(c) : The two French scientists, Jacob and Monod proposed the lac operon of E. coli. The lac operon (an inducible operon) contains a promoter, an operator, a regulator gene and three structural genes $z, y,$ and $a,$ coding for the enzym $\beta$-galactosidase, $\beta$­-galactoside permease, and $\beta$-­galactoside transacetylase, respectively. $\beta$-­galactoside permease “pumps” lactose into the cell, where $\beta$-­galactosidase cleaves it into glucose and galactose. The function of the transacetylase is still not clear. The lac regulator gene, designated the $i$ gene, codes for a repressor. In the absence of the inducer ($i.e.$, lactose, actuall yallolactose), the repressor binds to the lac operator sequence, preventing $RNA$ polymerase from binding to the promoter and transcribing the structural genes. The inducer of the operon, allolactose, is derived from lactose in a reaction that is catalyzed by $\beta$-galactosidase. Once formed, allolactose binds to therepressor, causing it to be released from the operator $\beta$ in doing so, it induces transcription of the $z, y$ and $a$ structural genes. $CAP$ is activator called catabolic activator protein. It exerts a positive control in lac operon because in its absence $RNA$ polymerase is unable to recognise promotor gene. $CAP$ activates lac genes only when glucose is absent. Such enzymes whose synthesis can be induced by adding the substrate are known as inducible enzymes and the genetic systems responsile for the synthesis of such an enzyme are known as inducible operons.