Transcript The lac Operon - kyoussef-mci

Control Mechanisms
 There are 42 000 genes that code for proteins in
humans
 However, not all proteins are required at all times.
 E.g. Insulin is only required in a cell when glucose
levels are high.

It would be inefficient and wasteful for a cell to transcribe,
translate the insulin gene when glucose levels are low
 Regulation is therefore vital to an organism’s survival
 Regulation – the turning on or off of specific genes
depending on the requirements of an organism
Why Turn Genes On and Off?
 Cell Specialization
 each cell of a multicellular eukaryote expresses only a
small fraction of its genes
 Development
 different genes needed at different points in life cycle
of an organism

afterwards need to be turned off permanently
 Responding to organism’s needs
 cells of multicellular organisms must continually turn
certain genes on & off in response to signals from their
external & internal environment
Signal
NUCLEUS
Chromatin
Chromatin modification:
DNA unpacking involving
histone acetylation and
DNA demethlation
DNA
Gene available
for transcription
 The control of gene expression can
Gene
Transcription
RNA
occur at any step in the pathway
from gene to functional protein
Exon
Primary transcript
Intron
RNA processing
Tail
Cap
mRNA in nucleus
Transport to cytoplasm
 The focus of today’s lesson will be
on the regulation of gene expression
at the transcriptional level.
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypetide
Cleavage
Chemical modification
Transport to cellular
destination
Active protein
Degradation of protein
Degraded protein
Figure 19.3
Gene Expression
Gene expression for all genes falls into one of two
categories. (prokaryotes and eukaryotes)
constitutive expression – genes which are always
turned on
1.

2.
known as housekeeping genes
induced expression – genes which are only turned
on as needed
lac Operon
 lac - lactose
 operon – several genes in a sequence all controlled
by a single promoter (mainly in prokaryotes; some
eukaryotes). It also includes an operator which is
the “on” and “off” switch.
promoter
gene 1
operator
gene 2
gene 3
lac Operon
Cells mainly use glucose as a source of energy.
The lac operon is only turned on when glucose is
absent, but lactose is present.
lac Operon Animation
lac Operon Details
Operon codes for 3 enzymes – found in E. coli
1. beta-galactosidase (lacZ gene)

enzyme which breaks down lactose
2. permease (lacY gene)

protein transporter which brings lactose into cell
3. transacetylase (lacA gene)

adds acetyl group to galactose
Repressor Protein
repressor protein (lacI gene)
 transcribed by a different gene from the lac operon
 binds to the operator portion in the presence of
glucose
 prevents RNA polymerase from transcribing genes
when bound to operator

Lactose is not needed as an energy source because
glucose is present
When do you want the repressor to bind / not bind to
the operon?
When lactose is absent:
 enzymes are not needed to metabolise lactose
 repressor binds to the operator to inhibit transcription
Promoter
Regulatory
gene
DNA
mRNA
Protein
Operator
lacl
5’
lacZ
3’ RNA
polymerase
No
RNA
made
Active
repressor
(a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in
the absence of lactose it switches off the operon by binding to the operator.
Figure 18.22a
When lactose is present:
 Lactose (or allactose) binds to the repressor protein
 lactose-repressor complex cannot bind to the operator
 transcription can proceed lac operon
DNA
lacl
lacz
3’
mRNA
5’
lacA
RNA
polymerase
mRNA 5'
5’
mRNA
β-Galactosidase
Protein
Allolactose
(inducer)
lacY
Permease
Transacetylase
Inactive
repressor
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses
the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
Figure 18.22b
Effector Molecules
Since lactose is the molecule that determines when
the operon is turned on or off, it is known as an
effector molecule.
 effector molecule – any molecule that can regulate
the activity of a protein
 inducer – effector molecule that binds repressor
protein to cause it to fall off operator
Regulatory
gene
Promoter
Operator
DNA
mRNA 5’
Protein
lacl
lacZ
No
RNA
made
3’ RNA
polymerase
Active
repressor
(a) Lactose absent, repressor active, operon off.
lac operon
DNA
lacl
mRNA 5’
lacz
lacY
lacA
RNA
3’ polymerase
mRNA 5'
5’
mRNA
Protein
Allolactose
(inducer)
Inactive
repressor
(b) Lactose present, repressor inactive, operon on.
lac Operon Animation
lac Operon Animation
trp Operon
 trp – tryptophan
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.
trp Operon
Operon codes for 5 genes – found in E. coli
Five polypeptides combine to make three enzymes.
 each enzyme participates in a step to make tryptophan
Repressor Protein
repressor protein (trpR)
 transcribed as a different gene from trp operon
 binds to operator when tryptophan is present
 prevents RNA polymerase from transcribing genes
when bound to operator
When do you want the repressor to bind / not bind to
the trp operon?
When tryptophan needs to be made:
 enzymes are required to make tryptophan
 repressor is NOT bound to operator
 transcription can proceed
trp operon
Promoter
DNA
Promoter
Genes of operon
trpD trpC
trpE
trpR
trpB
trpA
Operator
Regulatory
gene
mRNA
5’
RNA
3’ polymerase
mRNA 5’
E
Protein
Inactive
repressor
D
C
B
A
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the
DNA at the promoter and transcribes the operon’s genes.
When cell has enough tryptophan:
 tryptophan binds to repressor
 repressor can now bind operator to prevent
transcription
DNA
No RNA made
mRNA
RNA
Polymerase
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off. As
tryptophan accumulates, it inhibits its own production by
activating the repressor protein.
Effector Molecule
Since tryptophan is the molecule that determines when
the operon is turned on or off, it is known as an
effector molecule.
corepressor – effector molecule that binds repressor
protein to cause it to bind to the operator
DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active,
operon off.
trp operon
Promoter
DNA
Promoter
Genes of operon
trpE trpD trpC
Operator
trpR
3’
mRNA 5’
Protein
5’
E
Inactive
repressor
trpB trpA
D
C
B
A
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on.
http://www.youtube.com/watch?v=8aAYtMa
3GFU
Classwork/Homework
 Section 5.5 Questions pg. 258 #1-6