The operon theory proposes that genes are organized into operons - clusters of genes under the control of a single promoter. An operon contains a promoter, operator, and multiple structural genes. The lac operon in E. coli regulates genes involved in lactose metabolism. It is negatively regulated - in the absence of lactose, a repressor binds the operator to prevent transcription, but lactose or allolactose induce transcription by binding to the repressor. The operon theory explains how bacteria regulate gene expression in response to environmental conditions.

2. • The Operon theory was first proposed by the French microbiologists
François Jacob and Jacques Monod in the early 1960s.
• It is also known as Gene Regulation System.
• Operon is a functioning unit of DNA containing a cluster of genes
under the control of a single promoter.
PROMOTER OF
REGULATORY
GENE
REGULATORY
GENE
STRUCTURAL
GENE
PROMOTER
OF OPERON
SYSTEM
OPERATOR
SITE

3.  It helps to produce proteins only when and where they are required and
the Operon allows the cell to conserve energy.
 Each and every gene have an operating system.

4. An Operon is made of 3 basic DNA components:
1. Promoter – A nucleotide sequence that enables a gene to be transcribed.
The promoter is recognized by RNA polymerase, which then initiates
transcription.
2. Operator – A segment of DNA to which a repressor binds. It is a segment
between the promoter and the genes of the Operon. It plays important role in
the Operon function.

5. 3. Structural genes – The genes that are co-regulated by the Operon.
 Control of an Operon is a type of gene regulation that enables
organisms to regulate the expression of various genes depending on
environmental conditions.
 Operon regulation can be either negative or positive by induction or
repression.

6. With positive control, an activator
protein stimulates transcription by
binding to DNA.
Involves the binding of a repressor
to the operator to prevent
transcription.

8. Found in E.coli bacteria, is involved
in regulatory mechanism of operon
responsible for utilization of lactose.
Found in E. coli bacteria, is a group of
genes that encode biosynthetic
enzymes for the amino acid tryptophan.

11.  The regulatory mechanism of operon responsible for utilization
of lactose as carbon source is called lac operon.
 It was extensively studied by Jacob and Monad in 1961 with help
of E.coli.
 Lac operon can function only in the presence of lactose.

12. Operon provides a mechanism for the coordinate expression of structural genes
controlled by regulatory genes.
Products are required for
transport and metabolism of
lactose.
A regulator gene, regulator, gene is a
gene involved in controlling the
expression of one or more other genes.

13. +
Promoter of
Regulatory gene
(Lac I)
Lac I gene
(Regulatory
gene)
Lac P gene
(Promoter)
Lac O
(operator
gene)
Lac Z gene
(Galactosidase
enzyme)
Lac Y gene
(Permease
enzyme)
Lac A gene
(Transacetylase
enzyme)

14. Present at upstream of
promoter sequence and it
have its own promoter.
Lac I
Common promoter for
the operon system.
Lac P
Plays important role in
gene expression
Lac O

15. Encodes the enzyme
β-galactosidase.
(which splits lactose into
glucose and galactose).
Lac Z
Encodes lactose
Permease enzyme.
absorbs the lactose to
into the body of bacteria
Lac Y
Encodes a lactose
transacetylase enzyme.
Lac A

16. The repressor protein is produced by Lac I gene . The repressor protein
consist of identical polypeptide with binding sites. One for attachment with
DNA and another for attachment with the inducer.

17. •If there are no lactose, the repressor protein bind to operator site, so that RNA
polymerase doesn’t bind to promoter thus the enzyme is not secreted.

18. • Presence of lactose, it acts as an inducer and binds to the repressor ,
changes its configuration thus the repressor get released from the operator
site (Lac O) and initiates Transcription.

19. • Now the RNA polymerase binds to the promoter hence, transcription starts
and finally produces the mRNA & functional protein.
•Small amount of lactose is converted into allolactose the transcription
process continues…...

20. •When the lactose is exhausted the Lac I gene will synthesize the repressor
protein which binds to the operator gene and thus stops the transcription
process.
•Isopropylthiogalactoside (IPTG) are similar to allolactose which
can also act as Inducer.

22. • To control the amount of gene product present in the cell.
•To allow cells to adjust with changing conditions.
•Specializations and division of labor among cells.
•Expression of appropriate genes at proper time.
•Synchronized regulation of multiple genes encoding products with
interdependent activities.
•Prevent the wastage of cellular energy