When lac z gene is used as one of the selectable markers in presence of chromogenic substrate then the non-recombinant colonies

When the lacZ gene is used as a selectable marker in the presence of a chromogenic substrate, non-recombinant colonies can appear blue. This is often seen in molecular biology and microbiology techniques like bacterial cloning using plasmids or other genetic vectors.

Here's how it works:

lacZ Gene: The lacZ gene encodes β-galactosidase, an enzyme that can cleave a chromogenic substrate called X-gal (5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside). X-gal is a colorless compound until it is cleaved by β-galactosidase.

Recombinant vs. Non-Recombinant: In molecular biology experiments, researchers often insert their target DNA into a plasmid or vector that contains a selectable marker such as the lacZ gene. The target DNA can be inserted into the lacZ gene, disrupting its function. When the lacZ gene is disrupted (non-recombinant), β-galactosidase is not produced or is non-functional.

Chromogenic Substrate: When bacterial colonies containing these plasmids are grown on a medium containing X-gal, they will take up X-gal and, if the lacZ gene is disrupted (non-recombinant), they won't be able to cleave it. As a result, X-gal remains colorless, and the colonies appear white or colorless.

Blue Colonies: On the other hand, if the lacZ gene is intact (recombinant), β-galactosidase is functional, and it can cleave X-gal into a blue-colored product. So, colonies that have successfully taken up the intact lacZ-containing plasmid will appear blue.

This system is widely used in molecular biology for the selection of transformed or transfected cells, helping researchers identify colonies that have successfully taken up the plasmid of interest by their blue color. It's a convenient way to distinguish between recombinant and non-recombinant colonies on a plate.