Final answer:
The lac operon is both positively and negatively controlled. Negative control occurs when the operon is repressed, while positive control occurs when the operon is induced. Negative control is mediated by a repressor protein, while positive control is mediated by an activator protein called CAP.
Step-by-step explanation:
The lac operon is a regulatory system in bacteria that controls the expression of genes involved in lactose metabolism. It can be both positively controlled and negatively controlled.
Negative control occurs when the lac operon is repressed, meaning that the genes are turned off. This happens when a repressor protein binds to the operator sequence, blocking the binding of RNA polymerase and preventing transcription of the genes. In the absence of lactose, the lac operon is negatively controlled.
Positive control occurs when the lac operon is induced, meaning that the genes are turned on. This happens when an activator protein called Catabolite Activator Protein (CAP) binds to a specific site called the CAP binding site. CAP facilitates the binding of RNA polymerase to the promoter, increasing the rate of transcription. In the presence of lactose and low glucose levels, the lac operon is positively controlled.