AP Biology: Unit 6, Gene Expression & Regulation (Drill 22)

Question 1. Comparing Condition 1 to Condition 2, which of the following best describes the role of TF-D in regulating DRG1 expression?

• A) TF-D is required for high-level DRG1 expression, because loss of TF-D function reduces mRNA levels from 850 to 42 units even when AP is present. ✓
• B) TF-D represses DRG1 expression, because the loss-of-function mutant shows lower mRNA levels than the wild type when AP is removed.
• C) TF-D and AP together repress DRG1 expression, because removing AP (Condition 3) produces higher mRNA than the TF-D mutant (Condition 2) in this comparison.
• D) TF-D has no role in DRG1 regulation because a small amount of mRNA (42 units) is still produced in the TF-D mutant.

Explanation: Correct: (A) Condition 2 (TF-D loss-of-function mutant with AP present) produces only 42 units of DRG1 mRNA compared to 850 units in the wild type (Condition 1). This approximately 20-fold reduction demonstrates that TF-D is required for high-level expression of DRG1. Without a functional TF-D, even the presence of AP cannot drive robust transcription, identifying TF-D as an activating transcription factor. Incorrect: (B) A repressor would be expected to increase expression when it is absent. Loss of TF-D function dramatically decreases DRG1 mRNA, which is the opposite of what a repressor would do. TF-D is an activator, not a repressor. Incorrect: (C) Comparing Condition 3 (wild-type, no AP: 91 units) to Condition 2 (TF-D mutant, AP present: 42 units) shows that wild-type plants without AP still produce more mRNA than TF-D mutants with AP. This indicates TF-D drives basal activation even without AP, not that TF-D and AP together are repressors. Incorrect: (D) The residual 42 units in the TF-D mutant represents basal transcription from other regulatory elements, not evidence that TF-D has no role. The nearly 20-fold difference between Conditions 1 and 2 demonstrates that TF-D plays a major activating role.

Question 2. Comparing Condition 1 (wild-type plus AP: 850 units) to Condition 3 (wild-type, no AP: 91 units), which of the following best explains why DRG1 mRNA is still produced at a low level when AP is absent?

• A) TF-D is enzymatically converted into AP when AP is absent, allowing the same protein to take over the role of the missing activating protein.
• B) RNA polymerase nonspecifically transcribes all genes at low levels regardless of regulatory inputs, producing 91 units as background noise.
• C) TF-D retains partial ability to activate DRG1 transcription without AP, producing a low level of basal expression that is substantially enhanced when AP is also present. ✓
• D) In the absence of AP, TF-D binds the DRG1 promoter more tightly and drives higher transcription, but the mRNA is rapidly degraded, producing a lower apparent level under these conditions.

Explanation: Correct: (C) Many transcription factors retain some capacity to activate transcription on their own, even without co-activators. The data show 91 units in Condition 3 (TF-D present, no AP) compared to 42 units in Condition 2 (TF-D absent, AP present). Since wild-type plants without AP produce more than twice the mRNA of TF-D mutants with AP, TF-D alone drives meaningful basal transcription. AP amplifies this basal activity approximately 9-fold (from 91 to 850 units), likely by stabilizing TF-D binding to the promoter or recruiting additional transcription machinery. Incorrect: (A) The data do not support increased TF-D synthesis as an explanation. The experiment measures DRG1 mRNA, not TF-D protein levels. Proposing compensatory TF-D upregulation introduces a mechanism not mentioned in the stimulus and not testable from these data. Incorrect: (B) Nonspecific background transcription from RNA polymerase alone would be extremely low and approximately equal across all conditions. The 91 units in Condition 3 is far higher than the 42-unit baseline in the TF-D mutant, demonstrating that TF-D specifically drives this level of expression, not random polymerase activity. Incorrect: (D) The data do not support the idea that TF-D binds more tightly without AP. This is mechanistically speculative and contradicted by the observation that removing AP dramatically reduces total mRNA from 850 to 91 units, consistent with AP being required to maximize TF-D activity, not inhibit its binding.

Question 3. Conditions 3 and 4 produce similar DRG1 mRNA levels (91 and 78 units, respectively), even though Condition 4 includes AP. The inhibitor molecule in Condition 4 blocks the interaction between TF-D and AP without binding to DNA. Which of the following best explains why Condition 4 mRNA levels resemble Condition 3 rather than Condition 1?

• A) The inhibitor directly binds to the DRG1 promoter and blocks RNA polymerase from initiating transcription, reducing mRNA regardless of TF-D or AP activity.
• B) The inhibitor causes TF-D itself to be rapidly degraded, reducing its concentration below the level needed for even the basal activation of DRG1 transcription.
• C) The inhibitor increases the rate of DRG1 mRNA degradation, so mRNA produced at the Condition 1 rate is rapidly broken down before it can be measured.
• D) The inhibitor prevents AP from interacting with TF-D, so AP cannot enhance TF-D activity. TF-D still drives basal transcription alone, producing an mRNA level similar to Condition 3 where AP is simply absent. ✓

