(a) What is an operon ?
(b) Explain how a polycistronic structural gene is regulated by a common promoter and a combination of regulatory genes in a lac- opron.

Explanation

(a) An operon is a polycistronic structural gene 'I' which codes for the repressor protein promoter (P) and operator (O) are adjacent to gene.
Structural genes z,y,a code for enzymes (`beta` - galactosidase , peruecase and transacetylase respectively).
The regulator gene i- synthesize the repressor protein , in absence of inducer , the repressor protein binds the operator region of the operon , prevents transceiption.
The repressor is inactivated in the presence of an inducer that binds with it , this allows RNA polymerase access to promoter and transcription proceeds.