Question

The lac operon is turned on when allolactose molecules bind to:
A) Promoter site
B) Operator site
C) mRNA
D) Repressor protein

Answer 1

The lac operon is lactose operon. It is required for the transport and metabolism of lactose in E.coli and other bacteria. The lac operon helps in the digestion of lactose when glucose is not available.

Complete answer:

The lac operon consists of three structural genes, a promoter, regulator, terminator, and an operator.
The three structural genes arelacZ, lacY and lacA.
lacZand lacYare necessary for lactose catabolism.
Transcription of all genes starts with the binding of the enzyme RNA polymerase RNAP. The binding of the RNA polymerase to the promoter is aided bycAMP receptor protein.
The lacI gene (regulatory gene for lac operon)produces a protein that blocks RNAP from binding to the operator of the operon.
The allolactose can inactivate the protein by binding to it.
The protein which is produced by lacIgene is known as lac repressor.
This type of regulation of the lac operon is called as negative inducible.
The allolactose binds to the lac repressor and dissociates it from the lac operator.
The absence of the lac repressor can only allow the transcription of lac operon to proceed. So the lac >operon is turned on when the allolactose molecules bind to repressor protein.

Hence the correct answer is option ‘D’ i.e, repressor protein.

Note: Allolactose is a disaccharide which is similar to lactose. Allolactose is an inducer of lac operon in E.coli and many other bacteria.Allolactose binds to the subunit of lac repressor. It results in the conformational changes and reduces the binding affinity of lac repressor to lac operator.It helps in proceeding the transcription of lac operon.