Tay-Sachs disease is caused by a mutation of one nucleotide for a protein that disrupts the activity of an enzyme in the brain. This leads to a toxic level of a substance to build up in neurons in the brain and spinal cord, leading to severe brain damage and eventually death. Why do you think that the change in one nucleotide can cause Tay-Sachs disease to be fatal, whereas the change in one nucleotide causes sickle cell disease, a disease in which a person can lead a functional life?