Give an account of the Blue-White Method of selection of recombinants

Explanation

Blue-white method of selection is a powerful method of screening for the presence of recombinant plasmid. Foreign gene is inserted. This method is based upon the insertional inactivation of the lac Z gene present on the vector (e.g., PUC19). This gene expresses the enzyme Bgalactosidase whose activity can cleave a colourless substrate called X-gal into blue coloured product. Isopropyl B-D-1-thiogalacto-pyranoside (IPTG) is used along with X-gal for blue-white screening. IPTG is a non-metabolizable analog of galactose that induces the expression of lac Z gene. It should be noted that IPTG is not a substrate for B-galactosidase but only an inducer. If the lac Z gene is inactivated due to the presence of the insert (express active enzyme), then the enzyme is not expressed. Hence, if after a transformation experiment the E. coli host cells are plated on an ampicillin and X-gal containing solid media plate then colonies which appear blue are those which have transformed cells (antibiotic resistant) but do not have the insert (express active enzyme). Colonies which appear white are both ampicillin resistant and have the insert rDNA and thus are the cells to be used for future experiment.