explain genetic control of protein production in a prokaryote using the lac operon (knowledge of the role of cAMP is not expected)

Published by Patrick Mutisya · 14 days ago

Gene Control – The lac Operon (A‑Level Biology 9700)

Gene Control in Prokaryotes: The lac Operon

The lac operon of Escherichia coli is a classic example of an inducible

operon that allows the bacterium to metabolise lactose only when it is present.

It demonstrates how a single regulatory system can switch the production of

several proteins on or off at the transcriptional level.

Key Components

ComponentFunction
lacZEncodes β‑galactosidase – hydrolyses lactose into glucose and galactose.
lacYEncodes permease – transports lactose into the cell.
lacAEncodes transacetylase – role in lactose metabolism is minor.
Promoter (P)Binding site for RNA polymerase to start transcription.
Operator (O)DNA segment where the repressor protein binds to block transcription.
Regulatory gene (lacI)Produces the lac repressor protein (LacI).

Regulatory Mechanism

  1. Absence of lactose

    • The lac repressor (product of lacI) binds tightly to the operator.

    • Binding blocks RNA polymerase from transcribing the structural genes.

    • Result: No synthesis of β‑galactosidase, permease or transacetylase.

  2. Presence of lactose

    • Lactose is taken up at low levels by diffusion and is converted to allolactose

      (the true inducer).

    • Allolactose binds to the repressor, causing a conformational change that reduces

      its affinity for the operator.

    • The operator becomes free; RNA polymerase can now bind to the promoter and

      transcribe the lac genes.

    • β‑galactosidase is produced, increasing the rate of lactose hydrolysis,

      which in turn generates more allolactose – a positive feedback loop.

Summary of the Induction Process

The lac operon exemplifies an inducible system: transcription is normally

off and is switched on only when an appropriate inducer (allolactose) is present.

This ensures that the cell expends energy producing the lactose‑utilising enzymes

only when they are needed.

Suggested diagram: The lac operon showing the promoter, operator,

structural genes, repressor protein, and the effect of allolactose binding.

Key Points for Examination

  • Identify the structural genes and their products.

  • Explain the role of the promoter and operator.

  • Describe how the lac repressor controls transcription.

  • State why allolactose, not lactose itself, is the inducer.

  • Outline the sequence of events that leads to enzyme production when lactose is present.