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:

The correct answer is Statement I is incorrect but Statement II is correct

Concept:

• The differentiation between recombinant and non-recombinant colonies using blue-white screening is a widely used method in molecular biology.
• It is based on the insertional inactivation of the lacZ gene, which encodes the enzyme β-galactosidase.
• In this method, a chromogenic substrate such as X-gal is used. The β-galactosidase enzyme cleaves X-gal, producing a blue-colored product.
• Recombinant colonies are identified by the insertion of foreign DNA into the multiple cloning site of the plasmid, which disrupts the lacZ gene and prevents the production of β-galactosidase, resulting in white colonies.
• Non-recombinant colonies retain an intact lacZ gene and produce β-galactosidase, leading to blue-colored colonies.

Explanation:

Statement I: "The blue-colored colonies have DNA insert in the plasmid and they are identified as recombinant colonies."

• This statement is incorrect because the blue-colored colonies represent non-recombinant colonies.
• These colonies have an intact lacZ gene, which produces β-galactosidase, resulting in the cleavage of X-gal and the formation of blue color.
• No DNA insert is present in these colonies, and thus they are not recombinant.

Statement II: "The colonies without blue color have DNA insert in the plasmid and are identified as recombinant colonies."

• This statement is correct because the absence of blue color (white colonies) indicates the disruption of the lacZ gene by the insertion of foreign DNA.
• The lack of β-galactosidase activity results in no cleavage of X-gal, and thus no blue color is produced.
• These colonies are recombinant as they contain the inserted DNA fragment in the plasmid.