Question

Which of the following statements are true about lac operon?
I. Glucose binds to CAP binding site and activates the operon
II. In the absence of lactose, the repressor binds with the operator region
III. All three structural gene products are not essential for the metabolism of lactose
IV. The operator is a negative regulatory site bound by the lac repressor protein

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

Answer 1

The correct option is C II and IV

When glucose levels are low, cAMP molecules are accumulated in the cell. The cAMP attaches to CAP (Catabolite Activator Protein), allowing it to bind to the DNA. CAP helps RNA polymerase bind to the promoter, resulting in the high levels of transcription.

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

The repressor protein, which is coded by the regulatory gene (i gene), is synthesised constitutively. The main function of the repressor protein is to prevent the process of transcription. Therefore, 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.

Hence, statements II and IV are true about lac operon.