- Two different missense mutations are found in a gene for a kinase. Mutation X changes a cysteine residue involved in a critical disulfide bond to a serine. Mutation Y changes a valine to an isoleucine in a surface loop far from the active site. Which statement best predicts the phenotypic outcome?
Answer: Mutation X is likely to cause a severe loss of function, while Mutation Y is likely to be benign.
Mutation X disrupts a disulfide bond, which is often crucial for maintaining the correct three-dimensional structure of a protein. Replacing cysteine with serine removes the sulfhydryl group necessary for this bond, likely leading to misfolding and a severe loss of function. Mutation Y is a conservative substitution (valine to isoleucine, both nonpolar) in a non-critical region (a surface loop). This type of change is very unlikely to have a significant impact on the protein's function. Therefore, Mutation X is predicted to be much more severe than Mutation Y. Distractor C is incorrect because despite some structural similarity, the functional difference of the side group (SH vs OH) is profound in this context.
- The following mRNA sequence is part of a coding region: 5'-AUG GCU UAC GAG-3'. A single uracil (U) is inserted immediately after the third codon, UAC. What is the effect on translation from this point onward?
Answer: The insertion creates a premature stop codon, leading to termination.
The original sequence is read as AUG (Met), GCU (Ala), UAC (Tyr), GAG (Glu). The insertion of a U after UAC changes the sequence to 5'-AUG GCU UAC U GAG...-3'. The ribosome reads in triplets, so the new reading frame after UAC is UGA G... The codon UGA is a stop codon. Therefore, the insertion immediately creates a premature stop codon, and translation will terminate after the tyrosine is added. The resulting product would be a tripeptide (Met-Ala-Tyr). Distractor C fails to recognize the new stop codon. Distractors B and D misinterpret the effect of a frameshift-inducing insertion.
- A gene acquires a +1 frameshift mutation due to a single nucleotide insertion at codon 50. A researcher isolates a revertant strain that produces a near-functional protein. Sequencing reveals a second, intragenic mutation. Which of the following is the most plausible second mutation to explain the restored function?
Answer: A deletion of a single nucleotide at codon 55.
The original mutation is a +1 frameshift. To restore the reading frame, a compensating mutation that brings the frame back to normal is needed. A single nucleotide deletion (-1) downstream of the original insertion would cancel out the +1 shift, restoring the correct reading frame for the rest of the gene. This would result in a short segment of incorrect amino acids between codons 50 and 55, but the bulk of the protein would be correct, leading to a near-functional state. Distractor B would have no effect on the frameshift. Distractor C would remove an entire amino acid but would not correct the reading frame. Distractor D would create a +2 frameshift, which would not restore the original reading frame.
- A single nucleotide substitution occurs in the first exon of a gene, changing the DNA sequence from 3'-TAC-5' to 3'-TCC-5' on the template strand. What is the most likely effect on the protein product?
Answer: Translation will fail to initiate at the canonical start site, preventing synthesis of the correct protein.
The sequence 3'-TAC-5' on the DNA template strand is transcribed into the 5'-AUG-3' mRNA codon, which is the canonical start codon for translation and codes for methionine. The mutation changes the template strand to 3'-TCC-5', which is transcribed into 5'-AGG-3', a codon for arginine. The ribosome and initiation factors specifically recognize the AUG codon to begin translation. Replacing it with AGG will severely impair or completely prevent the initiation of translation at this site, leading to a failure to produce the intended protein. While translation could potentially start at a downstream AUG (Distractor B), the primary and most severe effect is the loss of the canonical start signal. Distractor A underestimates the importance of the start codon for initiation. Distractor D incorrectly identifies a substitution as a frameshift.
- A G-to-A transition creates a nonsense codon (UAG) at position 75 of a gene that normally codes for a 350-amino acid protein. This mutation is located in the second exon of a 7-exon gene. What is the most likely molecular consequence in a mammalian cell?
Answer: The mutant mRNA transcript will be targeted for degradation, resulting in little or no protein production.
Premature termination codons (PTCs) located upstream of the final exon-exon junction are typically recognized by the nonsense-mediated decay (NMD) surveillance pathway in mammalian cells. This pathway targets the faulty mRNA for rapid degradation to prevent the production of potentially harmful truncated proteins. Since the PTC at codon 75 in exon 2 is far from the final exon, NMD is the most likely outcome. Distractor A ignores the effect of NMD. Distractor B describes ribosomal read-through, which is a very rare event. Distractor C describes an alternative splicing outcome that is possible but less direct and certain than NMD activation.
