Final answer:
An operon is turned off by a process called repression, which occurs in response to certain molecules in the cell's environment. Repressible operons such as the trp operon are deactivated by the molecule they help to synthesize, while inducible operons like the lac operon are activated by the substrate they metabolize.
Step-by-step explanation:
When an operon is turned off in response to molecules present in the environment of the cell, it is undergoing repression. An operon is a set of genes that are regulated as a single unit. Repressible operons, like the trp operon, are typically turned off by the presence of a certain molecule, in this case, tryptophan.
When tryptophan is present, it binds to the repressor, which then binds to the operator and blocks RNA polymerase from transcribing the necessary genes for tryptophan synthesis. Conversely, inducible operons like the lac operon are turned on in the presence of a specific substrate, such as lactose. Allolactose, a form of lactose, binds to the repressor, causing it to release from the operator, and allowing RNA polymerase to transcribe genes involved in lactose metabolism.