Watson-Crick base pairs complementarity

Classic Watson-Crick base pairs are formed by unique hydrogen-bonding interactions between the nitrogenous bases of DNA and RNA. The purine adenine associates specifically with the pyrimidine thymine in DNA (or the related unmethylated analog, macil, in RNA), and the pmine guanine interacts with the pyrimidine cytosine. These complementarity rules. 

An examination of the crystal structures of various DNA polymerases reveals why shape complementarity is so important. First, residues of the enzyme form hydrogen bonds with the minor-groove side of the base pair in the active site (Figure 28.18). In the minor groove, hydrogen-bond acceptors are present in the same positions for all Watson—Crick base pairs. These interactions act as a ruler" that measures whether a properly spaced base pair has formed in the active site. 

Watson-Crick base pairing in complementary oligonucleotide strands keeps two rules of complementarity in both size and hydrogen-bonding patterns. Hydrophobicity and planarity in the bases also appear to be important for the stability of the double-helical structure. Designing new base pairs that vary in shape, size, and functionality has been usefiil in rmderstanding what is essential in the natural base pairing. 

Figure 7.6 Molecular models of the DNA base pairs Illustrating Watson-Crick hydrogen bonding complementarity.

The Crick-Watson base pairing with the complementarity C-G and A-T (for DNA) or A-U (for RNA) is a consequence of the delicate balance between aromatic resonance energies and bond energies for CC, CO and CN single and double bonds. 

---