Blue-white screening is used in cloning. What does a white colony indicate?

Blue-white screening is used in cloning. What does a white colony indicate?

• A. No insert
• B. White colonies indicate plasmid intact without insert
• C. White colony indicates disruption of lacZ gene due to insertion
• D. White colony indicates cell death

Answer: (C)

Blue-white screening uses the lacZ gene to report on inserts. When the plasmid’s lacZ is intact and functional, beta-galactosidase cleaves X-gal and colonies appear blue. If a DNA fragment is inserted into the cloning site within lacZ, the gene is disrupted and beta-galactosidase activity is lost, so X-gal is not cleaved and the colonies stay white. Therefore a white colony signals that the lacZ gene was disrupted by the insertion, indicating a recombinant plasmid. The other options don’t fit: no insert leaves lacZ intact and produces blue colonies; a plasmid intact without insert would be blue; and cell death isn’t what this screening reads.

Blue-white screening uses the lacZ gene to report on inserts. When the plasmid’s lacZ is intact and functional, beta-galactosidase cleaves X-gal and colonies appear blue. If a DNA fragment is inserted into the cloning site within lacZ, the gene is disrupted and beta-galactosidase activity is lost, so X-gal is not cleaved and the colonies stay white. Therefore a white colony signals that the lacZ gene was disrupted by the insertion, indicating a recombinant plasmid. The other options don’t fit: no insert leaves lacZ intact and produces blue colonies; a plasmid intact without insert would be blue; and cell death isn’t what this screening reads.