In the recombinant screening technique known as 'blue-white selection', recombinant bacterial colonies appear white on X-gal agar plates because:

Answer and explanation

Correct answer: C. Insertional inactivation of the -galactosidase gene () prevents cleavage of the chromogenic substrate X-gal

  1. Step 1: Cloning vectors like pUC19 carry a multiple cloning site located inside the gene, which encodes the -peptide of -galactosidase.
  2. Step 2: Non-recombinant plasmids express functional -galactosidase, which cleaves the chromogenic substrate X-gal to produce a brilliant blue indole product (blue colonies).
  3. Step 3: In recombinant plasmids, insertion of foreign DNA disrupts the coding sequence (insertional inactivation); no functional -galactosidase is made, so X-gal is not cleaved and the colonies remain white.
Answer
Insertional inactivation of the -galactosidase gene () prevents cleavage of the chromogenic substrate X-gal.
Why not the others: Secretion of white crystals is false; Cells failing to take up plasmid do not grow due to ampicillin selection; Active enzyme producing blue dye explains the non-recombinant colonies.

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