According to lac operon concept, an operator gene combines with Co-repressor to switch off transcription of structural genes.

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Inducer gene to switch on structural gene transcription.

Repressor genes to switch off transcription of structural genes.

Regulator protein to switch on structural gene transcription.

Regulator protein to switch off structural gene transcription.

answer is D.

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Detailed Solution

Concept: The lac operon, also known as the lactose operon, is a gene required for the transport and metabolism of lactose and is found in enteric bacteria like E. coli. Most bacteria use glucose as their primary carbon source, however when glucose is not present, the lac operon uses beta-galactosidase to aid in the digestion of lactose.
By using a transcription factor, regulator genes regulate the expression of structural genes. These transcription factors impede binding and transcription because they bind to the operator region found in the RNA polymerase's binding site.
Hence, option 4 is the correct answer.

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