Question

What acts as an inducer in the lac operon? How does it switch on the operon?

Answer 1

In the lac operon, lactose (specifically its isomer, allolactose) acts as the inducer. It is a chemical signal that indicates to the bacteria that a fuel source is available and the necessary enzymes should be produced.

How it switches “on” the operon:

1. Binding to the Repressor: When lactose enters the cell, it binds to the repressor protein (which is normally stuck to the operator, keeping the operon “off”).
2. Conformational Change: This binding changes the three-dimensional shape of the repressor.
3. Inactivation: Because of its new shape, the repressor can no longer hold onto the operator DNA and falls off.
4. Transcription Initiation: With the operator now clear, the RNA polymerase is free to move past the promoter and transcribe the structural genes (lacZ, lacY, and lacA) into mRNA.

This process ensures that the cell only spends energy making lactose-digesting enzymes when lactose is actually present.