Watson-Crick recognition pattern

Fortunately, the imino forms of A and C and the enol forms of G and T occur rarely. Most A C and G T mispairs observed to date in high-resolution crystal structures (e.g., References 22 and 23) associate through a wobble configuration (14), with the bases sheared past one another relative to the Watson-Crick configuration (Table 1 and Fig. 1). These structural perturbations (Table 1 and Table 2) alter the patterns of atomic charges and accessibihty that are presumably required for protein recognition and enzymatic action (see discussion below). 

Figure 16 A combination of Watson-Crick and Hoogsteen hydrogen-bonding patterns permit the recognition between the adenine and the diimide.

These complementarity rules owe their discovery to the chemical analysis of DNA by Chargaff and associates (3). The DNA from many different organisms shows the same patterns of base composition, namely A and T are always present in equal quantities, as are G and C. The immediate corollary of this observation, that a purine base (R) exists for every pyrimidine base (Y) and vice versa, led Watson and Crick to propose that two helical strands in DNA are held together by specific, intermolecular purine-pyrimidine (R Y) interactions (4). In turn, this unique chemical complementarity of the double-helical structure, proved to be a major breakthrough to understand the self-recognition and self-reproduction of DNA and forms the cornerstone of structmal biology as we know it today, more than half a century later. 

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