Final answer:
A change from UUU to UUC in an mRNA sequence does not alter the amino acid phenylalanine that is coded for, making this a silent mutation with no effect on the protein's function.
Step-by-step explanation:
A change in an mRNA nucleotide sequence from UUU to UUC results in a silent mutation. This is because both UUU and UUC codons encode for the amino acid phenylalanine. Since the change does not result in a different amino acid, the mutation is considered silent and typically has no effect on the protein's function. If the sequence mutation had resulted in a different amino acid, it could have been a missense mutation, and depending on the properties of the amino acid substituted, it could have been harmful or even beneficial. A nonsense mutation would occur if the change resulted in a stop codon, leading to an incomplete and likely non-functional protein.