Specifically describe how the lac operon (enzyme induction) and trp operon (enzyme repression) control gene expression. Use a diagram.

Question 1

Lac Operon: Enzyme Induction

Answer 1

Lac Operon: Enzyme Induction
- Function: Controls genes responsible for lactose metabolism in prokaryotes.
- Key Components:
1. Promoter: Site where RNA polymerase binds to start transcription.
2. Operator: Sequence controlling transcription via repressor binding.
3. Structural Genes: Code for enzymes like β-galactosidase (lactose metabolism).
4. Lac Repressor: Protein encoded upstream by the lacI gene; always present.

• Mechanism:
  
  • Without lactose:
    
    • Repressor binds to the operator.
    • RNA polymerase cannot bind to the promoter.
    • Transcription is blocked, and lactose metabolism enzymes are not produced.
  • With lactose:
    
    • Lactose acts as an inducer by binding to the repressor, inactivating it.
    • Repressor cannot bind the operator.
    • RNA polymerase binds the promoter, and enzymes for lactose metabolism are synthesized.

Question 2

Comparison

Answer 2

• Lac Operon: Off until lactose turns it on. Lactose helps make enzymes to break it down.
• Trp Operon: On until tryptophan turns it off. Tryptophan stops its own production when there’s enough.

Lac = Stopped until activated
Trp = Active until stopped

Question 3

Trp Operon: Enzyme Repression

Answer 3

• Function: Regulates genes for tryptophan synthesis in prokaryotes.
• Key Components:
  
  1. Promoter: Site for RNA polymerase binding to initiate transcription.
  2. Operator: Controls transcription by interacting with the repressor.
  3. Structural Genes: Code for enzymes synthesizing tryptophan.
  4. Trp Repressor: Protein activated by tryptophan.
• Mechanism:
  
  • Without tryptophan:
    
    • Repressor is inactive and does not bind to the operator.
    • RNA polymerase binds the promoter, allowing transcription and tryptophan synthesis.
  • With tryptophan:
    
    • Tryptophan acts as a corepressor by binding to the repressor, activating it.
    • Activated repressor binds to the operator, blocking transcription.
    • Energy is conserved by halting tryptophan synthesis.