Cells need control systems to turn the right genes on and off at the right time in order to manage the rate and amount of product produced. Thus, cells need to turn gene expression on and off in order to respond to environmental conditions. Operons provide such a management system in bacteria. These are clusters of genes that are regulated as a unit. An operon typically consists of an operator, a promoter and structural genes. This system allows gene expression only under the appropriate conditions. Thus, if the enzymes produced by the structural genes are actually needed, then a repressor is inactivated by the operator, allowing the RNA polymerase to access the structural genes and initiate transcription. Alternatively, if the products are not needed, then the repressor binds to the operator thereby preventing the RNA polymerase from accessing the promoter sequence and no product is produced.
There are now many such systems known in bacteria where enzymes switch on only when the appropriate substrate is available for the enzymes to work on. In each case there is a repressor protein which recognizes the appropriate control sequence but which is inactivated by the relevant substrate.
Some specialists say that this sophisticated system of controls is like the logic statements in computer programs. Indeed, Marcos Eberlin asked concerning these systems: “How is it possible for genes to evolve at random to then be recruited, juxtaposed on the chromosome, and assembled into operons so early in life’s history?” [Fore Sight: How the history of life reveals planning and purpose. 2016 pp. 171. See p. 64) Obviously, evolution had nothing to do with developing these systems. Purpose and planning from God were what was needed.