- A single base-pair substitution results in a missense mutation, changing an alanine (a small, nonpolar amino acid) to a threonine (a polar amino acid) on the surface of a protein, away from any known functional sites. Which of the following is the most accurate statement about the likely effect on protein function?
Answer: The functional consequence cannot be determined with certainty and could range from negligible to significant.
When evaluating the functional impact of amino acid substitutions, you must consider multiple factors: the chemical properties of the amino acids involved, the location within the protein structure, and the complexity of protein function itself.
The alanine-to-threonine substitution represents a significant chemical change—from a small, hydrophobic residue to a larger, polar one with a hydroxyl group. While the mutation occurs on the protein surface (away from functional sites), this doesn't automatically determine its impact. Surface residues can affect protein stability, folding kinetics, protein-protein interactions, and even distant conformational changes through allosteric effects. The polar threonine might form new hydrogen bonds with water or other residues, potentially stabilizing or destabilizing the protein, or it could disrupt existing surface interactions.
Answer A correctly acknowledges this uncertainty—without experimental data, the functional consequence could range from completely negligible to significantly harmful, depending on the specific protein context and cellular environment.
Answer B incorrectly assumes surface mutations are always silent. Surface residues frequently influence protein function through stability and interaction effects. Answer C represents an extreme overstatement—a single amino acid change, especially outside functional sites, rarely renders proteins completely non-functional. Answer D makes an unwarranted prediction about stability; while threonine can form favorable water interactions, it might also disrupt existing stabilizing interactions or create unfavorable steric clashes.
Remember: predicting mutation effects requires experimental validation. Be wary of answer choices that make definitive claims about protein function without considering the inherent complexity and context-dependence of protein structure-function relationships.
- A mutation alters the canonical GT dinucleotide at the 5' splice site of intron 2 in a eukaryotic gene containing 5 exons. What is the most probable effect on the mature mRNA transcript and the resulting protein?
Answer: Intron 2 is retained in the mature mRNA, likely causing a frameshift and creating a premature stop codon.
The 5' splice site (typically a GT sequence in the DNA, corresponding to GU in the pre-mRNA) is essential for the spliceosome to recognize and remove an intron. Mutating this site will most likely cause the splicing machinery to fail to recognize the intron-exon boundary. As a result, intron 2 will be retained in the mature mRNA. Since introns do not typically maintain the reading frame and often contain stop codons, their inclusion in the mRNA usually leads to a frameshift and premature termination of translation, producing a severely altered and non-functional protein. Distractor A is a common misconception; while introns are non-coding, their boundary sequences are critical. Distractor B confuses splicing with transcription. Distractor C describes exon skipping, which can happen with splice site mutations, but intron retention is a very common and direct consequence of a non-functional 5' site.
- A researcher introduces a single-nucleotide deletion into codon 10 of a gene. Sequence analysis of the resulting mRNA reveals that the next in-frame stop codon occurs at what would have been codon 15 in the original sequence. Which term best describes the overall effect of this mutation?
Answer: A frameshift mutation leading to premature termination.
When you encounter questions about nucleotide deletions and their effects on protein synthesis, focus on how the deletion impacts the reading frame and what happens downstream.
A single-nucleotide deletion in codon 10 shifts the reading frame for all subsequent codons. This frameshift causes the ribosome to read the mRNA sequence incorrectly from that point forward, creating entirely different amino acid sequences than intended. The fact that a stop codon appears at position 15 (where it shouldn't normally exist) confirms that this frameshift created a premature termination signal, cutting protein synthesis short.
Answer A correctly identifies this as a frameshift mutation leading to premature termination - exactly what we observe here.
Answer B is wrong because frameshift mutations don't produce "stable proteins." The completely altered amino acid sequence from codon 10 onward, plus early termination, would likely create a nonfunctional protein.
Answer C incorrectly calls this a "simple nonsense mutation." Nonsense mutations involve a single base change that directly creates a stop codon, not a deletion that shifts the entire reading frame.
Answer D mischaracterizes this as a missense mutation. While codons 10-14 do produce different amino acids due to the frameshift, this description ignores the crucial early termination and fails to recognize the frameshift mechanism.
Remember: single-nucleotide insertions or deletions (except in multiples of three) always cause frameshifts. Look for downstream effects like premature stop codons to determine the full impact of the mutation.
- A gene's coding sequence ends with the mRNA sequence 5'-...UGG GAC UAG...-3', where UAG is the stop codon. A deletion of the first guanine (G) in the tryptophan (UGG) codon occurs. What is the effect on the protein?
Answer: A frameshift replaces the C-terminal aspartic acid with a new sequence and extends the protein.
