= Deoxyribonucleic acid
= DNA
{c}
{synonym}
<Deoxyribonucleic acid>, or <DNA>, is a <nucleotide> <polymer> that stores biological information in its <nucleotide sequence>. The crucial variable is order: two <DNA> <molecules> can contain the same numbers of each <nucleobase> yet carry different <nucleotide sequences>. This makes a physical <molecule> an inheritable record, rather than merely a mixture of ingredients. <Genes> are functional regions of that record; the whole <DNA> <nucleotide sequence> is not one long <gene>. See https://www.genome.gov/about-genomics/fact-sheets/Deoxyribonucleic-Acid-Fact-Sheet[NHGRI: DNA fact sheet].
Each <DNA> strand has a backbone of alternating <deoxyribose> <sugars> and <phosphate> groups joined through <covalent bonds>. Attached <nucleobases> supply the familiar four-letter alphabet: <adenine> A, <thymine> T, <guanine> G, and <cytosine> C. In ordinary double-stranded <DNA>, <base pairing> matches A with T and G with C. The backbone connects successive <nucleotides> along a strand; <base pairing> connects partners across strands. Keeping these two connections distinct explains how strands can separate while each retains its own <nucleotide sequence>.
The two backbones have directions, conventionally labelled $5'$ and $3'$ after numbered <carbon> <chemical atoms> in <deoxyribose>. They form <antiparallel nucleic acid strands>: following one from $5'$ to $3'$ takes you opposite to the other's $5'$-to-$3'$ direction. For the chosen <DNA> <nucleotide sequence> $5'\text{-}\mathrm{ATGCC}\text{-}3'$, write its partner directly underneath as $3'\text{-}\mathrm{TACGG}\text{-}5'$. Now read that partner from its own $5'$ end: its <reverse complement> is $5'\text{-}\mathrm{GGCAT}\text{-}3'$. Complementing letters and reversing their order are separate operations. See https://www.ncbi.nlm.nih.gov/books/NBK26821/[Alberts et al., Molecular Biology of the Cell: the structure and function of DNA].
\Image[/dna-antiparallel-complement.png]
{title=<DNA> <base pairing> and <antiparallel nucleic acid strands>: an unrolled ladder for ATGCC and its <reverse complement>}
The diagram's rails represent <sugar>–<phosphate> backbones and its coloured blocks represent <nucleobases>, not individual <chemical atoms>. Reading left to right gives five matched positions: A–T, T–A, G–C, C–G, C–G. There are two A–T-type <base pairs> and three G–C-type <base pairs>; the complete two-strand example contains ten <nucleotides>. Reversing the bottom row changes the reading convention, not the physical partners. Real double-stranded <DNA> winds into a <nucleic acid double helix>; the flattened ladder emphasizes <base pairing> and direction without pretending to show its three-dimensional geometry.
The motivating consequence is copying by a template. After strands separate, each existing <DNA> strand specifies which incoming <nucleotides> should accompany it through <base pairing>. <DNA replication> uses <enzymes> to build complementary strands, allowing the record to persist through successive <biological cells>. <Base pairing> supplies a selection rule, while <enzymes> perform the chemistry: a matching diagram alone is not a complete copying mechanism. The connection between structure and copying was already central to the model published by Watson and Crick on 25 April 1953. See https://www.nature.com/articles/171737a0[Watson and Crick, Molecular Structure of Nucleic Acids, Nature 171, 737–738 (1953)].
Stored information needs interpretation. Some <DNA> regions specify <proteins> through <RNA>; others contribute to functional <RNA> or regulate <gene expression>, and some have no known function. <Non-coding DNA> means sequence that does not specify a <protein>'s sequence of <amino acids>, not automatically useless sequence. Nor does a <DNA> <nucleotide sequence> alone determine an <organism>'s fate: <phenotypes> also depend on biological context and environment. See https://www.genome.gov/genetics-glossary/Non-Coding-DNA[NHGRI: non-coding DNA]. Its double-stranded form is often modeled mechanically as a <worm-like chain>, with <elasticity> and effective <excluded volume> depending on solvent conditions.
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