Genetics Quiz: Post Transcriptional Regulation

What this quiz covers

This quiz focuses on Post Transcriptional Regulation, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The PTC is recognized by a miRNA that specifically targets and degrades aberrant transcripts.
2. The aberrant protein product is highly unstable and rapidly degraded by the proteasome.
3. The process of nonsense-mediated decay (NMD) degrades the mRNA after it is recognized by the ribosome. (correct answer)
4. The mutation leads to transcriptional attenuation, preventing the completion of the primary transcript.

Explanation: When you encounter a genetics question involving a premature termination codon (PTC) and dramatically reduced mRNA levels, think about quality control mechanisms that cells use to eliminate faulty transcripts before they can produce harmful proteins. The key insight here is that the mRNA level—not protein level—is severely reduced. This points to nonsense-mediated decay (NMD), a cellular surveillance mechanism that detects and degrades mRNAs containing PTCs. NMD works when ribosomes encounter a stop codon upstream of exon-exon junction complexes during the pioneer round of translation. Since the frameshift occurred early in exon 2 of a 6-exon gene, the resulting PTC would be positioned well upstream of multiple remaining exon junctions, triggering robust NMD activation. Choice C correctly identifies this mechanism. Choice A is incorrect because miRNAs typically don't specifically target frameshift-induced transcripts, and this level of mRNA reduction is characteristic of NMD, not miRNA-mediated degradation. Choice B focuses on protein stability, but the question emphasizes mRNA reduction—if this were the explanation, you'd expect normal mRNA levels with rapid protein turnover. Choice D describes transcriptional attenuation, which would affect transcript initiation or elongation, but frameshift mutations in exons don't typically cause transcriptional problems. Remember this pattern: PTC + dramatic mRNA reduction = think NMD first. This quality control pathway is evolution's way of preventing cells from wasting resources on truncated, potentially harmful proteins.

Question 2

1. An accumulation of pri-miRNAs in the nucleus due to blocked processing by Drosha.
2. Enhanced processing of pre-miRNAs by Dicer in the cytoplasm to compensate for the block.
3. An increase in mRNA degradation due to the activation of alternative decay pathways.
4. A global decrease in mature miRNA levels, leading to widespread de-repression of target genes. (correct answer)

Explanation: When you encounter questions about miRNA regulation, focus on the sequential pathway: pri-miRNA → pre-miRNA → mature miRNA, and identify where the disruption occurs. Exportin-5 (XPO5) serves as the critical transport bridge between nuclear pre-miRNA production and cytoplasmic miRNA maturation. When a viral protein sequesters XPO5, pre-miRNAs become trapped in the nucleus and cannot reach the cytoplasm where Dicer processes them into functional mature miRNAs. This creates a bottleneck that dramatically reduces the cell's overall mature miRNA population. Since mature miRNAs normally bind to target mRNAs and suppress their translation or promote their degradation, fewer mature miRNAs means widespread loss of this regulatory control. Target genes that were previously silenced by miRNAs will now be expressed at higher levels—this is called de-repression. Answer D correctly captures this global effect. Answer A misidentifies the problem location. Drosha processes pri-miRNAs to pre-miRNAs in the nucleus before XPO5 transport, so blocking XPO5 wouldn't affect Drosha's activity. Answer B suggests compensatory enhancement, but Dicer cannot enhance processing of pre-miRNAs that never reach the cytoplasm due to blocked transport. Answer C incorrectly predicts increased mRNA degradation when the opposite occurs—without mature miRNAs to target them, mRNAs become more stable. Remember this pattern: when nuclear export is blocked in the miRNA pathway, always trace the downstream effects to mature miRNA depletion and subsequent target gene de-repression. This is a common mechanism viruses exploit to alter host gene expression.

