNA with high GC-content is more stable than DNA with low GC-content; however, the hydrogen bonds do not stabilize the DNA significantly, and stabilization is due mainly to stacking interactions. 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 robustness endowed to the genetic materials in high GC organisms, it was commonly believed that the GC content played a vital part in adaptation temperatures, a hypothesis that has recently been refuted. 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 GC base pairs are more stable than AU base pairs, because GC bonds have 3 hydrogen bonds and AU only has 2 hydrogen bonds, which makes high-GC-content RNA structures more tolerant of high temperatures. More recently, the first large-scale systematic gene-centric association analysis demonstrated the correlation between GC content and temperature for certain genomic regions while not for others.

In PCR experiments, the GC-content of primers are used to predict their annealing temperature to the template DNA. A higher GC-content level indicates a higher melting temperature.