The document discusses gene regulation in prokaryotes through the use of operons. It describes the lac and tryptophan operons. The lac operon controls genes involved in lactose metabolism and is regulated by the presence of lactose. The tryptophan operon controls genes for tryptophan synthesis and is regulated by the presence of tryptophan. Operons allow for the coordinated expression of genes through a single promoter region and regulatory protein that can turn transcription on or off in response to environmental conditions.

2. • Introduction
• Genes
• Gene Regulation
• The Lactose Operon
• The Tryptophan Operon

3. • Always on-
Also known as ‘House Keeping Gene’.
• Regulate-
a) Normally off-
Also known as ‘Inducible Gene’. Because it
induces or switches on the gene.
b) Normally On-
Also known as ‘Repressible gene’. Because it
represses or switches off the gene.

4. POSITIVE REGULATION
Activator binds to regulatory site to stimulate
transcription; gene off in the absence of
activator.
NEGATIVE REGULATION
Repressor binds to regulatory site to stimulate
transcription; Gene is off in absence of
activator.

5. • 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.

6. 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.

7. • E. coli can use either glucose, which is a
monosaccharide, or lactose, which is a
disaccharide
• However, lactose needs to be hydrolysed
(digested) first
• So the bacterium prefers to use glucose when it
can.

8. • A repressor protein is continuously synthesised. It
sits on a sequence of DNA just in front of the lac
operon, the Operator site
• The repressor protein blocks the Promoter site
where the RNA polymerase settles before it starts
transcribing
Regulator
gene
lac operonOperator
site
z y a
DNA I
O
Repressor
protein RNA
polymeraseBlocke
d

9. • A small amount of a sugar allolactose is formed within the
bacterial cell. This fits onto the repressor protein at another
active site (allosteric site)
• This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the operator
site. RNA polymerase can now reach its promoter site
Promotor
site
z y a
DNA
I O

10. • The genes of the trp operon are used to make
the amino acid tryptophan.
• It is turned off when enough tryptophan is in
the cell.
• Tryptophan is the effector molecule.

11. • Operon codes for 5 genes in bacteria.
• Five polypeptides combine to make three
enzymes.
• Each enzyme participates in a step to make
tryptophan

12. • Repressor Protein (trpR) binds to operator
when tryptophan is present.
• Prevents RNA polymerase from transcribing
genes when bound to operator.
• Transcribed as a different gene from trp operon

13. • Enzymes are required to make tryptophan
• Repressor is NOT bound to operator
• Transcription can proceed

14. • Tryptophan binds to repressor.
• Repressor can now bind operator to prevent
transcription

15. Operon is set of gene that are coordinately
controlled by a regulatory protein and
transcribed as a single polycistronic message.
This mechanism allows for synchronization of
transcription and translation, a key element in
attenuation

16. • https://www.google.co.in/search?q=The+lac+operon
+in+e.coli.ppt&client=opera&hs=OtG&biw=1366&bih
=586&source=lnms&tbm=isch&sa=X&ei=OzQ0VJu1N4
2xuATqjIH4BQ&ved=0CAYQ_AUoAQ
• https://www.khanacademy.org/science/biology/gene-regulation/gene-
regulation-in-bacteria/a/the-trp-operon
• https://www.khanacademy.org/science/biology/gene-regulation/gene-
regulation-in-bacteria/a/the-lac-operon
• http://www.phschool.com/science/biology_place/biocoach/lacoperon/qui
z.html