Question 3

1. The endogenous miRNA pathway leading primarily to translational repression.
2. The RNA interference (RNAi) pathway, where RISC mediates target mRNA cleavage. (correct answer)
3. Nonsense-mediated decay (NMD) triggered by the introduction of a premature stop codon.
4. Transcriptional gene silencing through methylation of the Gene X promoter.

Explanation: Short, synthetic dsRNAs that are perfectly complementary to a target mRNA are the hallmark of RNA interference (RNAi), a pathway that utilizes siRNAs (short interfering RNAs). In mammals, perfect complementarity typically directs the Argonaute protein within the RISC complex to cleave the target mRNA, leading to its rapid degradation. (A) is less likely; while there is overlap with the miRNA pathway, perfect complementarity and significant mRNA reduction point strongly to cleavage-based RNAi rather than translational repression. (C) is incorrect as NMD is triggered by premature stop codons, not dsRNA. (D) describes transcriptional silencing, a different mechanism that would not cause a rapid reduction in existing mRNA levels.

Question 4

1. RNA splicing
2. translation initiation
3. RNA stability (correct answer)
4. polyadenylation

Explanation: When you encounter questions about RNA regulation, focus on distinguishing between the different levels of post-transcriptional control. The key clue here is that IRP1 binding "protects the transcript from degradation" — this directly points to RNA stability mechanisms. IRP1's binding to the 3' UTR of transferrin receptor mRNA creates a protective complex that prevents degradation enzymes from accessing and breaking down the transcript. By stabilizing the mRNA, more copies remain available for translation, effectively increasing protein production when iron levels are low. This is a classic example of post-transcriptional control of RNA stability, making C correct. Let's examine why the other options don't fit: A) RNA splicing occurs in the nucleus during pre-mRNA processing, not in the cytoplasm where mature mRNA interacts with regulatory proteins like IRP1. B) Translation initiation involves ribosome binding and start codon recognition — while IRP1 can affect translation indirectly by preserving mRNA, the primary mechanism described is protection from degradation, not direct control of ribosome binding. D) Polyadenylation happens during mRNA processing in the nucleus when the poly-A tail is added, not during cytoplasmic regulation by binding proteins. Remember this pattern: when you see binding proteins that "protect from degradation" or "stabilize transcripts," think RNA stability control. The 3' UTR location is also a strong hint, as this region commonly contains regulatory sequences that affect mRNA half-life rather than splicing or processing events.

Question 5

1. Co-transfection of a miR-21 mimic and a luciferase reporter fused to the wild-type PTEN 3' UTR shows reduced luciferase activity.
2. Co-transfection of a miR-21 mimic and a luciferase reporter fused to the PTEN 3' UTR with a mutated seed-matching site shows no change in luciferase activity.
3. Co-transfection of a miR-21 mimic and a luciferase reporter fused to the wild-type PTEN 3' UTR, and demonstrating that mutating the seed-matching site restores luciferase activity. (correct answer)
4. Showing that overexpression of miR-21 reduces endogenous PTEN protein levels in the cell.

Explanation: Rigorous proof of a direct miRNA-target interaction requires showing both the effect and its specificity. Option (C) accomplishes this in two steps. First, it shows that the miRNA represses the reporter containing the wild-type 3' UTR. Second, it demonstrates that this repression is abolished when the specific miRNA binding site is mutated. This 'rescue' experiment proves the effect is mediated through that exact sequence, confirming a direct interaction. (A) shows correlation but not causation. (B) is a necessary control, but alone it doesn't prove the wild-type site is targeted. (D) shows a downstream effect but does not prove the interaction is direct; it could be mediated by an intermediate factor.

Question 6

1. cytoplasmic 5'-to-3' exoribonucleases like XRN1. (correct answer)
2. the exosome complex acting in a 3'-to-5' direction.
3. sequence-specific endoribonucleases that cleave internally.
4. the RISC complex as part of miRNA-mediated silencing.

