Question

Some types of gene mutations result in a substitution of one DNA nucleotide for another. This is called a point mutation. Another type of mutation is a frameshift mutation. In a frameshift mutation, a nucleotide is inserted into or deleted from the nucleotide chain.

- Describe what happens to a protein produced as a result of a point mutation.


- Describe what happens to a protein produced as a result of a frameshift mutation.


- Which type of mutation is most likely to result in a functional protein? Explain.

Answer 1

- Point mutation (single nucleotide substitution):
- Can be silent (synonymous) — the altered codon still codes for the same amino acid because of genetic code redundancy; the protein is unchanged and usually functional.
- Can be missense — the altered codon codes for a different amino acid; the protein has one amino acid changed. That single change may have little effect (conservative substitution), a moderate effect, or a severe effect on protein structure and function (example: sickle-cell disease is caused by one missense mutation).
- Can be nonsense — the altered codon becomes a stop codon, producing a truncated protein that is often nonfunctional or rapidly degraded.

- Frameshift mutation (insertion or deletion of a nucleotide that is not a multiple of three):
- Shifts the reading frame for every codon downstream of the change, so the sequence of amino acids after the mutation is usually completely different.
- This typically produces a protein with many incorrect amino acids and often encounters a premature stop codon, yielding a severely shortened, nonfunctional protein.

- Which is more likely to yield a functional protein?
- Point mutations are more likely to result in a functional protein because they can be silent or produce only a single amino-acid change. Frameshifts change every downstream codon and almost always disrupt protein structure and function.