Question

MCAT Tutorial: Gene Regulation of Lactose & Tryptophan

Answer 1

\({\bf{The~Lactose~Operon:}}\)



consists of:
- a regulatory gene (codes for the repressor protein)
- operator region (controls the expression of the structural genes)
- structural genes (code for the enzymes required to digest lactose)
- promoter (binding site for RNA Polymerase)

when lactose absent:
- regulatory gene active, codes for the repressor
- repressor binds to the operator, blocking RNA polymerase from transcribing the genes

when lactose present:
- lactose binds to the repressor so the repressor cannot bind to the operator
- with repressor inactive, rna polymerase can bind to the promoter and proceed with transcription
- the mRNA for the digestive enzymes are transcribed/translated

Answer 2

\({\bf{The~Tryptophan~Operon:}}\)



when tryptophan absent:
- the repressor cannot bind to the operator
- transcription of structural genes proceeds as normal. synthesis of tryptophan continues.

when tryptophan present:
- tryptophan binds to the repressor, changing its conformation so it can bind at the operator
- RNA Polymerase is blocked and cannot continue transcription
- the structural genes are not transcribed and the enzyme is not produced. synthesis of tryptophan is halted.

Answer 3

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT