A point mutation can be completely harmless, or it can result in the death of a cell or organism. Explain why these types of mutations can have such varying effects

Question

What will be an ideal response?

Answer 1

A point mutation is a single base change in the nucleotide sequence of a cell's genome. The effects of a point mutation can depend on its location. Point mutations in noncoding regions of the genome are usually harmless. Even in coding regions, point mutations can be harmless if they result in silent mutations. Silent mutations preserve the sense of the amino acid code because of the concept of "wobble," in which two codons can code for the same amino acid by varying only at the third base of the codon. If the point mutation has occurred at this third base, then the amino acid sequence of the protein will remain unchanged. Point mutations occurring at the first or second base of the codon are almost always much more serious because they change the codon to a completely different amino acid. This type of point mutation is known as a missense mutation. Occasionally, changes in the first or second base are still silent, in the case of amino acids specified by several codons (e.g. CGG and AGG both code for Arginine). (Another exception to this occurs when the new amino acid is chemically similar to the previous amino acid, in which case the missense mutation usually causes little or no change in the overall structure or function of the protein.) Finally, one of the most serious types of point mutations is a nonsense mutation in which the codon has been changed to a stop codon. These types of mutations result in the abnormal termination of a protein sequence. In all such cases, if the protein affected by the mutation is an enzyme or some other vital protein required for proper cellular function, then the cell and/or organism may die as a result of these relatively simple mutations.
Bloom's Taxonomy: Analysis
Section: Mutations of Genes
Learning Outcome: 7.20, 7.21