Question: 179. The blue and white selectable markers have been developed which differentiate recombinant colonies from non-recombinant colonies on the basis of 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 DNA insert in the plasmid and are identified as recombinant colonies.
In the light of the above statements, choose the most appropriate answer from the options given below:
(1) Both Statement I and Statement II are correct
(2) Both Statement I and Statement II are incorrect
(3) Statement I is correct but Statement II is incorrect
(4) Statement I is incorrect but Statement II is correct
Answer: Option (4)
Explanation:
Blue-white screening is based on the activity of the enzyme \beta-galactosidase, which is encoded by the lacZ gene present in the plasmid.
When no foreign DNA is inserted into the plasmid, the lacZ gene remains intact and produces functional \beta-galactosidase.
In the presence of a chromogenic substrate such as X-gal, this enzyme breaks the substrate and produces a blue coloured product, resulting in blue colonies.
When a foreign DNA fragment is inserted into the plasmid, it disrupts the lacZ gene, preventing the synthesis of functional \beta-galactosidase.
As a result, the chromogenic substrate is not broken down and the colonies remain white.
Therefore, white colonies indicate recombinant colonies with DNA insert, while blue colonies indicate non-recombinant colonies without DNA insert.
Hence, Statement I is incorrect and Statement II is correct.