Final answer:
The lac operon is a stretch of DNA in E. coli consisting of three genes that code for proteins involved in lactose metabolism. It is controlled by a negative regulation strategy, where transcription is prevented by a repressor protein in the absence of lactose. When lactose is present, the repressor is inactivated and gene expression can occur.
Step-by-step explanation:
The lac operon is a stretch of DNA in E. coli that consists of three adjacent genes that code for proteins involved in the absorption and metabolism of lactose. The expression of these genes is controlled by a negative regulation strategy, meaning that their transcription is prevented by a repressor protein when lactose is not available. When lactose is present, an end product of lactose metabolism binds to the repressor protein and allows the RNA polymerase to bind to the promoter and transcribe the genes.