mRNA code can be derived as follows. In RNA instead of Thymine (T) you have Uracil (U)
mRNA string will have the complementary base pair to the DNA string
Complementary paring -
A-T here U
Therefore mRNA code is,
DNA - TAC/TTC/AAA/CCG/CGT/GGG/ATT
mRNA - AUG/AAG/UUU/GGC/GCA/CCC/UAA
To find the appropriate amino acid sequence we need a the codon code.
The amino acid squence is,
AUG is the start codon and it will represent Methionine.
UAA is the stop codon
Therefore the sequence is,
Methionine-Lysine - Phenylalanine-Glycine-Alanine-Proline (Stops)
Suppose the DNA code was changed like below through a mutation.
DNA - TAC/TTC/ATA/CCG/CGT/GGG/ATT
mRNA - AUG/AAG/UAU/GGC/GCA/CCC/UAA
Then the mRNA code also changes and protein will change as below.
Methionine-Lysine -Tyrosine-Glycine-Alanine-Proline (Stops)
Phenylalanine is replaced by tyrosine. Then the whole nature of the protein can be changed and it will have a different function.