Wobble: an example of a
four-fold degenerate site
Alanine (Ala) is a four-fold
degenerate amino acid, with four mRNA codons
N indicates that the last base can be any of the
four, so as to include 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
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.