In the absence of tryptophan, what happens to the genes within the trp operon? a. the regulator without tryptophan-binding prevents the genes from being transcribed. b. the regulator falls off the operator and structural genes get transcribed. c. lack of tryptophan increases the level of camp high, which leads to activation of cap protein and gene expression. d. the active repressor binds to the operator and genes do not get transcribed. e. the active activator binds to the operator and transcription of structural gene takes place.

The correct answer is b  The regulator gene falls off the operator and the structural genes get transcribed.

Explanation:

Tryptophan operon contain3 regulatory genes and 5 structural genes.when adequate amount of tryptophan is present at that time the tryptophan binds to the repressor molecule encoded by tryp R gene.Thus Tryptophan act as co repressor .The repressor molecule bound to tryptophan interacts with operator region resulting in blocking the expression of structural genes.

 But when tryptophan is absent at that time repressor molecule cannot function.As a result the repressor molecule fall off from the operator and structural genes get transcribed.