According to the Watson-Crick model, the sequence of bases in one strand of a double-stranded DNA (dsDNA) may be entirely arbitrary, but the sequence in the second strand is fixed by complementary base pairing. Each pyrimidine base forms a hydrogen-bonded base pair with only one of the two purine bases: C forms a base pair with G, and T pairs with A.
Opening image: G·C base pair. All four base pairs have the same overall dimensions. The distance separating the backbone C1′ atoms, shown in purple, is very important. In the "ideal" Watson-Crick model this distance is 10.85 Å for all four base pairs. However, observed values range from 10.4 to 10.9 Å. the distance between this pair.