Explanation: When you encounter questions about mRNA decay pathways, focus on the sequential nature of the process and how each step enables the next. In eukaryotic mRNA degradation, the order of events matters crucially for understanding which enzymes can act when. The major decay pathway follows a specific sequence: first deadenylation removes the poly(A) tail, then decapping removes the 5' cap structure. This decapping step is the key here because the 5' cap normally protects mRNA from degradation. Once removed, the mRNA becomes vulnerable to 5'-to-3' exoribonucleases, particularly XRN1, which is the primary cytoplasmic enzyme responsible for this activity. XRN1 can only degrade uncapped mRNA because it requires access to the 5' monophosphate end that's exposed after decapping. Choice A correctly identifies this relationship between decapping and XRN1 exposure. Choice B describes the exosome complex, which does degrade mRNA in a 3'-to-5' direction, but this typically occurs from the 3' end after deadenylation—decapping doesn't directly expose mRNA to exosome activity. Choice C mentions endoribonucleases that cleave internally, but decapping doesn't specifically expose internal sites; these enzymes can act on capped mRNA. Choice D refers to RISC-mediated degradation, which is part of the miRNA pathway and doesn't require prior decapping—RISC can target capped mRNAs. Remember that mRNA processing and degradation questions often test the sequential dependencies between steps. Always consider what each modification enables or prevents in terms of enzyme access.

Question 7

1. The transcript will be degraded by the nonsense-mediated decay (NMD) pathway in the cytoplasm. (correct answer)
2. The transcript will be stable and translated into a shorter, non-functional protein.
3. The transcript will be trapped in the nucleus and degraded by the nuclear exosome.
4. The transcript will be targeted for degradation by a miRNA that recognizes the new exon-exon junction.

Explanation: When you encounter questions about aberrant splicing and premature stop codons, think immediately about quality control mechanisms that cells use to prevent defective proteins from being made. The correct answer is A because this scenario perfectly describes a nonsense-mediated decay (NMD) trigger. When exon 4 is skipped, the resulting frameshift creates a premature termination codon (PTC) in exon 5. Since this gene has 8 exons total, the PTC occurs upstream of normal exon-exon junctions that still contain exon junction complexes (EJCs). During the pioneer round of translation in the cytoplasm, ribosomes will encounter this PTC while EJCs remain bound downstream—the hallmark signal for NMD activation. The transcript will be rapidly degraded. Answer B is wrong because cells don't allow aberrant transcripts with PTCs to be stably translated—NMD specifically evolved to prevent this wasteful and potentially harmful outcome. Answer C is incorrect because the transcript will successfully exit the nucleus; nuclear quality control doesn't typically recognize this type of splicing defect since the transcript appears properly processed. Answer D is wrong because miRNAs don't specifically target novel exon-exon junctions created by exon skipping, and this mechanism wouldn't be the primary response to PTC-containing transcripts. Remember this key principle: premature termination codons upstream of exon-exon junctions almost always trigger NMD. When you see frameshift mutations from exon skipping in multi-exon genes, NMD should be your first thought for the transcript's fate.

Question 8

1. Histone mRNAs will persist longer than normal, leading to histone synthesis outside of S-phase. (correct answer)
2. Histone mRNAs will be degraded more rapidly, leading to insufficient histone production for DNA replication.
3. The transcription of histone genes will be permanently silenced to compensate for the defect.
4. The poly-A tails of histone mRNAs will be extended, leading to more efficient translation initiation.

Explanation: Deadenylases are enzymes that shorten the poly-A tail of mRNAs, which is the rate-limiting step for their degradation. Histone mRNA levels are tightly controlled and must be rapidly degraded at the end of S-phase. A mutation inactivating the specific deadenylase responsible for this process would prevent efficient degradation. Consequently, the histone mRNAs would have an extended half-life and persist into G2/M, leading to inappropriate synthesis of histones outside of S-phase. (B) is the opposite of the expected effect. (C) is an indirect, compensatory mechanism, not a direct consequence. (D) is incorrect; deadenylase inactivation prevents shortening, it doesn't cause extension.

