The genes of prokaryotic organisms are organized into operons. An operon consists of several structural genes. It allows prokaryotes to synthesize products of several genes simultaneously. The structural genes in the operon are arranged one after another and have one common promoter, one terminator, and one operator that regulates their expression. An example of an operon is the lactose operon of Escherichia coli. It contains genes encoding the enzymes necessary for the metabolism of lactose. Unlike prokaryotic genes, genes in eukaryotic organisms do not form operons. Each of them has its promoter and terminator. In addition, the structure of these genes is more complex. They contain DNA sections that lack the information necessary for the synthesis of a gene product. Such sites are called introns. Those sites that contain the necessary information are called exons. Usually, eukaryotic genes contain several introns and exons.