Final answer:
When tryptophan is present in the cell, the tryptophan-repressor complex forms and binds to the trp operator, blocking RNA polymerase and preventing transcription of downstream genes. When tryptophan is not present, the repressor does not bind, allowing the operon to be active and tryptophan to be synthesized.
Step-by-step explanation:
When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor. This causes the repressor to change shape and bind to the trp operator. Binding of the tryptophan-repressor complex at the operator physically blocks the RNA polymerase from binding, and transcribing the downstream genes. Thus, when the cell has enough tryptophan, it is preventing from making more.
When tryptophan is not present in the cell, the repressor by itself does not bind to the operator; therefore, the operon is active and tryptophan is synthesized.