Wobble base pairing. (a) Wobble allows one tRNA to recognize two different codons. U normally pairs with A (top) but can form two hydrogen bonds with G to make a weak G–U wobble base pair, which occurs in the third position of the codon (bottom). (b) A tRNA pairs with two different codons through wobble pairing (shown in red) at the third nucleotide of the codon. The G of the G–U pair can be in either the anticodon or the codon.