Wobble: an example of a four-fold degenerate site
Alanine (Ala) is a four-fold
degenerate amino acid, with four mRNA codons
GCN (The
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
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.
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 © 2014 by Steven M. Carr