Now consider a single base mutation in a leucine codon that results in a phenylalanine codon. A true reversion would change the phenylalanine codon back to a leucine codon. Which of the following leucine codons could undergo a single mutation to form a phenylalanine codon and then sustain a second mutation at a different site to revert to a leucine codon? a. CUU b. CUC c. CUA d. UUG e. UUA f. CUG

To approach this question, we need to know the codons for both leucine and phenylalanine and understand that a true reversion involves two mutations - the first changing leucine to phenylalanine and the second reverting it back to leucine.

The codons for leucine are CUU, CUC, CUA, UUG, UUA, and CUG. The codons for phenylalanine are UUU and UUC. There needs to be a transition mutation (changing a purine to another purine or a pyrimidine to another pyrimidine) or a transversion mutation (changing a purine to a pyrimidine or vice versa) from leucine to phenylalanine and then another mutation at a different nucleotide position to revert back to leucine.

Let's check each leucine codon:

a. CUU can change to UUU by altering the first base (C to U). A second mutation at a different site cannot revert it back to a leucine codon without altering the base that was changed in the first mutation.

b. CUC can mutate to UUC (first base C to U), which is a phenylalanine codon. Another mutation at the third base (C to G), would result in CUG, which is a codon for leucine, achieving the true reversion.

c. CUA can change to UUA by altering the first base (C to U). Like with CUU, another mutation at a different site cannot revert it back to a leucine codon without changing the first base.

d. UUG can convert to UUU by changing the second base (U to U), however, this is not changing the base to a different one. A mutation back to a leucine codon would require altering the mutated base.

e. UUA cannot change to a phenylalanine codon via a single base change because it requires two base changes to align with UUU or UUC.

f. For CUG, a mutation from the first base (C to U) would result in UUG, which is still a codon for leucine, and not phenylalanine.

Based on this analysis, the correct answer is b. CUC, because it allows for a mutation to phenylalanine (CUC to UUC) and a subsequent true reversion back to a different leucine codon with another mutation at a different site (UUC to CUG).