According to Watson-Crick base-pairing, which forms the basis for the helical configuration of double-stranded DNA, DNA contains four bases: the two purines adenine (A) and guanine (G) and the two pyrimidines cytosine (C) and thymine (T).

In DNA, there are four nitrogenous base options: adenine (A), thymine (T), cytosine (C) and guanine (G). Each base can only bond with one other, A with T and C with G. This is called the complementary base pairing rule or Chargaff's rule.

The two strands are complementary rather than identical and are held together by hydrogen bonds between specific pairs of bases, A with T and C with G. That is, whenever an A base occurs in one strand, a T base occurs opposite it in the other strand; when a C base occurs in one, a G occurs in the other (Figure 28.3).

The bonds between the top string and the bottom string is called “base-pairing” where the term “base” refers to the nucleotides (because their pH is basic) and “pairing” refers to the hydrogen bonds. Note here that the top string goes from 5′ to 3′, left to right, while the bottom string goes from 5′ to 3′, right to left.