Molecular Basis Of Inheritance Practice Test

Which statement describes repressible operon regulation?

The operon is normally off and turned on by substrate The operon is normally active but can be turned off when the product is abundant; regulated by a co-repressor with repressor to block transcription. Regulated only by enzymes Not regulated at all by metabolites or factors

Correct Answer:

The operon is normally active but can be turned off when the product is abundant; regulated by a co-repressor with repressor to block transcription.

Explanation:

Repressible operons are normally on, producing enzymes for a biosynthetic pathway, and can be switched off when the end product is plentiful. The product acts as a co-repressor that binds to the repressor protein, activating it. The activated repressor-co-repressor complex then binds to the operator to block transcription, stopping synthesis of more enzyme. This negative feedback keeps production aligned with demand, conserving resources. The classic example is the tryptophan (trp) operon. The other ideas don’t fit this pattern: inducible systems are turned on by substrates, not repressed by end products; and regulation by enzymes alone or by factors other than metabolites doesn’t describe the regulatory mechanism at the transcriptional level in repressible operons.

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