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Q.101) The blue and white selectable markers have been developed, which differentiate recombinant colonies from non-recombinant colonies based on their ability to produce colour in the presence of a chromogenic substrate.
Given below are two statements about this method:
Statement I: The blue coloured colonies have DNA insert in the plasmid and they are identified as recombinant colonies.
Statement II: The colonies without blue colour have a DNA insert in the plasmid and are identified as recombinant colonies.
In light of the above statements, choose the most appropriate answer from the options given below:
A) Statement I is incorrect, but Statement II is correct
B) Both Statement I and Statement II are correct
C) Both Statement I and Statement II are incorrect
D) Statement I is correct, but Statement II is incorrect
Answers (1)
Q.101) In the blue-white screening method, a chromogenic substrate like X-gal is used.
- When the lacZ gene is intact (no DNA insert), the enzyme β-galactosidase is produced, and colonies turn blue — these are non-recombinant.
- When a foreign DNA is inserted into the lacZ gene, it is disrupted, and the enzyme is not produced. These colonies remain white — indicating recombinant bacteria.
Therefore:
- Statement I is incorrect: Blue colonies are non-recombinant.
- Statement II is correct: White colonies are recombinant as they contain the DNA insert.
Hence, the correct answer is Option (1).
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