How does the DNA sequence affect the structure of the protein?
Each amino acid in a protein is coded as three codons or bases.
If any one of the codons is incorrect, the amino acid will be also incorrect. There is a "wobble" effect in the last base so there can be some neutral effect.
But if the base is incorrect at the first or second spot in the codon, there is a good chance that the protein will be incorrect (a mutation). Most of our enzymes are proteins as well as many hormones and tissues. These will not work correctly and the result will be a malfunction where these proteins are needed.
Think of all the steps in the metabolic processes that will come to an abrupt halt.