Wobble base pair

A wobble base pair is a G-U and I-U / I-A / I-C pair fundamental in RNA secondary structure. Its thermodynamic stability is comparable to that of the Watson-Crick base pair. Wobble base pairs are critical for the proper translation of the genetic code. The genetic code makes up for disparities in the number of amino acids (20) for codons (64), by using modified base pairs in the first base of the anti-codon. One important modified base is inosine which can pair with three bases: uracil, adenine, and cytosine.

Another critical base pair is the G-U base pair, which allows uracil to pair with two bases: guanine and adenine.

tRNA Wobble
The fact that there are 61 amino-acid-coding codons and roughly 40 tRNA molecules presented a problem; in 1966 Francis Crick proposed the Wobble hypothesis to account for this. He postulated that the 5' base on the anti-codon was not as spatially confined as the other two bases, and could thus have non-standard base pairing. This would account for 60 codons for 40 tRNA.

As an example yeast tRNAPhe has the anticodon 5'-GmAA-3' and can recognize the codons 5'-UUC-3' and 5'-UUU-3'. It is, therefore, possible for non-Watson-Crick base pairing to occur at the third codon position, i.e. the 3' nucleotide of the mRNA codon and the 5' nucleotide of the tRNA anticodon.