Question 9

1. Mcl-1 protein levels will decrease due to a dominant effect of miR-125b repression.
2. Mcl-1 protein levels will increase due to competitive exclusion of miR-125b by the overexpressed HuR. (correct answer)
3. There will be no net change in Mcl-1 protein levels because the stabilizing and repressive effects will cancel each other out.
4. Mcl-1 protein levels will increase because HuR will directly enhance the transcription rate of the Mcl-1 gene.

Explanation: This scenario describes competitive binding for an overlapping site. Since HuR is highly overexpressed, it will likely outcompete miR-125b for binding to the Mcl-1 3' UTR. The predominant effect will therefore be that of HuR, which is mRNA stabilization. This stabilization increases the mRNA half-life, leading to more protein production. The repressive effect of miR-125b is effectively masked or prevented. (A) is incorrect because the high concentration of HuR makes it the likely winner of the competition. (C) is unlikely as one effect will probably dominate. (D) is incorrect because HuR is an RNA-binding protein that acts post-transcriptionally.

Question 10

1. The SNP occurs outside the seed region and strengthens the thermodynamic stability of the miRNA-mRNA duplex.
2. The SNP is located within the seed region sequence of the KRAS 3' UTR, disrupting the binding of let-7. (correct answer)
3. The SNP is located in the 3' UTR but several hundred bases away from the conserved let-7 binding site.
4. The SNP creates a new binding site for a different miRNA that also targets KRAS for repression.

Explanation: The 'seed region' (nucleotides 2-8 of the miRNA) is critical for target recognition. A mutation within the complementary site in the mRNA's 3' UTR would likely disrupt or prevent the binding of the let-7 miRNA. If let-7 can no longer bind efficiently, its repressive effect is lost, leading to increased translation of the KRAS mRNA and thus higher KRAS protein levels. (A) describes a scenario that would likely enhance repression, not reduce it. (C) describes a mutation unlikely to affect let-7 binding at its specific site. (D) would lead to more repression, not less.

Question 11

1. They will regulate completely different sets of target mRNAs due to the non-seed differences.
2. Only one of the two miRNAs can be loaded into the RISC complex at any given time in a cell.
3. miR-200a will bind to and inhibit the function of miR-200b through direct RNA-RNA interaction.
4. They will regulate a largely overlapping set of target mRNAs, with some minor differences in repression efficiency. (correct answer)

Explanation: When you encounter miRNA targeting questions, focus on the seed sequence - it's the primary determinant of target specificity. The seed sequence (nucleotides 2-8) is the most critical region for miRNA-mRNA binding, as it forms the most stable base pairs with target mRNA sequences in their 3' untranslated regions. Since miR-200a and miR-200b share identical seed sequences, they will recognize and bind to the same complementary sequences on target mRNAs. This means they'll regulate a largely overlapping set of targets. The two nucleotide differences at positions 12 and 19 are outside the seed region and will have minimal impact on target selection, though they may cause slight differences in binding stability or repression efficiency for some targets. Answer A incorrectly assumes that any sequence difference leads to completely different targets, ignoring the primacy of seed sequence matching. Answer B reflects a misunderstanding of RISC loading - multiple miRNAs can be loaded into different RISC complexes simultaneously within the same cell. Answer C describes an impossible mechanism - miRNAs don't typically inhibit each other through direct binding, especially family members that would likely be co-expressed. The correct answer is D because miRNAs with identical seeds target overlapping mRNA sets, with non-seed differences causing only minor variations in repression strength. Study tip: Remember that seed sequence identity predicts target overlap. When comparing miRNAs, always check the seed region first - identical seeds mean similar biological functions, regardless of differences elsewhere in the sequence.

