A mutation that causes a single letter change in the DNA sequence can affect your protein structure and function because the sequence of letters in a gene determines the sequence of amino acids in the protein that it encodes. A single letter change in the DNA alters the mRNA which alters the amino acid sequence of the resulting protein and sometimes that can cause serious results if it changes the way a protein works.
ex. sickle cell disease - a single letter swap in the gene that encodes the hemoglobin molecule results in deformed red blood cells
