Why don these safeguards stop the codon expansion before it becomes harmful?
DNA interacts with many compounds that act as safeguards to repair any mutations that may occur. However, sometimes the safeguards miss the mutation, allowing it to elicit a change in the organism involved. Hundreds of millions of years ago, the safeguards missed the mutations that allowed the first animals to have lungs that could breathe air. These mutations allowed life to emerge from the sea and onto land. The safeguards also missed the mutations that allowed the first humans to stand upright, as well as the mutations that allowed people to develop the tools necessary for speech. These amazing feats would never have been achieved if the mutations would have been repaired by the safeguards. But the safeguards can also miss mutations that can have harmful results. For instance, it appears that in people with HD, the expansion mutation in the Huntington gene is not repaired by the safeguards. One of these safeguards is called MSH2. MSH2 is what is known as a mismatch repair enzyme: it
