An operon is a functionating unit of genomic DNA containing a cluster of genes under the control of a single promoter. Thus, a single mRNA molecule is produced from this DNA segment that can be translated into several different proteins. Each gene is transcribed into its own cistron, a segment of mRNA that codes for a single polypeptide. Thus, transcription of mRNAs from the genes of an operon is usually tightly coordinated. Examples include the lac operon and trp operon in E. coli, which contain genes required for lactose metabolism and tryptophan biosynthesis, respectively.