THE LAC OPERON

TANVI BAGTHARIA

P O Z YI

KEY

P O Z YI

STRUCTURAL GENESCONTROL REGIONS

REGULATORY GENE

Structural Gene (Z) – codes for β- Galactosidase. Structural Gene (Y) – codes for Lactose Permease.

Both consist of base pairs that can be transcribed into a length of DNA

Promoter Region (P) – a length of DNA to which the enzyme RNA Polymerase binds to to begin

transcription of Z and Y.Operator (O) – a length of DNA next to the structural genes. It

can switch them on and off.

KEY

RP

Β-G

LP

RNA P

mRNA strand

Repressor protein

Ribosome

RNA Polymerase

β- Galactosidase

Lactose Permease

Glucose molecules

Lactose molecules

IN THE PRESENCE OF GLUCOSE

I P O Z Y

1) The regulator gene is expressed (by translation of mRNA on a ribosome) and the repressor protein is synthesised.

RP

2) The repressor protein binds to the operator.

IN THE PRESENCE OF GLUCOSE

I P O Z Y

RNA P

XRP

3) This prevents RNA Polymerase binding to the promoter region therefore the structural genes cannot be transcribed onto mRNA.

4) Without transcription, translation cannot occur and therefore β- Galactosidase and Lactose Permease cannot be formed.

IN THE PRESENCE OF LACTOSE

I P O Z Y

RNA P

5) Lactose molecules bind to the allosteric site of the repressor protein.

RP

6) This causes the repressor protein to undergo conformational change so it cannot bind to the operator and so it breaks away.

IN THE PRESENCE OF LACTOSE

I P O Z Y

RNA P

7) Now that the promoter region is unblocked, mRNA can bind to it and initiate transcription of MRNA for genes Z and Y.

8) Once genes Z and Y have been translated on a ribosome, they form the enzymes β- Galactosidase and Lactose Permease repectively.

Β-G LP

9) Lactose Permease increases the permeability of the cell to lactose and β-Galactosidase hydrolyses lactose into galactose and glucose.