AP Biology Questions: Lac Operon and Trp Operon Regulation

📚 Quick Study Guide

🧬 Operon: A cluster of genes under the control of a single promoter. 🧪 Lac Operon (E. coli): Regulates lactose metabolism.
🔍 Inducible system: Usually off, turned on by inducer (lactose). 📝 Key components: lacZ, lacY, lacA genes, promoter, operator, regulator gene (lacI). 💡 In the absence of lactose: Repressor protein binds to the operator, blocking transcription. 🌱 In the presence of lactose: Lactose (specifically allolactose) binds to the repressor, preventing it from binding to the operator, allowing transcription.
🧬 Trp Operon (E. coli): Regulates tryptophan synthesis.
🔍 Repressible system: Usually on, turned off by corepressor (tryptophan). 📝 Key components: trpE, trpD, trpC, trpB, trpA genes, promoter, operator, regulator gene (trpR). 💡 In the absence of tryptophan: Repressor protein is inactive, transcription occurs. 🌱 In the presence of tryptophan: Tryptophan binds to the repressor, activating it. The activated repressor binds to the operator, blocking transcription.
🔑 Key Difference: Lac operon is inducible (turned on by a substrate), while trp operon is repressible (turned off by a product).

🧪 Practice Quiz

1. Which of the following is true regarding the lac operon?
   1. A. It is an example of a repressible operon.
   2. B. It is typically turned on in the absence of lactose.
   3. C. It requires the presence of glucose for activation.
   4. D. It is an example of an inducible operon.
2. What is the role of allolactose in the regulation of the lac operon?
   1. A. It binds to the operator region, blocking transcription.
   2. B. It binds to the repressor protein, preventing it from binding to the operator.
   3. C. It increases the production of repressor protein.
   4. D. It inhibits the production of mRNA.
3. In the trp operon, what happens when tryptophan levels are high?
   1. A. The repressor protein becomes inactive.
   2. B. Transcription of the trp operon increases.
   3. C. Tryptophan binds to the repressor protein, activating it.
   4. D. The operator region is unblocked.
4. Which of the following genes is NOT part of the lac operon?
   1. A. lacZ
   2. B. lacI
   3. C. lacY
   4. D. lacA
5. The trp operon is primarily involved in the:
   1. A. Breakdown of lactose.
   2. B. Synthesis of tryptophan.
   3. C. Breakdown of tryptophan.
   4. D. Synthesis of lactose.
6. What would be the predicted effect of a mutation in the lacI gene that prevents the repressor from binding to lactose?
   1. A. The lac operon would be continuously repressed.
   2. B. The lac operon would be continuously transcribed.
   3. C. The lac operon would be unaffected.
   4. D. The lac operon would only be transcribed in the presence of glucose.
7. Which of the following is a characteristic of a repressible operon?
   1. A. It is usually off but can be turned on.
   2. B. It is usually on but can be turned off.
   3. C. It is always on.
   4. D. It is always off.