Question 12

1. Global levels of mature miR-34a will decrease due to a feedback mechanism.
2. The Cyclin D1 mRNA will become resistant to miR-34a-mediated repression, promoting cell proliferation. (correct answer)
3. The half-life of the Cyclin D1 protein will be significantly increased.
4. The transcription of the Cyclin D1 gene will be constitutively activated.

Explanation: The function of miR-34a is to repress Cyclin D1 expression by binding to its mRNA's 3' UTR. Deleting this binding site makes the Cyclin D1 mRNA invisible to miR-34a. Therefore, even when miR-34a levels are high (e.g., in a non-proliferating state), it will be unable to repress Cyclin D1. This leads to inappropriately high levels of Cyclin D1 protein, which would promote, not inhibit, cell proliferation. (A) is incorrect as the mutation is in the target gene, not the miRNA gene. (C) refers to protein stability, but miRNAs act on mRNA. (D) refers to transcription, but the regulation described is post-transcriptional.

Question 13

1. The transcription rate of the TNFα gene will be significantly decreased due to a lack of enhancer elements.
2. The TNFα mRNA will have a significantly longer half-life, leading to prolonged and elevated protein production. (correct answer)
3. The translation of TNFα mRNA will be less efficient, resulting in lower peak protein levels despite normal mRNA levels.
4. The TNFα mRNA will be rapidly degraded upon stimulus, preventing the production of the cytokine.

Explanation: AU-rich elements (AREs) are binding sites for RNA-binding proteins (like TTP) that target mRNAs for rapid degradation. Deleting these elements removes the signal for decay. This will increase the stability and half-life of the TNFα mRNA. As a result, each mRNA molecule will persist longer and be available for more rounds of translation, leading to prolonged and higher levels of TNFα protein. (A) is incorrect as AREs function post-transcriptionally. (C) is incorrect because AREs relate to stability, not inherent translational efficiency. (D) is the opposite of the expected effect.

Question 14

1. A significant reduction in the processing of pri-miRNAs to pre-miRNAs in the nucleus.
2. A widespread decrease in the levels of mature miRNAs, leading to a moderate de-repression of their target mRNAs. (correct answer)
3. A complete loss of miRNA-mediated gene silencing due to the failure of RISC complex loading.
4. An accumulation of unprocessed pre-miRNAs in the nucleus, blocking their export to the cytoplasm.

Explanation: Dicer is a key enzyme in the cytoplasm that cleaves pre-miRNAs into mature miRNAs. A 75% reduction in its catalytic efficiency means that this processing step will be significantly slowed, leading to a widespread decrease in the production of mature miRNAs. This, in turn, will lessen the degree of repression on their numerous target mRNAs, causing a de-repression (increase in expression). (A) is incorrect because Dicer acts on pre-miRNAs in the cytoplasm, while Drosha processes pri-miRNAs in the nucleus. (C) is too extreme; a hypomorphic mutation would reduce, not eliminate, silencing. (D) is incorrect because Dicer acts in the cytoplasm after the pre-miRNA has been exported from the nucleus.

Question 15

1. 4-fold higher (correct answer)
2. 2-fold higher
3. 90-fold higher
4. Unchanged, as transcription rate is constant

Explanation: When you encounter questions about mRNA steady-state levels, remember that these levels depend on the balance between production (transcription) and removal (decay). The key insight is understanding how half-life changes affect this equilibrium. At steady state, the rate of mRNA production equals the rate of decay. The decay rate is inversely proportional to half-life - when half-life increases, the decay rate decreases proportionally. Since the drug increases c-myc mRNA half-life from 30 to 120 minutes, that's a 4-fold increase in half-life, which means a 4-fold decrease in decay rate. With transcription rate unchanged but decay rate reduced by 4-fold, the mRNA will accumulate until a new equilibrium is reached where the slower decay rate again balances the constant transcription rate. This new steady state will have 4-fold higher mRNA levels than the original. Looking at the distractors: Choice B (2-fold higher) might tempt you if you confused the relationship between half-life and steady-state levels - perhaps thinking it's a square root relationship rather than linear. Choice C (90-fold higher) could result from incorrectly multiplying the time values (120 - 30 = 90) rather than understanding the proportional relationship. Choice D (unchanged) reflects the common misconception that only transcription rate matters for steady-state levels, ignoring the critical role of mRNA stability. Remember this principle: at steady state, mRNA levels are proportional to transcription rate divided by decay rate. When half-life changes, decay rate changes inversely by the same factor.

