Question

Which of the following events would occur in lac operon of E. coli when the growth medium has a high concentration of lactose?

• A. The structural genes fail to produce polycistronic mRNA
• B. The repressor protein binds to RNA polymerase and prevents translation
• C. The repressor protein attaches to the promoter sequence and derepresses the operator
• D. The inducer molecule binds to repressor protein and RNA polymerase binds to promoter sequence

Answer 1

The correct option is D The inducer molecule binds to repressor protein and RNA polymerase binds to promoter sequence

In lac operon, when lactose is present, it acts as an inducer molecule that binds to the repressor molecule. Thus, forming an inactive repressor. Due to which repressor will fail to bind the operator region and the RNA polymerase binds to the operator, transcribing mRNA. The concept of lac (lactose) operon of E. coli was given by Jacob and Monod.
mRNA produced is known to be polycistronic which on transcription, produces all three enzymes, i.e. $\beta$-galactosidase, permease and transacetylase. Hence, the operon will be 'switched on' in this situation.