Question

Blue-white screening is based on the activity of:

• A. Lac Repressor
• B. \(\beta\)-galactosidase
• C. Alkaline phosphatase
• D. \(\beta\)-galactoamidase

Hint:

Remember: \[ \beta\text{-galactosidase} + \text{X-gal} \rightarrow \text{Blue color} \]
• Blue colony \(\rightarrow\) Non-recombinant plasmid
• White colony \(\rightarrow\) Recombinant plasmid

Answer 1

The Correct Option is B

Concept: Blue-white screening is a molecular biology technique used to identify recombinant bacterial colonies. It is based on the activity of the enzyme: \[ \beta\text{-galactosidase} \] which is encoded by the lacZ gene. The method uses a chromogenic substrate called X-gal:
• Functional \(\beta\)-galactosidase breaks X-gal and produces blue colonies.
• Disrupted lacZ gene cannot produce active enzyme, resulting in white colonies.

Step 1: Understanding the basis of blue-white screening. In recombinant DNA technology:
• Foreign DNA is inserted into the lacZ gene of a plasmid.
• If insertion occurs, the lacZ gene becomes inactive. Therefore: \[ \text{No functional } \beta\text{-galactosidase} \Rightarrow \text{White colonies} \] If no insertion occurs: \[ \text{Active } \beta\text{-galactosidase} \Rightarrow \text{Blue colonies} \] Hence, blue-white screening depends on the activity of: \[ \boxed{\beta\text{-galactosidase}} \]

Step 2: Eliminating the incorrect options.
• Option (A): Lac repressor regulates the lac operon but is not directly responsible for blue-white screening.
• Option (C): Alkaline phosphatase is an enzyme used in molecular biology but not in blue-white selection.
• Option (D): \(\beta\)-galactoamidase is not involved in this screening process. Thus, the correct answer is \(\beta\)-galactosidase.