Question

Explain how the structural genes of the lac operon start transcription on the addition of lactose.

Answer 1

When lactose is added to the medium, it enters the E. coli cell and acts as an inducer by binding to the active repressor protein. This binding causes a conformational change in the repressor, making it inactive and causing it to detach from the operator region. With the operator no longer blocked, RNA polymerase is free to bind to the promoter and transcribe the structural genes (lacZ, lacY, and lacA) into a single polycistronic mRNA.