Practice question

IPTG, isopropyl-beta-D-thiogalactopyranoside, mimics allolactose geometry required for LacI binding pocket including galactose ring recognition but contains a thio linkage between isopropyl and galactose resistant to hydrolysis by beta-galactosidase enzyme. Cells therefore cannot catabolize IPTG as carbon source, so its intracellular concentration remains stable over many generations, driving persistent derepression without consumption of inducer or metabolic side effects. This gratuitous property contrasts with lactose, which induces transiently until eventually metabolized to glucose. Non-metabolizability makes IPTG ideal for laboratory induction of lac promoters and T7-driven recombinant protein overexpression systems producing stable transcription.