Write short note on lac operon
 

Francois Jacob and Jacque Monod in $1961$ were the first ones to propose the concept of transcriptionally regulated by a common promoter and regular genes.

Such an arrangement is referred as an operon eg. lac (lactose) operon, trp (tryptophan) operon, arn (arabinose) operon, his (histidine) operon and val (valine) operon.

Structure of 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 permeability of the cell to $\beta$-galactosides.

The $a$ gene encodes a transacetylase.

Hence all the three gene products in lac operon are required for 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 switching on and off of the operon.

Hence, it is termed as inducer.