In lac operon when the cell is using its normal energy source glucose, the ' ' gene transcribes a repressor , which after translation produces a repressor protein. It binds the operator region of the operon and prevents the RNA polymerase from transcribing the operon. As a result, -galactosidase is not produced and the operon is switched 'off'. In absence of glucose, if lactose is available as energy source for the bacteria then lactose enters the cell as a result of the activity of permease enzyme. It acts as inducer and binds with the repressor to inactivate it. As the repressor is inactivated, the RNA polymerase can bind itself to the operator site and transcribe the operon to produce lac RNA which enables formation of all the required three enzymes needed for lactose metabolism. Thus, lactose acts as an inducer and switches 'on' the lac operon.