Question

In lac operon, repressor protein binds to
a. Regulator gene
b. Operator gene
c. Promoter gene
d. Structural gene

Answer 1

Hint: lac operon is short for lactose operon which is required for transport and metabolism of lactose in E. coli and other prokaryotes. It is expressed when lactose is present, and glucose is absent.

Complete answer:
The idea of lac operon was proposed by Francois Jacob and Jacques Monod in 1961. It is based on the study of genes in E. coli that codes for enzymes that break down the lactose. It only occurs in prokaryotes. lac operon operates in the presence of lactose and absence of glucose.

lac operon consists of four main components:
• Structural genes
• Promoter – binding site of RNA polymerase
• Operator- binding site of lac repressor protein; the lac repressor protein inhibits the process of transcription.
• Regulator

The three structural genes are:
• lac A – encodes for β-galactoside transacetylase
• lac Y – encodes for β-galactoside permease
• lacZ- encodes for β-galactosidase

lac operon acts on presence or absence of lactose or glucose,
> When glucose is present, and lactose is absent: no transcription of the lac operon takes place because lac repressor protein binds to the operator and prevents transcription by RNA polymerase. In this, CAP is inactive and doesn’t bind DNA.

> When glucose is absent, and lactose is present: transcription occurs as lac repressor protein is released from the operator because inducer is present. In this, CAP is present and binds to the DNA and CAP also helps RNA polymerase bind to the promoter.

> When glucose is present, and lactose is present: low level transcription occurs. Lac repressor protein is released from the operator because the inducer is present. CAP remains inactive and cannot bind to the DNA.

> When glucose and lactose both are absent: no transcription occurs.
Thus, it is clear from the above context that repressor protein binds to the operator.

Hence, the correct answer is option (b).

Note: The change is lac operon is only caused by an isomer of lactose and allolactose. It binds to the lac repressor and changes its shape so DNA cannot bind. Allolactose is an example of an inducer.