In blue-white screening used to identify recombinant bacteria, why are white colonies considered desirable rather than blue colonies?

• A. White colonies are more resistant to antibiotics than blue colonies.
• B. White colonies result from bacteria expressing functional β-galactosidase.
• C. White colonies contain bacteria that have not taken up any plasmid.
• D. White colonies indicate successful insertion of foreign DNA into the plasmid, disrupting the lacZ gene.

Answer: (D)

Verified step by step guidance

1. Understand the principle of blue-white screening: it is a molecular biology technique used to identify bacteria that have taken up recombinant plasmids containing foreign DNA.
2. Recall that the plasmid used in this method contains the lacZ gene, which encodes the enzyme β-galactosidase. This enzyme can cleave a substrate called X-gal, producing a blue color.
3. Recognize that if the foreign DNA is successfully inserted into the plasmid, it disrupts the lacZ gene, preventing the production of functional β-galactosidase.
4. Therefore, bacteria with plasmids containing the inserted foreign DNA cannot cleave X-gal and form white colonies, while bacteria with intact lacZ genes produce blue colonies.
5. Conclude that white colonies are desirable because they indicate successful insertion of foreign DNA into the plasmid, which disrupts the lacZ gene and prevents blue color formation.