Question

Choose the correct answer from the alternatives given:
In E. coli the lac operon gets switched on when
A. Lactose is present and it binds to the repressor
B. Repressor binds to operator
C. RNA polymerase binds to the operator
D. Lactose is present and it binds to the RNA polymerase

Answer 1

The lac operon of E. coli contains genes involved in the metabolism of lactose. The genes are expressed only when lactose is present and glucose is absent. Usually, glucose is used as the main carbon source by most bacteria but when it’s not available, lactose is digested and used as an energy source. For this process lac operon is required. When glucose is available in plenty, the lac operon is switched off to prevent unnecessary digestion of lactose.

Complete answer:
Option A Lactose is present and it binds to the repressor: It is known that lac operon is switched on only when lactose is present and glucose is absent. The lactose molecule must bind to the repressor to make way for the RNA polymerase to bind to the promoter. This leads to the transcription of lac operon.
Option A is correct.

Additional Information:
Option B Repressor binds to operator: When the repressor is bound to the operator, the RNA polymerase cannot attach to the promoter. Hence the lac operon will be switched off.
Option B is incorrect.
Option C RNA polymerase binds to the operator: This option is wrong because the RNA polymerase does not bind to the operator but only to the promoter.
Option C is incorrect.
Option D Lactose is present and it binds to the RNA polymerase: This option is wrong because lactose does not bind with RNA polymerase but only with the repressor.
Option D is incorrect.

Hence option A. Lactose is present and it binds to the repressor is the correct answer

Note:
The lac operon is switched off when the repressor protein binds to the operator. The repressor remains active when there is no lactose and excess glucose is present. The repressor protein is encoded by a gene called lacI. When glucose is absent and lactose is present, it will bind to the repressor and lead to a change in the protein shape. This change will prevent the repressor from binding with the operator and the operon is switched on.