In blue-white screening using a plasmid vector containing the lacZ gene and X-gal medium, what explains the appearance of white colonies?

• A The lacZ gene is deleted from the plasmid
• B Foreign DNA insertion disrupts lacZ, preventing beta-galactosidase production ✓
• C The bacteria lack the lac operon repressor
• D X-gal is degraded by antibiotic resistance enzymes

Explanation

Blue-white screening relies on insertional inactivation of lacZ. Functional beta-galactosidase cleaves X-gal to produce a blue product. When foreign DNA inserts into the multiple cloning site within lacZ, the gene is disrupted, no functional enzyme is made, and colonies remain white. White colonies therefore indicate recombinant plasmids.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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