Question 16

1. The expression of miR-155 target genes will be decreased due to co-degradation with the non-coding RNA.
2. There will be no effect on target genes because the introduced RNA is not translated into a protein.
3. The expression of miR-155 target genes will be increased because mature miR-155 is being sequestered. (correct answer)
4. The processing of pri-miR-155 into mature miR-155 will be inhibited by a negative feedback mechanism.

Explanation: This question tests your understanding of microRNA regulation and competitive endogenous RNA (ceRNA) mechanisms. When you encounter scenarios involving non-coding RNAs with miRNA binding sites, think about how they might compete for miRNA binding. The introduced long non-coding RNA acts as a "molecular sponge" for miR-155. Since it contains multiple high-affinity binding sites for miR-155 and is continuously expressed, it will sequester (bind up) many copies of mature miR-155 molecules. This reduces the amount of free miR-155 available to bind its natural target mRNAs. With less miR-155 binding to target mRNAs, there's reduced translational repression and mRNA degradation of those targets, leading to increased expression of miR-155 target genes. Answer A is incorrect because the non-coding RNA doesn't cause co-degradation of targets—it actually protects them by sequestering the miRNA. Answer B reflects a fundamental misunderstanding: non-coding RNAs can have significant regulatory effects without being translated into proteins. The ceRNA mechanism depends entirely on RNA-RNA interactions. Answer D describes a different regulatory mechanism (feedback inhibition of miRNA processing) that isn't supported by the experimental setup described. For genetics exams, remember that non-coding RNAs are functional molecules even without translation. When you see scenarios involving RNAs with miRNA binding sites, consider whether they're acting as competitive inhibitors or "sponges" that sequester miRNAs away from their intended targets.

Question 17

1. RBP-Y stabilizes the mRNA, and phosphorylation inhibits this stabilizing activity.
2. RBP-Y promotes transcription, and phosphorylation inhibits its nuclear localization.
3. RBP-Y enhances translation, and phosphorylation switches its function to promote degradation.
4. RBP-Y promotes mRNA degradation, and phosphorylation enhances this destabilizing activity. (correct answer)

Explanation: When you encounter questions about RNA-binding proteins (RBPs) and post-transcriptional regulation, focus on the relationship between protein function and mRNA stability. The key clue here is that high RBP-Y levels correlate with low target mRNA levels—this inverse relationship suggests RBP-Y destabilizes or promotes degradation of the mRNA. The model states that phosphorylation increases RBP-Y's affinity for AU-rich elements (AREs) in the 3' UTR. AREs are well-known destabilizing sequences that recruit proteins involved in mRNA decay. Since phosphorylation enhances binding to these degradation signals, and we know RBP-Y presence correlates with reduced mRNA levels, RBP-Y must promote mRNA degradation. Phosphorylation enhances this activity by increasing binding affinity. Answer D correctly identifies both functions: RBP-Y promotes mRNA degradation, and phosphorylation enhances this destabilizing activity. Answer A contradicts the data—if RBP-Y stabilized mRNA, high RBP-Y levels would correlate with high mRNA levels, not low levels. Answer B incorrectly focuses on transcription rather than post-transcriptional regulation, and the 3' UTR context clearly indicates mRNA processing, not nuclear events. Answer C suggests a functional switch from translation enhancement to degradation, but there's no evidence RBP-Y ever enhanced translation—the correlation shows it's consistently associated with reduced mRNA levels. Remember: when analyzing RBP function, always connect the binding site (especially AREs) with the observed effect on mRNA levels. AU-rich elements typically signal instability, making degradation the most likely mechanism.

