Question

Screening markers use __ gene that turns the recombinant cells white in a media that is rich is X-Gal

Answer 1

For screening the clones containing recombinant DNA, LacZ gene is used as a screening marker. LacZ gene produces the enzyme β-galactosidase which breaks down lactose to glucose and galactose. A multiple cloning site is introduced within this gene, i.e., the recombinant plasmid will contain the gene of interest within this gene. Since the gene of interest is within the LacZ gene, the gene product or beta-galactosidase will not be produced by transformed cells. The cells are grown on agar plates containing a chromogenic substrate known as X-gal which is homologous to lactose. If $\beta$-galactosidase is produced, X-gal is hydrolyzed to form an insoluble blue pigment. Therefore, the colonies formed by non-recombinant cells will appear blue in colour while the recombinant ones appear white. The desired recombinant colonies can be easily picked and cultured.