Worked examplesThree real regulation of gene expression questions
From the practice pool, not the mock papers — each with the reasoning that produces the answer.
73 practice questions sit on this single topic — enough that it is worth working through by topic rather than meeting it scattered through a unit review.
86% of them come with a figure. Reading the graph or diagram correctly is most of the work here before any content knowledge applies.
Concept Explanation
In the lac operon, lactose is absent and glucose is low. Which outcome is most likely?
- ALactose binds CAP and blocks RNA polymerase.
- BAbsence of lactose keeps the repressor bound to the operatorcorrect
- CThe structural genes are translated before they are transcribed.
- DThe operon is transcribed at its highest level because glucose is low.
Why B is correct
When lactose is absent, the lac repressor remains active and blocks transcription. Low glucose alone cannot produce high transcription if the repressor is bound.
The stem asks students to explain a specific mechanism, not just recall a unit term: "In the lac operon, lactose is absent and glucose is low. Which outcome is most likely". The correct choice is supported by When lactose is absent, the lac repressor remains active and blocks transcription. The tempting alternatives such as Lactose binds CAP and blocks RNA polymerase or structural genes are translated before they are transcribed fail because they do not produce the same outcome described in the stem.
Argumentation
A mutation deletes an enhancer near gene Y. The coding sequence of gene Y is unchanged, but gene Y mRNA decreases. Which explanation is best supported?
- AThe mutation increased translation by adding ribosomes to mRNA.
- BThe coding sequence changed because all regulatory mutations alter exons.
- CReduced transcription-factor binding lowered transcription of gene Y.correct
- DA regulatory-sequence change cannot affect expression if the protein-coding sequence is unchanged.
Why C is correct
Enhancers can increase transcription by binding regulatory proteins. Deleting an enhancer can reduce transcription even when the coding sequence is unchanged.
The stem asks students to explain a specific mechanism, not just recall a unit term: "A mutation deletes an enhancer near gene Y. The coding sequence of gene Y is unchanged, but gene Y mRNA decreases....". The correct choice is supported by Deleting an enhancer can reduce transcription even when the coding sequence is unchanged. The tempting alternatives such as mutation increased translation by adding ribosomes to mRNA or coding sequence changed because all regulatory mutations alter exons fail because they do not produce the same outcome described in the stem.
Concept Explanation
Histone acetylation often loosens chromatin structure. Which effect is most likely for nearby genes?
- ATranscription must stop because ribosomes cannot bind histones.
- BIncreased accessibility can promote transcriptioncorrect
- CA regulatory-sequence change cannot affect expression if the protein-coding sequence is unchanged.
- DChromatin accessibility proves protein abundance
Why B is correct
Looser chromatin can make DNA more accessible to transcription factors and RNA polymerase, increasing transcription of nearby genes.
The stem asks students to explain a specific mechanism, not just recall a unit term: "Histone acetylation often loosens chromatin structure. Which effect is most likely for nearby genes". The correct choice is supported by Looser chromatin can make DNA more accessible to transcription factors and RNA polymerase, increasing transcription of nearby genes. The tempting alternatives such as Transcription must stop because ribosomes cannot bind histones or regulatory-sequence change cannot affect expression if the... fail because they do not produce the same outcome described in the stem.
73 questions on this topic, free to start.
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Last reviewed 2026-08-28. Topic and unit names follow the
College Board course framework. Question counts describe the PrepScore practice bank, not the exam.