Lac Operon
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Direct answer
The lac operon of Escherichia coli, described by Francois Jacob and Jacques Monod in 1961, is the classic model of gene regulation. It consists of a regulatory gene (i) that encodes a repressor, a promoter where RNA polymerase binds, an operator that controls access, and three structural genes transcribed together — z coding beta-galactosidase, y coding permease and a coding transacetylase. When lactose is absent the repressor blocks the operator and transcription stops; lactose, the inducer, inactivates the repressor so the operon switches on.
What you must remember
- Structural genes and products: gene z gives beta-galactosidase which splits lactose into glucose and galactose; gene y gives permease which raises the cell's permeability to lactose; gene a gives transacetylase.
- Regulatory gene: the i gene is expressed constitutively; its repressor protein, a tetramer in the standard account, binds the operator sequence with high affinity.
- Switch-off state: with no lactose, the repressor sits on the operator and prevents RNA polymerase bound at the promoter from transcribing the structural genes.
- Switch-on state: lactose (inducer) binds the repressor, changing its shape so it can no longer hold the operator; RNA polymerase then transcribes all three genes as one polycistronic mRNA.
- Coordinate expression: because the three genes share one promoter and operator, their products appear together in a fixed ratio whenever lactose is present.
- Concept of negative control: regulation here is negative because the default influence of the regulatory protein is to shut transcription off, with the inducer lifting that inhibition.
Common confusion
The three control elements are habitually interchanged: the promoter is the RNA polymerase landing site, the operator is the repressor's docking sequence between promoter and genes, and the regulatory gene lies outside the operon proper and codes for the repressor itself. Students also reverse the logic of induction — lactose does not activate transcription directly but removes the obstacle by inactivating the repressor.
Exam-focused takeaway
NEET-UG reliably asks a match-the-following between z, y and a genes and their enzyme products, and direct MCQs on who proposed the operon model. Assertion-reason stems run along the lines "when lactose is removed from the medium the operon shuts off because the repressor again binds the operator". Statement-based questions test the meaning of polycistronic mRNA, constitutive expression of the i gene, and the identity of the inducer. This is one chapter where the NCERT figure itself is examinable — practise labelling it from memory.
Frequently asked questions
Who proposed the operon model?
Francois Jacob and Jacques Monod in 1961, from their work on lactose metabolism in Escherichia coli.
What are the three structural genes of the lac operon?
Gene z coding for beta-galactosidase, gene y for permease and gene a for transacetylase, all transcribed as a single polycistronic mRNA.
What is the role of the repressor protein?
Encoded by the regulatory i gene, it binds the operator region when lactose is absent and blocks RNA polymerase from transcribing the structural genes.
Why is lactose called an inducer?
Because by binding and inactivating the repressor it switches on the operon; in its absence the operon remains repressed.
Which enzyme splits lactose into glucose and galactose?
Beta-galactosidase, the product of the z gene.