The production of beta-galactosidase by the lac operon is started when:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Lactose binds to the repressor protein and inactivate it, thus RNA polymerase gets access to the promoter and then transcription proceeds where z gene codes for beta-galactosidase.
The repressor, made by the i gene, normally sits on the operator and blocks RNA polymerase. Lactose (or allolactose) acts as the inducer. It binds to the repressor and inactivates it, so RNA polymerase reaches the promoter and transcription proceeds.
Option (1) Lactose binds to the repressor protein and inactivate it, thus RNA polymerase gets access to the promoter and then transcription proceeds where y gene codes for beta-galactosidase. — Incorrect. The switching on is right, but the y gene codes for permease. Permease makes the cell more permeable to β-galactosides.
Option (2) Lactose binds to the repressor protein and inactivate it, thus RNA polymerase gets access to the promoter and then transcription proceeds where z gene codes for beta-galactosidase. — Correct. Both parts are right: the inducer inactivates the repressor, and the z gene codes for beta-galactosidase. This enzyme splits lactose into galactose and glucose.
Option (3) Lactose binds to the repressor protein and inactivate it, thus RNA polymerase gets access to the promoter and then transcription proceeds where a gene codes for beta-galactosidase. — Incorrect. The a gene codes for transacetylase, not beta-galactosidase.
Option (4) Lactose binds to promoter and activate it, thus RNA polymerase gets full access to the promoter and then transcription proceeds where z gene codes for beta-galactosidase. — Incorrect. Lactose binds to the repressor, not to the promoter. The promoter is the site where RNA polymerase itself binds; the inducer works by removing the repressor from the operator.
NCERT Extract:
The Lac Operon – …
