Write a short note on the lac operon.

(N/A) Francois Jacob and Jacque Monod in $1961$ were the first to propose the concept of transcriptionally regulated systems by a common promoter and regulatory genes.
Such an arrangement is referred to as an operon,e.g.,lac (lactose) operon,trp (tryptophan) operon,ara (arabinose) operon,his (histidine) operon,and val (valine) operon.
Structure of the lac operon: The lac operon consists of one regulatory gene (the $i$ gene—here the term $i$ does not refer to inducer,rather it is derived from the word inhibitor) and three structural genes ($z$,$y$,and $a$).
The $i$ gene codes for the repressor of the lac operon.
The $z$ gene codes for beta-galactosidase ($\beta$-gal),which is primarily responsible for the hydrolysis of the disaccharide,lactose,into its monomeric units,galactose and glucose.
The $y$ gene codes for permease,which increases the permeability of the cell to $\beta$-galactosides.
The $a$ gene encodes a transacetylase.
Hence,all three gene products in the lac operon are required for the metabolism of lactose.
In most other operons as well,the genes present in the operon are needed together to function in the same or related metabolic pathway.
Lactose is the substrate for the enzyme beta-galactosidase,and it regulates the switching on and off of the operon.
Hence,it is termed as an inducer.