Lac Operon

• What do regulatory genes control?

  Structural genes and protein production levels
• How do regulatory genes affect structural genes?

  They can control several structural genes at once
• When does gene control occur at the transcriptional level?

  If structural genes are involved in transcription
• What can structural genes in prokaryotes form?

  An operon controlled by the same promoter

  
• What does the lac operon control?

  The production of the enzyme lactase

  
• What is the function of lactase?

  It breaks down lactose for energy
• Why is lactase considered an inducible enzyme?

  It is synthesized only when lactose is present
• What is the benefit of lactase being an inducible enzyme?

  It prevents waste of energy and materials
• What is the order of components in the lac operon?

  1. Promoter for structural genes
  2. Operator
  3. Structural gene lacZ (lactase)
  4. Structural gene lacY (permease)
  5. Structural gene lacA (transacetylase)

  
• What is located upstream of the lac operon?

  Promoter for regulatory gene

  
• What does the regulatory gene lacI code for?

  The lac repressor protein

  
• What are the two binding sites of the lac repressor protein?

  Operator and lactose binding sites

  
• What happens when the lac repressor protein binds to the operator?

  It prevents transcription of structural genes

  
• What occurs when the lac repressor protein binds to lactose?

  It distorts and cannot bind to the operator

  
• What happens when lactose is absent in the medium?

  No lactase enzyme is synthesized
• What processes occur when lactose is present?

  Lactose binds to repressor, allowing transcription

  
• What is produced when transcription takes place in the presence of lactose?

  mRNA from all three structural genes
• What is the role of the binding of lactose to the repressor protein?

  It frees the operator for RNA polymerase binding

  
• How does the genetic control of an inducible enzyme differ from a repressible enzyme?

  Inducible enzyme:

  • Synthesized when substrate is present
  • Example: lac operon

  
  

  Repressible enzyme:

  • Synthesized when substrate is absent
  • Binding helps repressor bind to operator
• What happens when there is less of the effector molecule in repressible enzymes?

  Transcription of structural genes proceeds