Wobble: an example of a four-fold degenerate site

    Alanine (Ala) is a four-fold degenerate amino acid, with four mRNA codons GCN (which includes GCA, GCG, GCC, & GCT) that are read by two species of Alanine tRNAs, tRNAAla1 and tRNAAla2. The two codons with a pyrmidine in the 3'-most position [C / U in the left-hand diagram] are read by tRNAAla1 , which has a G in the corresponding 5' position of its anticodon. Note that the G pairs with the C codon as usual, and the unusual pairing with the U codon retains a 2+1 ring structure. Similarly, in the right-hand diagram, the U in the 5' position of the tRNA-Ala2 anticodon reads either A or G in the third position of the codon, retaining a 1+2 ring structure.

    That is, the purine in the critical position of the first tRNA can read either pyrimidine, and the pyrimidine in the second tRNA can read either purine. Two-fold degenerate amino acids usually use only one or the other of these combinations.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2012 by Steven M. Carr