Question

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 color in the presence of a chromogenic substrate. Given below are two statements about this method: Statement I: The blue-colored colonies have DNA insert in the plasmid, and they are identified as recombinant colonies. Statement II: The colonies without blue color have DNA insert in the plasmid, and are identified as recombinant colonies.

• A. Both Statement I and Statement II are incorrect.
• B. Statement I is correct but Statement II is incorrect.
• C. Statement I is incorrect but Statement II is correct.
• D. Both Statement I and Statement II are correct.

Hint:

Blue colonies = No insert. White colonies = Insert.

Answer 1

The Correct Option is C

Statement I is incorrect: Blue colonies typically indicate \textit{non-recombinant} plasmids. The blue color is produced by a functional lacZ gene ($\beta$-galactosidase), which is disrupted when a foreign DNA insert is successfully cloned into the plasmid.
Statement II is correct: White colonies usually indicate recombinant plasmids. The foreign DNA insert disrupts the lacZ gene, preventing the production of the blue color.