The elucidation of the lac operon was also a result of a close association between a geneticist, Francois Jacob and a biochemist, Jacque Monod in $1961 .$ They were the first to elucidate a transcriptionally regulated system. In lac operon (here lac refers to lactose), a polycistronic structural gene is regulated by a common promoter and regulatory genes.

Such arrangement is very common in bacteria and is referred to as operon. To name few such examples, $lac$ operon, $trp $operon, $ara$ operon, $his $operon, $val$ operon, etc. $Lac$ operon is a type of inducible operon. In inducible operon, presence of a chemical switch on the 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$, $\mathrm{y}$ and $a$).

The $z$ gene codes for beta-galactosidase ($p$-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 permeability of the cell to $p$-galactosides. The a-gene encodes a transacetylase.