Q.In E. coli, the lac operon gets switched on when:

In the absence of lactose, the repressor protein, synthesized by the i gene, binds to the operator region of the lac operon. This binding physically blocks RNA polymerase from transcribing the structural genes, keeping the operon switched off. When lactose is present, it acts as an inducer. Lactose (specifically allolactose, its isomer) binds to the repressor protein, causing a conformational change. This altered repressor can no longer bind to the operator. With the operator free, RNA polymerase can now bind to the promoter and initiate transcription of the lac stru …

The lac operon is a classic example of gene regulation in prokaryotes, specifically in the bacterium Escherichia coli. Its primary function is to allow E. coli to efficiently metabolize lactose only when glucose, its preferred energy source, is unavailable. This system ensures that the cell doesn't waste energy producing enzymes for lactose metabolism if lactose isn't present or if a better sugar (glucose) is at hand.

At its core, the lac operon consists of several key genetic components:

• A regulator gene (i gene): This gene is located upstream of the operon and constitutively produces a repressor protein.
• A promoter (p): This is the binding site for RNA polymerase, the enzyme responsible for initiating transcription.
• An operator (o): Situated between the promoter and the structural genes, this is the binding site for the repressor protein.
• Structural genes: These include lacZ, lacY, and lacA.
  • lacZ codes for $\beta$-galactosidase, which hydrolyzes lactose into glucose and galactose.
  • lacY codes for permease, which increases the cell's permeability to lactose, allowing it to enter the cell.
  • lacA codes for transacetylase, whose exact role in lactose metabolism is not fully understood but is part of the operon.

When lactose is absent from the environment, the lac operon remains switched off. The regulator (i) gene continuously synthesizes the repressor protein. This repressor protein is active and readily binds to the operator region of the operon. When the repressor is bound to the operator, it physically blocks RNA polymerase from moving along the DNA and transcribing the structural genes. Consequently, no enzymes for lactose metabolism are produced, conserving the cell's resources.

The situation changes dramatically when lactose becomes available. Lactose, or more precisely, its isomer allolactose (which is formed in small amounts from lactose by $\beta$-galactosidase), acts as an inducer. When allolactose is present, it binds to the repressor protein. This binding causes a conformational change in the repressor protein, altering its shape.

Once the repressor protein's conformation changes, it can no longer bind to the operator region. With the operator site now free, RNA polymerase can bind to the promoter and proceed to transcribe the structural genes (lacZ, lacY, lacA). This leads to the production of $\beta$-galactosidase, permease, and transacetylase, enabling the E. coli cell to take up and metabolize lactose. This entire process, where the presence of a substrate (lactose) leads to the expression of genes required for its metabolism, is known as induction. …

Model the lac operon as a simple switch with one deciding action: transcription is 'off' whenever the repressor sits on the operator, and 'on' only once something removes it. Check each option for whether it actually detaches the repressor from the operator: only lactose binding the repressor changes the repressor's own shape enough to release it from the DNA. Option (b) describes the 'off' state itself, (c) put …