Explain how the lac operon of E. coli works when lactose is absent and when it is present. Draw labelled diagrams.

1. 1 · Define an operon. Polycistronic structural genes regulated by a common promoter and regulatory genes; lac operon explained by Jacob and Monod.
2. 2 · List the parts.
   • i: repressor
   • p: promoter
   • o: operator
   • z: β-galactosidase
   • y: permease
   • a: transacetylase
3. 3 · Lactose absent. The repressor binds the operator and blocks RNA polymerase; the structural genes are not transcribed.
4. 4 · Lactose present. Lactose (inducer) binds and inactivates the repressor; RNA polymerase transcribes z, y and a; lactose is broken down.
5. 5 · Draw both states. [Diagrams: (a) repressor on operator, no transcription; (b) inducer-repressor complex, polymerase on promoter, polycistronic mRNA, three enzymes]
6. 6 · Name the control. Control by the repressor is negative regulation.
7. 7 · Link to real life. Lactose-intolerant people lack enough lactase, the enzyme that does β-galactosidase's job in us.
8. 8 · Close in one line. So E. coli makes lactose-digesting enzymes only when lactose is there to digest.

Scoring tip: Draw two separate diagrams, off and on, with every part labelled. Diagrams carry about two of the five marks.