Q.An operon is a:

Gene expression can be switched on or off, most commonly at the transcriptional level, in response to metabolite signals. Clusters of functionally related genes transcribed together as one mRNA are called operons (E. coli groups roughly 260 genes into about 75 operons); each operon has one or more structural genes, a promoter (where RNA polymerase binds), and an operator (where a repressor protein binds, positioned between promoter and structural genes). Jacob and Monod's classical lac operon model, from E. coli, is the standard example. Metabolising lactose requires three enzymes: permease (lets lactose enter the cell), beta-galactosidase (hydrolyses lactose into glucose and galactose), and transacetylase. The lac operon has a regulatory 'i' gene, a promoter (p) and an operator (o), plus three structural genes, lac z, lac y and lac a, coding for these three enzymes respectively, all transcribed together as one polycistronic mRNA. When glucose (not lactose) is available, the 'i' gene product, the repressor protein, binds the operator and blocks RNA polymerase, so th …