Question

Glucose or galactose cannot act as inducers for lac operon. Can you think for how long the lac operon would be expressed in the presence of lactose?

Answer 1

• The Lac operon is the classic operon example and is responsible for the degradation of the milk protein lactose. The Lac operon is an inducible operon; in the absence of lactose, the operator is blocked by a repressor protein.
• In order for the lac operon to be turned on, an inducer molecule must inactivate the repressor protein. The inducer molecule in this system is allolactose, an isomer of lactose. When lactose and its isomer are present in the cell, allolactose will bind to allosteric sites on the repressor protein, changing its conformation and rendering it inactive.
• As the repressor protein detaches from the operator, RNA polymerase can bind to the promoter, transcription can occur, and the three lactose degradation genes can be synthesized.
• The Lactose operon expresses as long as the Lactose is present. When all lactose is converted into glucose and galactose, the reaction stops.