Which one of the following events is true about the lac operon in the absence of an inducer?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Repressor binds to the operator and prevents RNA polymerase from transcribing the operon
The lac operon of E. coli controls the genes for using lactose: z (beta-galactosidase), y (permease) and a (transacetylase). The regulator gene i makes a repressor all the time. An inducer (lactose or allolactose) switches the operon on by inactivating the repressor.
Option (1) The repressor is inactivated — Incorrect. The repressor is inactivated only when the inducer binds to it, so this happens in the presence of an inducer.
Option (2) Repressor binds to the operator and prevents RNA polymerase from transcribing the operon — Correct. With no inducer, the active repressor sits on the operator and blocks RNA polymerase. This is negative regulation.
Option (3) Beta-galactosidase, transacetylase and permease will be produced — Incorrect. These enzymes are made only when the operon is transcribed, which needs the inducer.
Option (4) RNA polymerase have access to the promoter and transcription proceeds — Incorrect. This is what happens after the inducer has inactivated the repressor, not in its absence.
NCERT Extract:
The Lac Operon – …
