Practice question

Blue-white screening exploits α-complementation of lacZ encoding β-galactosidase in Escherichia coli. Plasmids carrying lacZα produce functional enzyme when induced by IPTG, cleaving X-gal to 5,5'-dibromo-4,4'-dichloro-indigo yielding intensely blue colonies on indicator plates. Insertion of gene of interest within the multiple cloning site located inside lacZα causes insertional inactivation, abolishing enzyme activity and preventing complementation. No cleavage of X-gal occurs. Therefore white colonies harbor recombinant plasmids with insert, while blue colonies retain empty vector, allowing rapid visual identification of successful cloning events.