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NEETZoologyMolecular Basis of Inheritance

When lactose is added to a culture of Escherichia coli, it acts as an inducer to 'switch on' the lac operon. By what precise molecular mechanism does the inducer allow transcription of the structural genes to proceed?

Options

  1. AIt binds directly to the operator region to physically block the repressor protein from attaching.
  2. BIt binds to the promoter region to increase the binding affinity of RNA polymerase.
  3. CIt binds to the i gene to inhibit the synthesis of the repressor protein mRNA.
  4. DIt binds to the repressor protein, inactivating it so that it can no longer bind to the operator region.

Correct answer

D. It binds to the repressor protein, inactivating it so that it can no longer bind to the operator region.

Step-by-step solution

Lactose acts as an inducer in the lac operon. It binds to the active repressor protein (which is continuously synthesized by the i gene) and changes its conformation, rendering it inactive. This inactive repressor is unable to bind to the operator region. As a result, the operator is free, allowing RNA polymerase to access the promoter and transcribe the structural genes. The inducer does not bind to the operator, the promoter, or the i gene itself. Answer: It binds to the repressor protein, inactivating it so that

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