In prokaryotes, unlike eukaryotes, protein synthesis is constantly occurring, meaning that DNA is constantly being copied into RNA, which allows for the creation of proteins.

An operon is a series of genes that self-regulate and work together in order to form proteins. Within the operon lies the promoter and operator, which regulate but do not code for protein synthesis. In order for DNA to be copied, or transcribed, into RNA, RNA polymerase must first bind to the promoter segment of the DNA. The process of transcription continues until repressor proteins bind to the operator; the repressor prevents access to the rest of the protein-producing genes. However, if an inducer molecule is present and binds to the repressor, it causes the repressor to change shape and release from the operator. When this happens, RNA polymerase can continue to transcribe DNA into RNA until another repressor binds to the operator. 

There are three main kinds of operons that exist. The first can be modeled by the lac operon, which follows a regulatory gene that continually produces repressor proteins that bind to the operator and regulate transcription. When lactose is consumed, lactose molecules rush into the cell and bind to the repressor proteins, which allows for the initiation of transcription by RNA polymerase.

The trp operon is different (almost the opposite); the repressor is active only when bonded to a specific molecule, but for the remainder of the time, it remains unbonded and allows for transcription.

The final kind of operon uses activators, which bond directly to DNA and enhance transcription. When the activators are absent, RNA polymerase proceeds with transcription, but at a much slower rate.