Operon. Definition. noun, plural: operons. A group of genes or a segment of DNA that functions as a single transcription unit. It is comprised of an operator, a promoter, and one or more structural genes that are transcribed into one polycistronic mRNA.peron. Definition. noun, plural: operons. A group of genes or a segment of DNA that functions as a single transcription unit. It is comprised of an operator, a promoter, and one or more structural genes that are transcribed into one polycistronic mRNA.