Questions: Point Mutations: Silent, Missense, and Nonsense

Question 1

A mutation changes the second nucleotide of a codon from U to A, converting UUA to UAA. What is the most likely consequence?

• A. A silent mutation, because the wobble position allows flexible coding at the second position
• B. A nonsense mutation, because UAA is a stop codon that terminates translation prematurely
• C. A conservative missense mutation because only one nucleotide changed
• D. No functional effect, because single nucleotide changes at internal positions never halt translation

Answer: (B)

UAA is one of the three stop codons (UAA, UAG, UGA). A mutation that creates a stop codon within a protein-coding sequence is a nonsense mutation, which truncates the protein at that point. The wobble position (third position) is where degeneracy is concentrated and silent mutations are most common — not the second position. Second-position changes are among the most consequential because they almost always change the amino acid or, as here, can create a stop codon.

Question 2

A geneticist finds two mutations in a critical enzyme: Mutation A creates a premature stop codon near the middle of the protein, and Mutation B changes a single amino acid in the enzyme's catalytic active site. Which mutation is necessarily more damaging?

• A. Mutation A, because premature stop codons always abolish all protein function
• B. Neither is necessarily worse — Mutation B could destroy active site function entirely, while Mutation A might yield a truncated protein that retains some activity
• C. Mutation A, because any protein shorter than the wild type is nonfunctional
• D. Mutation B, because missense mutations always produce dominant negative effects that are worse than truncations

Answer: (B)

Mutation severity cannot be predicted from type alone. A premature stop codon early in the protein typically abolishes function, but one near the end may yield a nearly complete protein with partial or full activity. Conversely, a missense mutation at a critical catalytic residue can completely destroy enzymatic function even though the protein is the correct length. Sickle cell disease is caused by a single missense mutation (Glu→Val) — not a nonsense mutation — yet it produces one of the most clinically significant genetic diseases. Context determines impact.

Question 3

Silent mutations predominantly occur at the third (wobble) position of codons because the genetic code concentrates its degeneracy at that position.

• T. True
• F. False

Answer: True

The genetic code is structured so that most synonymous codons (those encoding the same amino acid) differ only at the third position. For example, all six leucine codons share UU at the first two positions and differ at the third. This is not random — the third position accommodates 'wobble' base pairing in the ribosome, and it appears to be an evolved feature of the code that minimizes the damage from the most common types of mutation. First- and second-position changes almost always alter the amino acid, while third-position changes are often silent.

Question 4

Nonsense mutations are generally more damaging than missense mutations because they terminate translation early and produce a shorter, incomplete protein.

• T. True
• F. False

Answer: False

The severity of any mutation depends on context, not type. A missense mutation at a critical active-site residue can completely destroy protein function; the sickle cell hemoglobin mutation (a missense) is among the most consequential mutations known. Meanwhile, a late-occurring nonsense mutation might produce a nearly complete protein with substantial residual function. Additionally, some missense mutations produce dominant negative effects — where the altered protein actively interferes with the normal protein — which can be worse than simply losing one functional copy of the gene.

Question 5

Why can't you predict the severity of a point mutation from its type (silent, missense, or nonsense) alone, and what additional information do you need?

Model answer: Severity depends on the specific amino acid change, the structural and functional role of that residue in the protein, the position within the coding sequence, and whether the change is conserved across species. A missense mutation at a catalytic active site can be lethal; one at a surface-exposed residue far from any functional domain may be nearly neutral. A nonsense mutation near the end of a gene may produce a nearly functional protein, while one early in the sequence abolishes it completely.

This is why modern genetics has moved from classifying mutations by type to evaluating each variant in its molecular context — the field of variant effect prediction. Tools like SIFT, PolyPhen, and deep mutational scanning try to estimate the functional impact of specific amino acid changes by combining evolutionary conservation data, protein structure, and experimental measurements. The same type of mutation (e.g., missense) can range from completely neutral to severely pathogenic depending entirely on which residue is changed and what it does.