Six-fold degeneracy of Leucine codons

The genetic code for leucine (Leu) can be either CUN or UUR. Consequently, either UUA or CUA, as well as either UUG or CUG code for Leu. That is, these first position changes do not change the amino acid, and the only exceptions to the rule that first positions 'always' lead to amino acid substitutions in the Universal Code.

Homework: There are two other amino acids encoded by six codons each: explain why they don't violate the rule

Figure © 2000 by Griffiths et al. ; text © 2009 by Steven M. Carr