Watson-Crick Base Pairing
A double stranded nucleic acid (AKA DNA) has millions of base pairs.  One base pair consist of 2 nucleobases bound to each other by hydrogen bonds. How they form the structure follows a natural rule: guanine(G) and cytosine(C) like to bond to each other, and adenine(A) and its buddy thymine (T) . GC base pair contains 3 hydrogen bonds, which means that the more GC pairs one have, the more likely it will tolerate high temperature. (Reference: Base pair - Wikipedia)

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DNA Double Helix
Two DNA single strand twist around each other to from the double helix structure. Based on Watson-Crick base paring rules, C pairs with G and A must pair with T. That means, if we are given one DNA strand sequence, we will know the other half’s sequence. (reference: N.C. Seeman, J Theor Biol. 99, 237-247, (1982) )

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Base pair - Wikipedia

N.C. Seeman, J Theor Biol. 99, 237-247, (1982)