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a) when lactose is absent, the repressor is active, RNA Polymerase cannot bind to the promoter and the structural gene are not formed.

b) when lactose is present, the lac repressor is inactive, RNA Polymerase can bind to the promotor and the structural genes are formed.

Explanation:

The lac operon in E.coli has genes required for lactose metabolism.

It is expressed only when lactose is present in the medium and glucose is absent.

According to the glucose and lactose level the operon is switched on or off.

The lac repressor or allolactose and catabolite activator protein is present in lac operon.

Glucose is the primary source of energy but bacteria loves to fuel down lactose. To use lactose as an energy source lac operon works.

a) when lactose is absent, the repressor is active, RNA Polymerase cannot bind to the promoter and the structural gene are not formed.

b) when lactose is present, the lac repressor is inactive, RNA Polymerase can bind to the promotor and the structural genes are formed.

The lac repressor is a protein that inhibits transcription of lac operon in the absence of lactose.

energy and particles

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explanation: