Question

Recombinants with insertional inactivation of $\beta$-galactosidase are identified by their inability to:

• A. grow on the galactose-rich medium
• B. grow on lactose rich medium
• C. develop resistance to antibiotics
• D. produce color in the presence of a chromogenic substrate

Hint:

Blue = Functional LacZ (Non-recombinant). White = Inactivated LacZ (Recombinant).

Answer 1

The Correct Option is D

Step 1: Concept
Blue-white screening is a technique to distinguish between recombinant and non-recombinant plasmids.

Step 2: Meaning
The lacZ gene encodes $\beta$-galactosidase, which cleaves X-gal (a chromogenic substrate) to produce a blue color.

Step 3: Analysis
When a foreign DNA insert is placed into the lacZ gene (insertional inactivation), the gene is disrupted and no functional $\beta$-galactosidase is made. Consequently, these recombinant colonies cannot cleave X-gal and remain white.

Step 4: Conclusion
Recombinants are identified by the absence of blue color production on media containing X-gal. Final Answer: (D)