The original sequence codes for Trp (UGG) - Asp (GAC) - Stop (UAG). Deleting the first G of the UGG codon results in the new sequence 5'-...UG GAC UAG...-3'. The reading frame shifts. The ribosome now reads the codons as UGG (Trp), followed by ACU (Threonine), followed by AG... (whatever follows in the 3' UTR). The original stop codon UAG is no longer in frame. This frameshift mutation not only changes the C-terminal amino acid from Asp to Thr but also causes the ribosome to read past the original termination site, extending the protein until a new stop codon is encountered in the shifted frame. Distractor A misunderstands how deletions affect the reading frame. Distractor C confuses a deletion with a substitution. Distractor D is incorrect.
- A gene contains the DNA sequence 5'-GGT-3' on the coding strand, which codes for glycine. A point mutation changes this sequence to 5'-GGC-3'. What is the effect of this mutation on the protein?
Answer: A silent mutation resulting in no change to the amino acid sequence.
The DNA coding strand has the same sequence as the mRNA, with T instead of U. The original codon, corresponding to 5'-GGT-3', is GGU in mRNA. The mutated codon, corresponding to 5'-GGC-3', is GGC in mRNA. Due to the degeneracy of the genetic code (the 'wobble' in the third position), both GGU and GGC code for the amino acid glycine. Therefore, this substitution has no effect on the amino acid sequence and is a silent mutation. The distractors describe other types of mutations that do not apply here.
- The mRNA sequence for the C-terminus of a wild-type protein is 5'-...GAA UAC UAA GCU-3'. A point mutation in the gene's DNA results in a transcript with the sequence 5'-...GAA UAC UCA GCU-3'. What is the most probable effect on the polypeptide chain?
Answer: The protein will be extended until the next in-frame stop codon is reached.
The original sequence has a UAA codon, which is a stop codon. The mutation changes this to UCA, which codes for the amino acid Serine. Therefore, translation will no longer terminate at this position. Instead, a serine will be added, and the ribosome will continue translating the 3' untranslated region (UTR) of the mRNA until it encounters the next in-frame stop codon. This results in an extended protein. Distractor A describes the effect of a nonsense mutation, not the mutation of a stop codon. Distractor B incorrectly assumes UAA coded for an amino acid. Distractor D is incorrect because termination will occur at a defined downstream stop codon, not randomly.
- A missense mutation occurs in a gene encoding a critical enzyme. Based on the provided table of amino acid properties, which of the following substitutions is most likely to result in a functional, or near-functional, enzyme?
Answer: Valine (nonpolar, aliphatic) is replaced by isoleucine (nonpolar, aliphatic).
A 'conservative' missense mutation, one that substitutes an amino acid with another of similar chemical properties, is least likely to disrupt protein function. Valine and isoleucine are both nonpolar, aliphatic amino acids with similar sizes. Distractor A represents a drastic change from a negatively charged to a positively charged residue. Distractor B is a change from a large, nonpolar residue to a small, polar one. Distractor D replaces the most flexible amino acid (glycine) with the most rigid one (proline), which can severely disrupt protein secondary structure.
- A gene contains a coding region with a CAG trinucleotide repeat. An error during DNA replication leads to the insertion of two additional CAG repeats within this region. Which of the following best describes the effect on the encoded protein?
Answer: Two glutamine residues will be added to the protein's amino acid sequence.
The insertion consists of two CAG repeats, totaling 6 nucleotides. Since 6 is a multiple of 3, the insertion is 'in-frame' and does not cause a frameshift (eliminating A). The mRNA codon CAG codes for the amino acid glutamine. Adding two CAG repeats to the coding sequence will result in the addition of two glutamine residues to the polypeptide chain at that position. This is not a silent mutation, as the protein's primary structure is altered (eliminating B). While such an expansion can affect function, it does not directly create a stop codon (eliminating D). This mechanism is characteristic of trinucleotide repeat disorders.
- A researcher identifies a genetic variant involving a 3-base-pair deletion that removes the codon for phenylalanine at position 150 of a 400-amino-acid protein. The deletion occurs in a region coding for a flexible surface loop, distant from any active or binding sites. What is the most likely consequence for the protein's function?
Answer: A minor alteration in protein structure with a high probability of retained function.
A deletion of 3 base pairs removes exactly one codon. This is an 'in-frame' deletion, meaning the reading frame for all subsequent codons remains unchanged. The resulting protein will be missing one amino acid (phenylalanine) but will be otherwise identical to the wild-type protein. Since the deletion is in a non-critical, flexible loop, this small change is unlikely to disrupt the overall structure or function of the protein. Distractor A is the most common error, assuming any deletion causes a frameshift. Distractor C confuses a deletion with a nonsense mutation. Distractor D is unlikely given the location of the deletion specified in the stem.
