1. The mRNA sequence with the mutant C:
AUGAAA CUUUCA CGCGUUUUUUAU
This mutation results in the substitution of codon UUC by codon UUU. As both codons encode the same amino acid phenylalanine (Phe), the amino acid sequence is not changed:
Met Leu Iso Leu Thr Leu Leu Val
2. If the third U becomes A, the mRNA sequence is as following:
AUGUAGAUA CUUACA CUUCUGGUC
Mutant codon UAG is a stop codon. As a result, the mRNA cannot be translated as the second triplet encodes a stop codon.