When an incorrect base is inserted during DNA replication, the resulting point mutation can affect the gene product, the protein, in various ways. In the case illustrated, we are looking at the mRNA that has been made from the DNA. The 3rd base in the codon shown is supposed to be C. If a mutation occurred which caused it to be a U, the codon would still code for the amino acid tyrosine. This is a silent mutation because the protein is unchanged. If the 3rd base changed to a G, this would result in a stop codon which would halt protein production at that point. If the first base was changed to a C, the codon would code for a different amino acid, histidine. The protein would have one amino acid wrong which might, or might not, affect its function.