Final answer:
An operon is a group of two or more genes under the transcriptional control of a single promoter. It plays a key role in the regulation of gene expression in prokaryotic cells.
Step-by-step explanation:
Operon is a group of two or more genes under the transcriptional control of a single promoter. In prokaryotic cells, operons play a key role in the regulation of gene expression. An operon consists of the promoter, operator, and structural genes. The promoter is the site where RNA polymerase binds to initiate transcription, and the operator is the region where regulatory proteins can bind to control the transcription of the operon genes.