AcornPrep

AP Biology Unit 6: Gene Expression and Regulation — Worked Examples

Predicting Gene Expression with Operator Mutations in the Lac Operon

Hard

An E. coli strain carries a mutation in the operator region of the lac operon such that the lac repressor protein can no longer bind to the operator. The bacteria are grown in a medium containing glucose but no lactose. What pattern of lac operon gene expression will be observed?

  1. No expression, because glucose represses the operon through catabolite repression regardless of operator status
  2. Low-level constitutive expression, because the repressor cannot bind but CAP activation is absent due to low cAMP ✓ Correct
  3. High-level constitutive expression, because both the repressor and glucose effects are eliminated by the operator mutation
  4. Normal inducible expression, because the operator mutation has no effect when lactose is absent
Solution

Two regulatory systems control the lac operon: negative regulation (repressor binding the operator blocks transcription) and positive regulation (CAP-cAMP binding enhances RNA polymerase binding). The operator mutation makes the repressor unable to bind, so the operon is constitutively expressed (negative regulation is lost). However, glucose is present, so cAMP levels are low, meaning CAP cannot bind the CAP site. Without CAP activation, RNA polymerase binds the promoter weakly, producing only low-level (basal) transcription. Choice A incorrectly assumes glucose completely silences the operon — catabolite repression reduces but does not eliminate expression when the repressor is absent. Choice C incorrectly assumes the operator mutation eliminates glucose effects — CAP regulation operates at the promoter, not the operator. Choice D incorrectly assumes the operator mutation is irrelevant without lactose.

miRNA Regulation of Gene Expression

Hard

Researchers measure both mRNA levels and protein levels of Gene X in wild-type cells and in cells where a specific miRNA (miR-22) has been knocked out. The results are shown below. What is the most likely mechanism by which miR-22 regulates Gene X?

  1. miR-22 promotes transcription of Gene X, so its removal decreases mRNA levels
  2. miR-22 degrades Gene X mRNA, so its removal increases mRNA levels
  3. miR-22 inhibits translation of Gene X mRNA without degrading it ✓ Correct
  4. miR-22 promotes translation of Gene X, so its removal decreases protein levels
Solution

The key observation is that mRNA levels are identical in both conditions (1.0), but protein levels are 5-fold higher in the knockout. This means miR-22 does not affect mRNA levels (ruling out transcription regulation and mRNA degradation) but suppresses protein production. Since mRNA is present but protein is low in wild-type cells, miR-22 must inhibit translation without degrading the mRNA. Choice A is wrong because mRNA levels do not change. Choice B is wrong because mRNA is not degraded (levels remain at 1.0). Choice D gets the direction wrong — protein increases when miR-22 is removed, meaning miR-22 normally suppresses (not promotes) translation.