The document discusses the operon concept in prokaryotes. It defines an operon as a cluster of genes located together on the chromosome that are transcribed together. Operons contain structural genes whose products work together in a metabolic pathway, as well as a regulatory gene that controls expression of the structural genes. The basic operon structure includes structural genes, an operator site that binds a repressor, and a promoter where RNA polymerase binds. The lac operon codes for enzymes that break down lactose and is induced by the presence of lactose. It contains genes, an operator, and a promoter, as well as the lac repressor gene that produces a repressor protein to block transcription unless lactose is present.

1. Operon concept
Dr. Nibedita Pradhan
Department of Life Sciences
Kristu Jayanti College
Bangalore

2. Background of Operon Concept:
Regulation of Gene Expression in Prokaryotes
• All the organisms include prokaryotes need some mechanism to control
their gene expression to save energy and resource
• Expression of genes are tightly regulated based on when protein products
are needed
• Prokaryotes use OPERONS to control gene expression!

3. • Inducible or Catabolic operon: gene products usually function
in catabolic pathways (lac operon)
• Repressible or Anabolic operon: gene products usually function in anabolic
pathways (trp operon)
Types of gene regulation

4. What is Operon?
▪ Operon is operating units which can be defined as
the cluster of genes located together on the
chromosomes & transcribed together.
▪
• It is group of closely linked structural genes &
associated control gene which regulate the
metabolic activity.
▪
• All the genes of an operon are coordinately
controlled by a mechanism 1st described in 1961 by
Francois Jacob & Jaques Monod of the Pasture
institute of Paris.

5. Basic Operon Structure
Regulatory
gene Gene C
Gene B
Gene A
operator
promoter
Structural genes
Always expressed
together
Or repressed together
• Structural genes share a single
promoter because they work
together in a metabolic pathway
towards a single outcome

6. Basic Operon Structure
Regulatory
gene Gene C
Gene B
Gene A
operator
promoter
Structural genes
Always expressed
together
Or repressed together
Repressor
DNA Binding
Domain
• Environmental factors make
repressor active or inactive

7. Basic Operon Structure
• repressor active
Regulatory
gene Gene C
Gene B
Gene A
operator
promoter
Repressor
RNA
polymerase
Active repressor effectively expression of downstream structural gene by
Preventing the binding of RNA polymerase to the promoter site of the
operon

8. • repressor inactive
Repressor Repressor
Inactive
Active
Regulatory
gene Gene C
Gene B
Gene A
operator
promoter
RNA
polymerase
protein A protein B protein C

9. • The lactose operon designated as lac operon
• The lac operon codes for enzymes involved in the
catabolism (degradation) of lactose
• Lactose is the disaccharide which is made up
of glucose & galactose.
• It is the inducible operon since the presence
of lactose induce the operon to switched on.
Lac Operon

10. What is operon?

13. Element Purpose
Operator (lac O) Binding site for repressor
Promoter (lac P) Binding site for RNA
Polymerase
Repressor Gene encoding the lac repressor
protein.
Binds to DNA at the operator &
blocks binding of RNA
Polymerase at the promoter.
lac I Controls production of the
repressor protein

14. How it works?

15. Positive Gene Regulation
• Activator protein turns on Lac operon
• catabolite activator protein (CAP)
Glucose high cAMP low
Glucose low cAMP high
CAP-cAMP binds Lac
promoter and induces transcription
When would this occur,
when glucose is high or low?
Low

16. Lactose and Glucose both present

17. ►An Activator and a Repressor Together Control the lac Genes
The activator is called CAP( Catabolite Activator Protein ) .CAP can bind DNA and activate the
lac genes only in the absence of glucose.
The lac repressor can bind DNA and repress transcription only in the absence of lactose.
Both CAP and lac repressor are DNA-binding proteins and each binds to a specific site n
DNA at or near the lac promoter.
CAP binds as a dimer to a site
similar in length to the lac
operator, but different in
sequence and location.
CAP has separate activating
and DNA-binding surfaces.
At the promoter, where there is no UP-element, a CTD binds to
CAP and adjacent DNA instead.

18. The LAC operon
Lac operon –
In presence
and absence of
Glucose

19. Thank You