1. THE LAC OPERON
TANVI BAGTHARIA I P O Z Y

2. KEY
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.
REGULATORY GENE
CONTROL REGIONS
STRUCTURAL GENES I P O Z Y

3. KEY mRNA strand Β-G β- Galactosidase Repressor protein RP LP
Lactose Permease
Ribosome
Glucose molecules
RNA P
RNA Polymerase
Lactose molecules

4. IN THE PRESENCE OF GLUCOSE
2) The repressor protein binds to the operator. I P O Z Y RP
1) The regulator gene is expressed (by translation of mRNA on a ribosome) and the repressor protein is synthesised.

5. IN THE PRESENCE OF GLUCOSEX
4) Without transcription, translation cannot occur and therefore β- Galactosidase and Lactose Permease cannot be formed.
RNA P RP I P O Z Y
3) This prevents RNA Polymerase binding to the promoter region therefore the structural genes cannot be transcribed onto mRNA.

6. IN THE PRESENCE OF LACTOSE
6) This causes the repressor protein to undergo conformational change so it cannot bind to the operator and so it breaks away.
RNA P RP I P O Z Y
5) Lactose molecules bind to the allosteric site of the repressor protein.

7. IN THE PRESENCE OF LACTOSE
9) Lactose Permease increases the permeability of the cell to lactose and β-Galactosidase hydrolyses lactose into galactose and glucose.
Β-G LP
RNA P I P O Z Y
8) Once genes Z and Y have been translated on a ribosome, they form the enzymes β- Galactosidase and Lactose Permease repectively.
7) Now that the promoter region is unblocked, mRNA can bind to it and initiate transcription of MRNA for genes Z and Y.