Question 18

1. The SNP occurs outside the seed region and strengthens the thermodynamic stability of the miRNA-mRNA duplex.
2. The SNP is located within the seed region sequence of the KRAS 3' UTR, disrupting the binding of let-7. (correct answer)
3. The SNP is located in the 3' UTR but several hundred bases away from the conserved let-7 binding site.
4. The SNP creates a new binding site for a different miRNA that also targets KRAS for repression.

Explanation: The 'seed region' (nucleotides 2-8 of the miRNA) is critical for target recognition. A mutation within the complementary site in the mRNA's 3' UTR would likely disrupt or prevent the binding of the let-7 miRNA. If let-7 can no longer bind efficiently, its repressive effect is lost, leading to increased translation of the KRAS mRNA and thus higher KRAS protein levels. (A) describes a scenario that would likely enhance repression, not reduce it. (C) describes a mutation unlikely to affect let-7 binding at its specific site. (D) would lead to more repression, not less.

Question 19

1. Mcl-1 protein levels will decrease due to a dominant effect of miR-125b repression.
2. Mcl-1 protein levels will increase due to competitive exclusion of miR-125b by the overexpressed HuR. (correct answer)
3. There will be no net change in Mcl-1 protein levels because the stabilizing and repressive effects will cancel each other out.
4. Mcl-1 protein levels will increase because HuR will directly enhance the transcription rate of the Mcl-1 gene.

Explanation: This scenario describes competitive binding for an overlapping site. Since HuR is highly overexpressed, it will likely outcompete miR-125b for binding to the Mcl-1 3' UTR. The predominant effect will therefore be that of HuR, which is mRNA stabilization. This stabilization increases the mRNA half-life, leading to more protein production. The repressive effect of miR-125b is effectively masked or prevented. (A) is incorrect because the high concentration of HuR makes it the likely winner of the competition. (C) is unlikely as one effect will probably dominate. (D) is incorrect because HuR is an RNA-binding protein that acts post-transcriptionally.

Question 20

1. The PTC is recognized by a miRNA that specifically targets and degrades aberrant transcripts.
2. The aberrant protein product is highly unstable and rapidly degraded by the proteasome.
3. The process of nonsense-mediated decay (NMD) degrades the mRNA after it is recognized by the ribosome. (correct answer)
4. The mutation leads to transcriptional attenuation, preventing the completion of the primary transcript.

Explanation: When you encounter a genetics question involving a premature termination codon (PTC) and dramatically reduced mRNA levels, think about quality control mechanisms that cells use to eliminate faulty transcripts before they can produce harmful proteins. The key insight here is that the mRNA level—not protein level—is severely reduced. This points to nonsense-mediated decay (NMD), a cellular surveillance mechanism that detects and degrades mRNAs containing PTCs. NMD works when ribosomes encounter a stop codon upstream of exon-exon junction complexes during the pioneer round of translation. Since the frameshift occurred early in exon 2 of a 6-exon gene, the resulting PTC would be positioned well upstream of multiple remaining exon junctions, triggering robust NMD activation. Choice C correctly identifies this mechanism. Choice A is incorrect because miRNAs typically don't specifically target frameshift-induced transcripts, and this level of mRNA reduction is characteristic of NMD, not miRNA-mediated degradation. Choice B focuses on protein stability, but the question emphasizes mRNA reduction—if this were the explanation, you'd expect normal mRNA levels with rapid protein turnover. Choice D describes transcriptional attenuation, which would affect transcript initiation or elongation, but frameshift mutations in exons don't typically cause transcriptional problems. Remember this pattern: PTC + dramatic mRNA reduction = think NMD first. This quality control pathway is evolution's way of preventing cells from wasting resources on truncated, potentially harmful proteins.