The document discusses the lac operon in bacteria. The lac operon is a cluster of genes involved in lactose metabolism. It includes three structural genes that encode enzymes for breaking down lactose. The operon is induced in the presence of lactose and repressed in its absence by a lac repressor protein. The lac operon demonstrates how bacteria efficiently regulate gene expression in response to environmental conditions through an inducible operon system.

2. Synopsis
Introduction to operon
Operon in prokaryotes
Structural of operon
Structural genes - lac operon
Functional genes - lac operon
Absence & presence of lactose
Lac operon
Absence & presence of glucose
CRP and cAMP

3. A bacterial cell lives in direct contact with its environment,
which may change dramatically in chemical composition from one
moment to the next. At certain times, a particular compound may be
present, while at other times that compound is absent. A number of
strategies an organism use to regulate expression of individual genes.
By controlling transcription initiation, a cell can regulate which
proteins it produces and how rapidly.
Introduction - Operon

4. Define Operon
 An operon is a group of genes that are transcribed at
the same time.
 They usually control an important biochemical
process.
 They are only found in prokaryotes.

5. The Bacterial Operon:-
In bacteria, the genes that encode the enzymes of a metabolic
pathway are usually clustered together on the chromosome in a
functional complex called an operon.
A typical bacterial operon consists of:-
 Structural genes
 Promoter
 Operator
 Regulatory

7. Lac Operon
 It is the Inducible operon since the presence of lactose
the induce the operon to switch on.
 The lac operon consists of three genes each involved in
processing the sugar lactose.
 One of them is the gene for the enzyme β- galactosidase.
 This enzyme hydrolyses lactose into glucose and
galactose.
The lac operon - the cluster of genes that regulates production of the
enzymes needed to degrade lactose in Escherichia coli and some other enteric
bacteria

8.  Lactose is a disaccharide composed of galactose and
glucose.
 In its natural environment, the lac operon allows for
the effective digestion of lactose.
 It achieves this with the lac repressor which halts the
production in the absence of lactose.
 The Catabolite activator protein(CAP), which assists in
production in the absence of glucose.

9. Lac Operon

10. Lac Operon
Repressor Promoter Operator Lac Z Lac Y Lac A
β-galactosidase
Lactose permease
transacetylase
RNA
polymerase

11. Operator binding site for repressor
Promoter Binding site for RNA polymerase
Repressor
Gene encoding for the repressor protein.
Binds to DNA at the operator site and
blocks the transcription
Lac I
controls the production of repressor
protein
Functional genes

12. Gene Codes for enzyme
Function of the
enzyme
Lac Z β-galactosidase
Breaks down lactose
into glucose & galactose.
Lac Y galactose permease
This protein, found in
the E.coli cytoplasmic
membrane, actively
transports lactose into
the cells.
Lac A
Thio-galactoside
trans acetylase
The function of this
enzyme is not known.
Structural genes

13. Carbohydrates
Activator
protein
Repressor
protein
RNA
polymerase
Lac operon
+ GLUCOSE
+ LACTOSE
Not bound to
DNA
Lifted off
operator site
Keeps falling off
promoter site
No transcription
+ GLUCOSE
- LACTOSE
Not bound to
DNA
Bound to
operator site
Blocked by the
repressor
No transcription
- GLUCOSE
- LACTOSE
Bound to DNA Bound to
operator site
Blocked by the
repressor
No transcription
- GLUCOSE
+ LACTOSE
Bound to DNA Lifted off
operator site
Sits on the
promotor site
Transcription

14. Presence of lactose
The repressor gene produces repressor, in the
presence of lactose it binds to the repressor.
This will not blocks the action of RNA
polymerase, then the transcription may proceed and
the enzyme will be synthesized.

15. Lac Operon
Repressor Promoter Operator Lac Z Lac Y Lac A
β-galactosidase
Lactose permease
transacetylase
RNA
polymerase
Lactose
Repressor
protein
M RNA

16. Absence of lactose
The repressor gene produces repressor, which
binds to the operator.
This blocks the action of RNA polymerase,
thereby preventing transcription.

17. Lac Operon
Repressor Promoter Operator Lac Z Lac Y Lac A
RNA
polymerase
Repressor
protein
M RNA
No transcription

18. Presence of glucose
The repressor gene produces repressor, in the
presence of glucose which the repressor binds to the
operator.
This will also blocks the action of RNA
polymerase, there by preventing transcription.

19. Lac Operon
Repressor Promoter Operator Lac Z Lac Y Lac A
RNA
polymerase
Repressor
protein
M RNA
No transcription
Glucose

20. Absence of glucose
The repressor gene produces repressor, which
binds to the operator.
This blocks the action of RNA polymerase,
thereby preventing transcription.

21. Lac Operon
Repressor Promoter Operator Lac Z Lac Y Lac A
RNA
polymerase
Repressor
protein
M RNA
No transcription

22. In condition of low glucose
CRP and cAMP will induces the RNA polymerase to
transcript the DNA for the enzymes / proteins
CRP cAMP

23. Lac Operon
CRP
binding site
Promoter Operator Lac Z Lac Y Lac A
β-galactosidase
Lactose permease
transacetylase
RNA
polymerase

24. CONCLUSION
Lac operon is a cluster of gene that regulate
lactose metabolism. It is an inducible operon. Lac
operon is transcribed only when lactose is single source
of energy, in order to conserve cellular resource and
energy by not producing the enzyme, when there is no
need to metabolize lactose.