The lac operon in E. coli contains structural genes lacZ, lacY and lacA, a promoter and an operator. The i gene encodes a repressor; without lactose, active repressor binds the operator and blocks expression. In the presence of lactose, the inducer derived from lactose binds the repressor and reduces its operator binding, permitting transcription of structural genes.

LacZ is associated with beta-galactosidase, lacY with permease and lacA with transacetylase. Calling the operator an enzyme or the repressor a DNA template confuses parts. The simple NCERT inducer model asks how presence or absence of lactose changes repression; expression can also depend on glucose via additional control, so do not claim lactose is the only influence under every real condition.

Worked example: Without lactose: repressor on operator, structural genes off; with inducer: repressor released, genes can be transcribed.

NCERT trap: Inducer acts on the repressor, while the operator is a DNA control site.

NEET method: Draw i → repressor → operator and lactose → repressor before predicting ON or OFF.

Reading estimate; allow another 3–5 minutes for each set of three checks.

Molecular Basis of Inheritance

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