AP Biology: Unit 6, Mutations (Drill 26)

Question 1. What type of mutation is represented by Mutation B (GAG->GAA)?

• A) Missense mutation, because the codon changes but the amino acid remains the same.
• B) Nonsense mutation, because the codon change introduces a premature stop.
• C) Silent mutation, because the codon changes but the amino acid sequence is unchanged. ✓
• D) Frameshift mutation, because a single nucleotide substitution shifts the reading frame.

Explanation: Mutation B changes GAG to GAA, both of which code for glutamic acid -- this is a synonymous substitution producing no change in amino acid sequence. This is the definition of a silent mutation. The 98% activity reflects experimental variation, not a functional change. A is incorrect because a missense mutation changes the amino acid; Mutation B does not. B is incorrect because a nonsense mutation introduces a stop codon -- that describes Mutation A. D is incorrect because a frameshift requires insertion or deletion of nucleotides, not a substitution.

Question 2. Based on the table, which conclusion is most directly supported by comparing the effects of the stop codon mutation and the charge-reversal mutation at position 47?

• A) Introducing a premature stop codon at position 47 is more disruptive to enzyme activity than replacing glutamic acid with lysine at that position. ✓
• B) The charge of the amino acid at position 47 is more important for enzyme activity than the size of the amino acid.
• C) Both mutations eliminate enzyme activity completely, confirming that position 47 is absolutely essential for any enzyme function to occur in this mutation comparison.
• D) Replacing glutamic acid with lysine is nearly as disruptive as a premature stop codon because both destabilize the active site equally.

Explanation: The stop codon mutation (0% activity) is more disruptive than the charge-reversal mutation (4% activity) -- this conclusion is directly supported by the table without requiring assumptions beyond the data. B is not supported because the table does not isolate amino acid size as a variable. C is incorrect because the charge-reversal mutation retains 4% activity, so it does not eliminate enzyme function entirely. D is contradicted by the data -- the two mutations produce clearly different levels of disruption (0% vs. 4%).

Question 3. A researcher claims that the 4% residual activity in the charge-reversal mutation is due to the positive charge of lysine partially compensating for the loss of negative charge at position 47. Which additional experiment would most directly test this claim?

• A) Introduce a mutation changing position 47 to a stop codon and test whether the stop codon itself supplies a positive charge in the scenario described.
• B) Introduce a mutation at position 47 that substitutes glutamic acid with alanine (nonpolar, uncharged) and measure enzyme activity. ✓
• C) Test enzyme activity at different pH levels to determine whether ionic conditions affect the wild-type enzyme.
• D) Compare the three-dimensional structure of the wild-type and charge-reversal mutant enzymes using X-ray crystallography.

Explanation: If positive charge partially compensates for lost negative charge, then substituting a nonpolar, uncharged amino acid at position 47 should produce activity lower than 4% -- because alanine provides neither positive nor negative charge, eliminating even partial ionic interaction. This directly tests whether charge drives the residual activity. A confirms the mutation exists but does not test whether charge is the mechanism. C tests pH effects on the wild-type but does not isolate the role of position 47 charge in the mutant. D provides structural information but does not directly measure the functional role of charge.

Question 4. The stop codon mutation at position 47 produces a truncated polypeptide of 46 amino acids. A student predicts this truncated protein will have no enzymatic activity. Which reasoning best supports this prediction?

• A) Stop codons are recognized by release factors, which chemically denature the truncated polypeptide.
• B) The truncated polypeptide terminates before position 47, so the active site residue is never incorporated and the substrate-binding geometry cannot form. ✓
• C) UAG is an amber stop codon, which is inherently more disruptive than UAA or UGA stop codons.
• D) Truncated polypeptides are immediately degraded by the ribosome itself before they have any chance to fold into a functional shape.

Explanation: The active site requires a negatively charged residue at position 47, but the truncated polypeptide terminates at position 46 -- one amino acid before the critical residue is incorporated. Without position 47 and the remaining 74 amino acids, the three-dimensional structure required for substrate binding cannot form. A is incorrect because release factors facilitate polypeptide release, not denaturation. C is incorrect because all three stop codons function equivalently in standard translation termination. D is incorrect because ribosomes do not degrade polypeptides -- that is the role of proteasomes, and degradation is not immediate or automatic.

Question 5. Which mutation type, if it occurred earlier in the coding sequence rather than at position 47, would most likely affect the largest number of downstream amino acids?

• A) Silent mutation occurring earlier in the coding sequence in the coding sequence described
• B) Missense mutation
• C) Nonsense mutation
• D) Frameshift mutation caused by a single nucleotide insertion ✓

Explanation: A frameshift mutation caused by a single nucleotide insertion shifts the reading frame for every codon downstream of the insertion point, typically altering every subsequent amino acid before often encountering a premature stop codon. A silent mutation changes a codon but not the amino acid, leaving all downstream sequence unaffected. A missense mutation changes one amino acid but leaves all downstream codons intact. Frameshifts typically alter many more downstream codons, whereas a nonsense mutation stops translation at a single defined position.