Explanation: Correct: (D) The stimulus specifies that the inhibitor blocks the TF-D and AP interaction without binding to DNA. This means TF-D is still present and functional but cannot partner with AP. Without that interaction, AP cannot enhance TF-D activity. TF-D alone still drives its characteristic basal level of DRG1 expression (approximately 91 units in Condition 3). Condition 4 produces 78 units, very similar to Condition 3's 91 units, because the effective state of the regulatory system is the same in both conditions: TF-D present but unable to interact with AP. The similar values suggest that blocking TF-D and AP interaction leaves TF-D functioning at approximately its basal level. Incorrect: (A) The stimulus explicitly states the inhibitor does not bind to DNA. Blocking a protein-protein interaction is distinct from blocking the promoter. If the inhibitor bound the promoter, mRNA would likely fall below the TF-D-alone level of 91 units, not produce a similar value. Incorrect: (B) If the inhibitor degraded TF-D, mRNA levels would be expected to fall to approximately the 42-unit level seen in Condition 2 (the TF-D mutant). The fact that Condition 4 produces 78 units, much closer to 91 than to 42, indicates TF-D is still present and functional. Incorrect: (C) If the inhibitor accelerated mRNA degradation, all conditions with the inhibitor would show reduced mRNA. This mechanism would not produce the specific pattern of Condition 4 resembling Condition 3, and the stimulus does not describe mRNA degradation as the mode of action.

Question 4. A researcher wants to determine whether TF-D increases DRG1 transcription by directly binding the DRG1 promoter or by binding to another protein that in turn contacts the promoter. Which of the following experimental designs would most directly test whether TF-D physically contacts the DRG1 promoter DNA?

• A) Introduce multiple point mutations throughout the TF-D coding sequence and measure DRG1 mRNA levels to identify which regions of TF-D are required for activation in this instance.
• B) Perform a chromatin immunoprecipitation (ChIP) assay using an antibody against TF-D to determine whether TF-D is physically associated with the DRG1 promoter region in vivo. ✓
• C) Overexpress TF-D in the TF-D loss-of-function mutant and measure whether DRG1 mRNA levels are restored to wild-type levels under the same growth conditions.
• D) Compare DRG1 mRNA levels in wild-type plants grown at different temperatures to determine whether temperature affects TF-D-mediated activation.

Explanation: Correct: (B) Chromatin immunoprecipitation (ChIP) is specifically designed to determine whether a given protein physically associates with a specific DNA region in living cells. The technique cross-links proteins to DNA, fragments the chromatin, immunoprecipitates with an antibody against the protein of interest (TF-D), and then identifies the DNA sequences that co-precipitate. If the DRG1 promoter sequence is enriched in the TF-D ChIP sample, it directly demonstrates physical association between TF-D and that promoter region. This is the most direct available test of promoter binding. Incorrect: (A) Mutating the TF-D coding sequence and measuring mRNA levels would identify which protein domains are required for TF-D function, but would not distinguish between direct DNA binding and indirect activation through another protein. A domain required for activation could be a DNA-binding domain or a protein-protein interaction domain. Incorrect: (C) Rescuing DRG1 expression by restoring TF-D confirms that TF-D is required for expression, which the existing data already show, but does not address whether TF-D contacts the promoter DNA directly or acts through an intermediary protein. Incorrect: (D) Measuring mRNA levels at different temperatures tests whether temperature affects TF-D activity, but does not determine whether TF-D contacts DNA directly. Temperature could affect protein folding, protein-protein interactions, or many other aspects of gene regulation without providing mechanistic information about DNA-protein contact.

Question 5. DRG1 encodes a protein that helps the plant retain water during drought conditions. A spontaneous mutation increases TF-D activity, causing DRG1 to be expressed at high levels even when AP is absent and when the plant is not experiencing drought. Assuming the plant population lives in an environment with unpredictable rainfall, which of the following best predicts the long-term effect of this mutation on the fitness of individuals carrying it?

• A) The mutation will increase fitness in all environmental conditions because constantly producing the DRG1 drought protein keeps the plant prepared.
• B) The mutation will have no effect on fitness because gene regulation only affects mRNA levels, not the functional protein that determines phenotype.
• C) The mutation may reduce fitness under well-watered conditions if constitutive DRG1 expression imposes a metabolic cost, even though it may increase fitness during drought. ✓
• D) The mutation will be immediately eliminated from the population by natural selection because unregulated gene expression is always deleterious.

Explanation: Correct: (C) Gene regulation exists precisely because producing proteins has metabolic costs. Proteins require energy and raw materials to synthesize, fold, and maintain. A plant constitutively expressing a drought-response protein at high levels is diverting resources from other biological processes. Under well-watered conditions where DRG1 protein confers no benefit, this metabolic cost is a net disadvantage. During drought, however, the constitutively high DRG1 expression could be advantageous. In an environment with unpredictable rainfall, the fitness effect of the mutation would depend on the relative frequency and severity of drought versus well-watered conditions, making the outcome context-dependent rather than universally beneficial or harmful. Incorrect: (A) Continuous production of a drought-response protein is not costless. Metabolic resources allocated to DRG1 protein are unavailable for growth, reproduction, and other cellular functions. If drought is not constant, constitutive expression provides no additional benefit during well-watered periods while still incurring metabolic costs. Incorrect: (B) mRNA levels directly determine how much protein is produced, all else being equal. Higher DRG1 mRNA leads to more DRG1 protein, which alters the plant's water-retention phenotype. Gene regulation is the primary mechanism by which cells control protein levels, so changes in transcription factor activity do affect the functional protein and therefore phenotype. Incorrect: (D) Not all constitutive expression is immediately deleterious. The fitness effect depends on the protein's function, the energetic cost of its production, and the environmental context. Many organisms constitutively express genes that are essential or broadly beneficial. The statement 'always deleterious' is an overgeneralization that biology does not support.