FIGURE 5-21 Nonstandard base pairing at the wobble position. The base in the third (or wobble) position of an mRNA codon often forms a nonstandard base pair with the base in the first (or wobble) position of a tRNA anticodon. Wobble pairing allows a tRNA to recognize more than one mRNA codon (top); conversely, it allows a codon to be recognized by more than one kind of tRNA (bottom), although each of those tRNAs will bear the same amino acid. Note that a tRNA with I (inosine) in the wobble position can “read” (become paired with) three different codons, and a tRNA with G or U in the wobble position can read two codons. Although A is theoretically possible in the wobble position of the anticodon, it is almost never found in nature. (Note that this is a simplified diagram of a tRNA. The actual structure of a tRNA is shown in Figure 5-20b.)