- Consider two separate single-nucleotide substitution mutations in a gene encoding a 500-amino-acid enzyme. Mutation A is a nonsense mutation at codon 25. Mutation B is a missense mutation at codon 450, changing a valine to an isoleucine. Which statement accurately compares the likely effects of these mutations?
Answer: Mutation A is almost certain to produce a non-functional protein, while Mutation B is likely to have a minimal effect on function.
Mutation A is a nonsense mutation very early in the coding sequence (codon 25 of 500). This will lead to a severely truncated protein that is almost certainly non-functional. Mutation B is a missense mutation that is 'conservative'—it replaces valine with isoleucine, which are both small, nonpolar amino acids with similar properties. Such a substitution, especially late in the protein sequence, is very likely to have little or no effect on the protein's structure and function. Therefore, Mutation A is far more severe than Mutation B. The other options contain incorrect reasoning about mutation severity.
- A missense mutation in the hydrophobic core of a globular protein replaces a leucine residue with a glutamic acid residue. What is the most probable consequence for the protein?
Answer: The protein will likely misfold or become unstable due to the unfavorable substitution.
The hydrophobic core is a critical structural feature of globular proteins, stabilized by the exclusion of water and favorable interactions between nonpolar side chains. Leucine is a hydrophobic amino acid, well-suited for this environment. Glutamic acid is hydrophilic and negatively charged. Placing a charged, water-loving residue into the oily, nonpolar core is energetically very unfavorable. This is a non-conservative substitution in a critical location, and it will likely disrupt the normal folding pattern and decrease the overall stability of the protein. Distractor A underestimates the importance of chemical properties. Distractors B and D propose positive outcomes that are biochemically implausible in this context.
- A segment of the template strand of a gene is 3'-TAC TGG CCT AAT-5'. If a transition mutation occurs at the 7th nucleotide, changing it from C to T, what is the resulting change in the polypeptide?
Answer: A missense mutation changing Glycine to Arginine.
First, transcribe the original template strand (3'-TAC TGG CCT AAT-5') to mRNA: 5'-AUG ACC GGA UUA-3'. This codes for Met-Thr-Gly-Leu. The mutation changes the 7th base of the template strand from C to T, so the new template is 3'-TAC TGG TCT AAT-5'. The new mRNA is 5'-AUG ACC AGA UUA-3'. The codon AGA codes for Arginine. Thus, the mutation changes Glycine (GGA) to Arginine (AGA), which is a missense mutation. Distractor C results from incorrectly translating the DNA template strand directly (CCT=Pro, TCT=Ser). Distractor A is incorrect because the amino acid changes. Distractor D is incorrect because AGA codes for an amino acid, not a stop signal.
- A gene undergoes a mutation that results in the deletion of 6 consecutive base pairs from an exon. Which of the following is the most direct and certain outcome for the protein product?
Answer: The protein will be missing two adjacent amino acids but the downstream reading frame will be correct.
A deletion of 6 base pairs is a deletion of a multiple of 3. This means that exactly two codons are removed. This is an 'in-frame' deletion. The reading frame for the codons downstream of the deletion is not affected. Therefore, the resulting protein will be shorter by two amino acids, but the sequence of all other amino acids will be correct. Distractor A is incorrect because there is no frameshift. Distractor C is a possible secondary consequence if the deletion brings two parts of the protein together in a way that destabilizes it, but it is not the direct result of the deletion itself, which is in-frame. Distractor D makes an assumption about function that cannot be known for certain; the deletion could be catastrophic or benign depending on the location.
- A gene encoding a 300-amino acid protein suffers a single nucleotide deletion at the codon corresponding to amino acid 298. What is the most likely outcome for the resulting protein?
Answer: A protein truncated at or shortly after position 298, with an altered C-terminal sequence.
A single nucleotide deletion causes a frameshift. This alters the reading frame from position 298 onward. The new reading frame will likely encounter a premature stop codon, leading to a truncated protein. The few amino acids translated after the deletion but before the new stop codon will also be incorrect. Because the mutation is so close to the natural C-terminus, the protein is only altered at its very end. Distractor A confuses a deletion with a substitution. Distractor C overstates the effect; a frameshift at the end of a gene is much less damaging than one at the beginning. Distractor D is incorrect because nonsense-mediated decay is typically not triggered by premature stop codons located in the final exon or very close to the natural stop codon.
