Question

(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-operon. [5]

Answer 1

(a) An operon is a polycistronic structural gene which is regulated by a common promoter and regulator gene. [1]

(b) Lac operon consists of one regulatory gene (i gene) which codes for the repressor protein, promoter (P), operator (O) and three structural genes (z, y and a). [1]

Structural genes z, y and a code for enzymes beta-galactosidase, permease and transacetylace, respectively. The regulator gene i synthesises the repressor protein. In the absence of the inducer, the repressor protein binds to the operator region of the operon and prevents transcription by RNA polymerase. The repressor is inactivated in the presence of an inducer (lactose) that binds with it. This allows RNA polymerase access to the promoter. In this manner transcription proceeds. [3]