In prokaryotes, genes are turned on and off using operons. An operon includes a series of structural genes along a segment of DNA and two other portions of DNA, a promoter and an operator. When the structural gene products (proteins) are required by the cell, the operator area is open which allows the RNA polymerase to attach at the promoter and transcription occurs. If the proteins are not needed by the cell, the operator area is blocked by the attachment of a repressor protein and transcription can not occur. The lac operon and the trp operon are described in the next two screens.