A2 Genetic Control of Proteins and Gene Expression Practice Test 2026 - Free Practice Questions and Study Guide

The main concept is what happens when a single amino acid in a protein is changed. This is a missense mutation, where one amino acid is substituted for another in the polypeptide chain. Replacing valine with alanine alters the side chain at that position. Valine has a larger, branched hydrophobic side chain, while alanine is smaller. This change can affect how the protein folds because folding depends on how side chains pack together and interact in the interior and at interfaces. Even a smaller, similar nonpolar substitution can tweak packing enough to alter local structure, stability, or function, so the folding could change.

The other possibilities describe different types of mutations that aren’t occurring here: a silent mutation would leave the amino acid unchanged, a stop codon introduction would truncate the protein (nonsense mutation), and a large insertion would disrupt the reading frame (frameshift).