= Nucleic acid
{wiki}
A <nucleic acid> is a <polymer> built from linked <nucleotides>. The two principal kinds are <DNA> and <RNA>. Each <nucleotide> contributes a <sugar>, a <nucleobase>, and <phosphate> to the chain; the <nucleobase> is only the letter-bearing component, not the entire <nucleotide>. <DNA> uses <deoxyribose>, whereas <RNA> uses <ribose>. A chain of six <nucleotides> is therefore not six free <nucleobases> placed beside one another. See https://www.genome.gov/genetics-glossary/Nucleic-Acids[NHGRI: nucleic acids].
The chemical scaffold and the variable lettering do different jobs. <Covalent bonds> maintain the <sugar>–<phosphate> backbone, while the order of attached <nucleobases> specifies the <nucleic acid sequence>. A <phosphodiester bond> links the $3'$ position of one <sugar> to the $5'$ position of the next through <phosphate>. These position numbers refer to <carbon> <chemical atoms> in the <sugar>; they define a strand's direction even when it bends or forms a circle. See https://www.ncbi.nlm.nih.gov/books/NBK21134/[Brown, Genomes, 2nd edition: the human genome].
This division is useful: the backbone supplies a connected physical record, and <base pairing> allows its letters to recognize partners. <DNA replication> copies that record, and <transcription> uses a <DNA> template to make <RNA>. <RNA> can also fold into functional shapes through <base pairing> within one chain; a <nucleic acid sequence> specifies chemical order, not a requirement for a straight shape. The beauty is that the same kind of <polymer> can carry a message and form a structure that acts on messages.
Back to article page
