Final answer:
The mutation is a transversion at the first nucleotide position of the leucine codon CUG, resulting in the methionine codon AUG.
Step-by-step explanation:
The consequence of a single-base substitution in a codon that specifies leucine, producing a codon that specifies methionine. Since the methionine codon is AUG, the only leucine codon that can change in the first position to create this is CUG. Changing the C to an A through a single-base substitution will result in AUG, the codon for methionine. This type of mutation is known as a transversion, where a purine is replaced by a pyrimidine or vice versa. Since C (a pyrimidine) is being replaced by A (a purine), it is a transversion occurring at the first nucleotide position of the leucine codon.