An operon is any of a group of genes on a chromosome comprising structural genes and an operator gene. Structural genes direct the synthesis of enzymes involved in the formation of a cell constituent or the utilization of a nutrient. The operator gene responds to a repressor molecule, and can exist open or closed. When the operator gene is open, the genes is controls are functional, producing proteins. When interacting with the repressor, the operator gene is closed. Regulatory genes control the operons.