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Within a double helix each molecule, though far from identical, nevertheless specifies the sequence of the other.
A double helix of DNA consists of two molecules of polynucleic acid running in opposite directions (i.e., 5' to 3' for one strand and 3' to 5' for the other strand). The sequence of nucleotides making up each of these strands are vastly different but nevertheless constrained by the other. In particular, at each base pair, one strand specifies the other with each A specifying a T, each G specifying a C, each C specifying a G, and each T specifying A. The sequence of nucleotides on each strand thus are said to be each other's complement, giving rise to the concept of strand complementarity.
One sees strand complementarity other than just within the double helix of DNA. Some viruses, for example, have double-stranded RNA genomes where again strand complementarity occurs. The process of transcription involves brief complementarity between DNA base and RNA base (though with slightly different rules of complementarity than one sees with DNA alone). Even within individual polynucleic acid molecules there can be base pairing, resulting in the secondary structure of ribosomal RNAs and transfer RNAs as well as to a lesser extent secondary structure within messenger RNA molecules and even the double-stranded DNA of a double helix.
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