Question

Explain the method of Blue-White selection used for screening of recombinant cells containing desired plasmid with gene of interest.

Hint:

Blue-White screening uses lacZ disruption and X-gal substrate to distinguish recombinant (white) from non-recombinant (blue) bacterial colonies.

Answer 1

The Blue-White screening method is employed to detect bacterial cells that have successfully integrated recombinant plasmids carrying the target gene. Underlying Principle: This approach capitalizes on the inactivation of the *lacZ* gene, which codes for the enzyme *β-galactosidase*. The plasmid features a multiple cloning site within the *lacZ* gene. Insertion of foreign DNA into this site disrupts the functionality of *lacZ*. Methodology:

• Bacteria are transformed with plasmids and cultured on agar plates supplemented with an antibiotic (to verify plasmid acquisition) and *X-gal*, a chromogenic substrate for \(\beta\)-galactosidase.
• In the absence of an insert (non-recombinant plasmid), active \(\beta\)-galactosidase hydrolyzes X-gal, yielding blue colonies.
• In the presence of an insert that inactivates *lacZ* (recombinant plasmid), \(\beta\)-galactosidase is inactive, resulting in white colonies.

Outcome:

• Blue colonies: Indicate non-recombinant cells lacking the gene of interest.
• White colonies: Denote recombinant cells containing the desired gene insert.

This technique facilitates straightforward visual identification of recombinant bacterial colonies.