Question

Which of the following statements about lac-operon is correct ?

• A. Galactose can act as an inducer of lac operon
• B. Gene i is constitutively expressed
• C. Lactose activates repressor to bind to the operator
• D. Genes i, z, y and a share single common promoter

Hint:

• Constitutive expression means that the gene is continuously expressed to maintain basic cellular functions, independent of external environmental triggers.

• The structural genes of the lac operon and their corresponding proteins are: itemize

• \(z\) gene \(\rightarrow\) \(\beta\)-galactosidase (hydrolyzes lactose)

• \(y\) gene \(\rightarrow\) permease (increases permeability of the cell to \(\beta\)-galactosides)

• \(a\) gene \(\rightarrow\) transacetylase (transfers an acetyl group to \(\beta\)-galactosides)
itemize

Answer 1

The Correct Option is B

Concept:

• The lac operon (lactose operon) is a polycistronic gene regulation system in Escherichia coli that regulates the transport and metabolism of lactose.

• Its key components include: itemize

• Regulatory gene (\(i\) gene): Codes for the repressor protein.

• Promoter (\(p\)): Binding site for RNA polymerase.

• Operator (\(o\)): Binding site for the repressor protein.

• Structural genes (\(z, y, a\)): Code for metabolic enzymes.
itemize

Step 1: Evaluate each statement individually for accuracy

• Option (A): Galactose is a monosaccharide product of lactose hydrolysis, but it cannot bind to the repressor protein to initiate operon transcription; only lactose (or its isomer allolactose) functions as the inducer. Thus, this statement is incorrect.

• Option (B): The regulatory \(i\) gene is transcribed and translated at a continuous, baseline level regardless of the presence of lactose, which is known as constitutive expression. This ensures repressor protein is always present in the cell to regulate the system. This statement is correct.

• Option (C): When lactose enters the cell, it binds to the repressor and causes a conformational change that inactivates the repressor. This prevents the repressor from binding to the operator, allowing transcription. Thus, this statement is incorrect.

• Option (D): The regulator gene (\(i\) gene) has its own independent promoter, while the structural genes (\(z, y, a\)) share a single, separate promoter. Thus, this statement is incorrect.

Step 2: Identify the correct option
Based on the evaluations, only statement (B) is biologically accurate.