Based on the data, which of the following mutations would most likely result in a nonfunctional protein?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Protein function emerges from precise three-dimensional folding driven by amino acid side-chain interactions: hydrophobic residues collapse into the interior core to escape aqueous solvent, while charged and polar residues form salt bridges, hydrogen bonds, and electrostatic interactions on the surface. The ribosome reads mRNA codons sequentially in the 5' to 3' direction during translation, recruiting charged tRNA molecules via elongation factors eEF1A and eEF2 in eukaryotes. Release factors (eRF1 in eukaryotes, RF1 and RF2 in prokaryotes) recognize the three stop codons—UAA, UAG, and UGA—and catalyze hydrolysis of the ester bond linking the completed polypeptide to the P-site tRNA, terminating translation.

Why Other Options Are Wrong

A nonsense mutation converts a sense codon into one of these stop codons through a single nucleotide substitution. When the ribosome encounters this premature stop codon during elongation, release factors bind and catalyze premature polypeptide release. The resulting truncated protein lacks all downstream amino acid residues, eliminating any functional domains, active-site residues, or structural motifs encoded beyond the mutation site. For example, a premature stop codon in the gene encoding β-globin (HBB) would truncate the hemoglobin subunit before residues His-F8 (the proximal histidine that coordinates the heme iron), abolishing oxygen binding entirely. Additionally, eukaryotic cells possess nonsense-mediated mRNA decay (NMD), a surveillance pathway in which the exon-junction complex (EJC) marks transcripts with premature termination codons for degradation by intracellular nucleases, reducing even the mutant protein's abundance.

PILLAR 2 — STEP-BY-STEP LOGIC

The question demands identification of which mutation category most predictably generates a nonfunctional protein product. Evaluating each mutation type through the lens of polypeptide structure–function relationships reveals a clear hierarchy of severity. A nonsense mutation producing a premature stop codon eliminates all C-terminal sequence downstream of the mutation site. Because functional protein domains—such as the catalytic triad of serine proteases (His-Asp-Ser) or the DNA-binding helix-turn-helix motif of lac repressor—require complete primary structure to achieve proper tertiary folding, truncation invariably abolishes any domain encoded distal to the mutation. Even if the N-terminal portion folds independently, it cannot execute the full biological function. The predictability and completeness of functional loss distinguish nonsense mutations from other mutation classes, making option B the strongest answer.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A describes a missense mutation with a conservative amino acid substitution—for instance, replacing aspartate with glutamate, both negatively charged residues at physiological pH. Students selecting this option conflate any amino acid change with total functional loss. In reality, conservative substitutions often preserve side-chain chemistry (charge, hydrophobicity, size), allowing the mutant protein to fold and function nearly identically to wild type. Hemoglobin variants like HbC (Glu→Lys at position 6) retain oxygen-binding capacity despite altered surface charge, illustrating this principle.

Option C describes a frameshift mutation from a small deletion. Students recognize frameshifts as devastating and intuitively rank them highest. However, the question asks which mutation most likely yields a nonfunctional protein. A small deletion—depending on its precise position—could occur near the 3' end of the coding sequence, preserving most upstream functional domains. Furthermore, some frameshift readthrough or compensatory mutations can partially restore the reading frame. While frameshifts are generally severe, their functional consequences vary with position, making them less uniformly devastating than premature termination.

Option D describes a silent mutation yielding no amino acid change. Students selecting this option may misunderstand the central dogma, believing any DNA sequence alteration necessarily compromises protein function. Because the genetic code is degenerate—multiple codons specify the same amino acid, such as both GAA and GAG encoding glutamate—a silent mutation leaves the polypeptide entirely unchanged, and the protein remains fully functional.