The right answer is D. Operon.
For the sake of saving energy, the genome of prokaryotes is organized so that the enzymes involved in the same metabolic pathway are grouped in the form of functional units called Operons.
An operon is a genetic unit of coordinated expression. It comprises a group of adjacent structural genes encoding the enzymes of the same metabolic pathway, as well as the regulatory sequences affecting the transcription of these structural genes. The mRNA resulting from this set is synthesized in a single block called polycistronic RNA.
It is composed of:
Structural genes/cistrons: code for the enzymes participating in the same metabolic pathway, they are co-transcribed into a single polycistronic mRNA.
• Promoter common to all structural genes: this is the binding site of RNA polymerase.
• The operator: it is a regulatory sequence, it is the site of binding of the protein of the regulation, it controls the transcription of the gene downstream.
• The regulatory gene: located upstream of the operon promoter, it encodes regulatory proteins capable of binding on the operator. He owns his own promoter.
