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.

A. An operon is a segment of DNA which consists of a set of genes. These genes include structural genes, an operator gene, and a regulatory gene. It is a functional unit of transcription and gene regulation. Operons are found in prokaryotic organisms such bacteria.

B. Lac operon consists of one regulatory gene (i), a promoter (p), an operator (o), and three structural genes z, y and a. The regulatory gene codes for repressor protein. Promoter initiates transcription and operator which is adjacent to promoter where specific repressor protein binds. These structural genes z, y, a code for enzymes (α-galactosidase, permease and transacetylase respectively.

The lactose induces operon in the following ways:

Regulator gene (i) codes for repressor protein. Repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon.

In presence of an inducer, such as lactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds.