In the Blue-White screening method, lacZ gene is inserted in the vector. It codes for the enzyme beta-galactosidase, which breaks a colourless substance X-gal to an insoluble blue product. The lacZ gene also contains restriction sites. In this screening method, what does a blue colony indicate?

The lacZ gene containing restriction sites is used as a marker gene. The target DNA is inserted in between this gene. In a recombined vector, insertion inactivates lacZ gene, so the enzyme beta galactosidase will not be produced and hence, X-gal will not be converted to the blue insoluble product. So, a blue colony indicates that insertion has not taken place and hence, the enzyme beta galactosidase is produced. It indicates the presence of non-recombined vector.