In a cloning vector carrying lacZ alpha, recombinant colonies grown on X-gal containing medium appear white while non-recombinant colonies appear blue. The white colour indicates that:

• A The cells failed to take up any plasmid DNA
• B The inserted DNA disrupted the lacZ coding sequence, abolishing beta-galactosidase activity ✓
• C Beta-galactosidase activity is abnormally high in these colonies
• D X-gal was degraded by the antibiotic resistance gene product

Explanation

Blue-white screening relies on alpha-complementation of beta-galactosidase. The vector supplies the lacZ alpha fragment whose multiple cloning site lies within it. Insertion of foreign DNA disrupts this fragment, so functional beta-galactosidase is not produced, X-gal is not hydrolysed, and colonies stay white. Non-recombinant vectors retain intact lacZ alpha, cleave X-gal, and yield blue colonies. Cells without plasmid would not grow under antibiotic selection at all.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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