= Ribonucleic acid
= RNA
{c}
{synonym}
<Ribonucleic acid>, or <RNA>, is a <nucleotide> <polymer> whose <nucleotide sequence> can both carry information and organize a working molecular shape. Its backbone contains alternating <ribose> <sugars> and <phosphate> groups connected through <covalent bonds>. Its usual <nucleobases> are <adenine> A, <uracil> U, <guanine> G, and <cytosine> C. Thus <RNA> uses U where <DNA> uses T; its <ribose> also differs chemically from <DNA>'s <deoxyribose>. See https://www.genome.gov/genetics-glossary/Ribonucleic-Acid-RNA[NHGRI: RNA].
In <transcription (biology)>, an <enzyme> uses one <DNA> strand as a template for <RNA>. For example, the <DNA> template $3'\text{-}\mathrm{TACGG}\text{-}5'$ specifies $5'\text{-}\mathrm{AUGCC}\text{-}3'$ in <RNA>. The <base pairing> rules now include A–U rather than A–T, while G–C remains. This <RNA> matches the other <DNA> strand's $5'$-to-$3'$ lettering after T is replaced by U. Identifying the template strand matters: replacing T by U in an arbitrarily chosen <DNA> strand is not a general method for deriving the <RNA> product. See https://www.ncbi.nlm.nih.gov/books/NBK26887/[Alberts et al., Molecular Biology of the Cell: from DNA to RNA].
<RNA> is often a single strand, but single-stranded does not mean straight or entirely unpaired. Complementary regions within one <RNA> <molecule> can undergo <base pairing>, bringing distant positions together. Consider the illustrative twelve-<nucleotide> <nucleotide sequence> $5'\text{-}\mathrm{GCAUUUUUAUGC}\text{-}3'$. Pair positions 1 with 12, 2 with 11, 3 with 10, and 4 with 9. The resulting <stem-loop> has four <base pairs>: G–C, C–G, A–U, U–A. Positions 5–8 remain unpaired in the loop. Both arms belong to the same continuous backbone, and the paired arms have the directions of <antiparallel nucleic acid strands>.
\Image[/rna-sequence-and-folding.png]
{title=One <RNA> <nucleotide sequence>, two drawings: numbered letters and an illustrative <stem-loop> with four <base pairs>}
The diagram proposes a possible pairing pattern, not a prediction that this short <RNA> must adopt that shape under every condition. Actual <RNA> folding depends on additional interactions, <temperature>, <ions>, and associated <proteins>; the full three-dimensional arrangement contains more information than the drawn <base pairs>. The example's purpose is to show how a linear <nucleotide sequence> can specify contacts within itself.
Different <RNA> roles make this flexibility useful. <Messenger RNA> carries a message that a <ribosome> reads during <translation (biology)> to build a <protein>. <Transfer RNA> helps match that message to <amino acid> building blocks; see https://www.genome.gov/genetics-glossary/Transfer-RNA-tRNA[NHGRI: transfer RNA]. <Ribosomal RNA> forms structural and catalytic parts of the <ribosome>. Other <RNA> participates in <gene regulatory networks>, changing how strongly <genes> are expressed. Many <RNA> <molecules> therefore function without being translated into <proteins>; for some <genes>, functional <RNA> is the product itself.
Most strikingly, some folded <RNA> acts as an <enzyme>: these catalytic <RNA> <molecules> are <ribozymes>. The <ribosome>'s bond-forming <active site> is built from <RNA>, although the complete <ribosome> also contains <proteins>. Structural evidence appears in https://pubmed.ncbi.nlm.nih.gov/10937990/[Nissen et al., The structural basis of ribosome activity in peptide bond synthesis, Science 289, 920–930 (2000)]. <RNA> therefore connects a readable record with physical recognition, regulation, and catalysis: the same kind of <polymer> can be a message, an adaptor, or part of molecular machinery.
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