Watson-Crick pair universality

In three dimensions, tRNAs fold into an L-shaped structure in which the acceptor stem and T C arm coaxially stack to form one part of the L known as the minihelix, and the D and anticodon arms likewise stack to form the other part of the molecule. This structure is facilitated and stabilized by tertiary interactions at the corner of the L that bring together the D and variable loops. The nucleotides involved in these interactions are typically invariant or semi-invariant, indicating that the tRNA L shape is universal. While most base pairs in tRNA helices are canonical Watson-Crick pairs, the tertiary interactions at the corner of the L make use of some unusual hydrogen-bonding conformations. For example, nearly all tRNAs contain a U8 A14 reverse Hoogsteen base pair, and several base triples (where three bases are paired together) are also typically present at the core of the structure. 

Why is wobble tolerated in the third position of the codon but not in the first two. " The 30S subunit has three universally conserved bases—adenine 1492, adenine 1493, and guanine S30—in the 16S KNA that form hydrogen bonds on the minor-groove side but only witFi correctly formed base pairs of the codon=anticodon duplex (Figure 30.21). These interactions serve to check whether Watson=Crick base pairs are present in the first two... 

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