Question

MCAT Tutorial: Lac and Trp Operons

Answer 1

\({\bf{Lac~Operon:}}\)
components
- P: promoter, RNA pol binds and initiates transcription
- O: operator, Lac repressor binds and stops transcription
- Z: codes for beta-galactosidase (cleaves lactose)
- Y: codes for permease (transports lactose into cell)
- A: codes for transacetylase (transfers acetyl group from acetyl-CoA to beta-galactosides)
- CAP: binds cAMP

Answer 2

Other genes:
- crp: codes for CAP
- I gene: codes for repressor

\({\bf{In~High~Glucose,~Low~Lactose}}\)
- lac operon is off (inducible)
- adenyl cyclase is inactivated
- cAMP low
- repressor is bound to operator

\({\bf{In~Low~Glucose,~High~Lactose}}\)
- lac operon is on
- adenyl cyclase is activated
- cAMP high
- repressor is not bound to operator
- repressor is bound to lactose, not to the operator, so transcription occurs

Answer 3

\({\bf{Trp~Operon}}\)
repressible (normally transcription is on)
the actual genes are less important to know except maybe trpR which codes for the repressor



in low tryptophan concentrations, the repressor cannot bind to the operator, thus transcription proceeds as normal
in high tryptophan concentrations, repressor and tryptophan bind to the repressor and inhibit transcription

In general, catabolic processes use inducible operons and anabolic processes use repressible operons

Answer 4

Source material is Chapter 4.8 of MCAT: Biology Review, 2nd edition by Princeton Review