The pairings between nucleic bases, forming "Watson-Crick" base pairs, are, on the one hand, adenine with thymine, and, on the other hand, guanine with cytosine. In addition, two nucleic acid sequences can match only antiparallel, that is to say between segments of opposite directions. Therefore, two nucleic acid sequences are complementary when the nucleic bases of one can pair with the nucleic bases of the other in the opposite direction to form two paired antiparallel segments.
The selective pairing of adenine (A) with thymine (T) and guanine (G) with cytosine (C) is based on the number of hydrogen bonds established between one of the purine bases and the one of the pyrimidine bases:
* Two hydrogen bonds between adenine (purine) and thymine (pyrimidines);
* Three hydrogen bonds between guanine (purine) and cytosine (pyrimidine).
Purity pairs, pyrimidine pairs, adenine-cytosine pair, or guanine-thymine pair are not normally formed. This selectivity is crucial for a large number of biochemical processes involving nucleic acids.
