Mechanism
The lac Z fragment, whose synthesis can be induced by IPTG, is capable of intra-allelic complementation with a defective form of β-galactosidase enzyme encoded by host chromosome (mutation lacZDM15). In the presence of IPTG in growth medium, bacteria synthesise both fragments of the enzyme. Both the fragments can together hydrolyse X-gal (5-bromo-4-chloro-3-indolyl- beta-D-galactopyranoside) and form blue colonies on media with X-gal.
Insertion of foreign DNA into the MCS located within the lac Z gene causes insertional inactivation of this gene at the N-terminal fragment of beta-galactosidase and abolishes intra-allelic complementation. Thus bacteria carrying recombinant plasmids in the MCS cannot hydrolyse X-gal, giving rise to white colonies, which can be distinguished on culture media from non-recombinant cells, which are blue.
Therefore the media used should contain ampicillin, IPTG, and X-gal.
Read more about this topic: PUC19
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