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Antiparallel Strands

DNA strands run in opposite directions (5’ to 3’ and 3’ to 5’), influencing base pairing and overall structure.
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The statement of the theorem

Consider two DNA strands, Strand1\text{Strand}_1 and Strand2\text{Strand}_2, represented by sequences of nucleotides indexed by ii. The directionality is defined by the indices ii. For Strand1\text{Strand}_1, the sequence runs from i=1i=1 to NN (the 5′→3′5' \to 3' direction). For Strand2\text{Strand}_2, the corresponding sequence must run from j=1j=1 to NN (the 5′→3′5' \to 3' direction). The antiparallel constraint requires that the ii-th nucleotide of Strand1\text{Strand}_1 pairs with the (N−i+1)(N-i+1)-th nucleotide of Strand2\text{Strand}_2. Mathematically, if B1(i)B_1(i) and B2(j)B_2(j) are the bases, then for a paired segment of length NN: \nB1(i) pairs with B2(N−i+1)B_1(i) \text{ pairs with } B_2(N-i+1) \nThis implies that the pairing index jj must be a linear function of the index ii such that j=N−i+1j = N-i+1.
Source: Wikipedia