a) The new mRNA sequence resulting from the mutation would be GUCUAACU. According to the codon chart, this codon (GUC) codes for the amino acid Valine (Val).
b) This mutation would not result in a phenotypic change because the codon GUC still codes for the same amino acid Valine (Val) as the original codon GUU. A substitution mutation in the third position of a codon often does not lead to a change in the amino acid sequence, as there is redundancy in the genetic code.