When lactose is present, which molecule binds to the repressor to inactivate it?

When lactose is present, which molecule binds to the repressor to inactivate it?

• A. Lactose
• B. Promoter
• C. RNA polymerase
• D. Operator

Answer: (A)

When a repressor binds the operator, it blocks RNA polymerase and shuts down transcription of the lac operon. The presence of lactose triggers production of a derivative, allolactose, which binds to the repressor and changes its shape so it can no longer attach to the operator. This release allows RNA polymerase to proceed with transcription of the operon genes needed to metabolize lactose. Among the given options, lactose is the molecule that acts as the inducer in this system (via its derivative allolactose) to inactivate the repressor. The other choices—promoter, RNA polymerase, and operator—do not bind the repressor to inactivate it.

When a repressor binds the operator, it blocks RNA polymerase and shuts down transcription of the lac operon. The presence of lactose triggers production of a derivative, allolactose, which binds to the repressor and changes its shape so it can no longer attach to the operator. This release allows RNA polymerase to proceed with transcription of the operon genes needed to metabolize lactose. Among the given options, lactose is the molecule that acts as the inducer in this system (via its derivative allolactose) to inactivate the repressor. The other choices—promoter, RNA polymerase, and operator—do not bind the repressor to inactivate it.