Question

Which of the following statements are true about lac operon?
I. In the absence of lactose, the repressor binds with the operator region.
II. All three structural genes in the operon are not essential for the metabolism of lactose.
III. The operator is a negative regulatory site bound by the lac repressor protein.

• A. II and III
• B. I and II
• C. II only
• D. I and III

Answer 1

The correct option is D I and III

The lac operon contains three structural genes: lac Z, lacY and lac A. All three structural genes are essential for the metabolism, entry and breakdown of lactose into its smaller monomeric units such as the glucose and galactose.

The repressor protein is coded by the regulatory gene (i gene). In the absence of lactose the lac repressor protein binds to the operator region (negative regulatory site) of the operon. Thereby preventing the RNA polymerase from transcribing the operon.

In the presence of inducers like lactose and allolactose, the repressor protein gets inactivated. Whereas, in the absence of lactose or a suitable inducer, the repressor binds with the operator region and prevents the process of transcription of the enzymes required for lactose metabolism.
Hence, statements I and III are true about lac operon.