How could a change in one nitrogen base alter a protein

So it starts with one nitrogen base change in DNA
The process of transcription occurs, where mRNA is made
Then translation, where the codons of mRNA is translated to amino acids.
1 codon = 3 nucleotides in mRNA
1 different nucleotide => 1 different codon
1 different codon = 1 different amino acid (more than one codon can code for an amino acid so it is possible there is no amino acid change, but in this case the question is saying the protein is altered)
Amino acid > Polypeptide > Protein
One different amino acid can change the structure of the protein.