Final answer:
An operon is a cluster of structural genes with related functions controlled by a single promoter and an operator in prokaryotes, allowing for efficient transcription regulation of multiple genes.
Step-by-step explanation:
The gene cluster described with a single promoter, two or more structural genes, and an operator is called an operon. In prokaryotes, structural genes with related functions are often organized into operons, which are transcribed from a single promoter. This efficiency is partly due to the formation of polycistronic mRNA, which is an mRNA molecule that carries information encoding for multiple polypeptides. The operon's regulatory region, composed of the promoter and the operator, allows for the coordinated regulation of these genes. Repressor proteins can bind to the operator sequence to prevent transcription, while activators can enhance it.
Within an operon, promoter regions signal the initiation of transcription, whereas the operator, located between the promoter and the first coding gene, acts as a binding site for repressor proteins, affecting transcription regulation. Such operons enable prokaryotes to efficiently control the expression of multiple genes that are involved in a common pathway or process, ensuring that the proteins/RNA are produced only when needed.