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Control, Genomes and
Environment
Cellular Control – The lac operon
Starter
• Cystic fibrosis (CF) in humans is caused by
mutations of a gene coding for transmembrane
protein (CFTR) which acts as an ion pump. A large
number of different mutations of the gene have been
found. Explain what is meant by a gene mutation. (2
marks)
• a change in the genetic material; unpredictable / AW;
extra detail; e.g. addition / substitution / deletion /
frame shift / small part of chromosome / may code for
different protein /may code for no protein
Learning Outcomes
• Explain genetic control of protein production in
a prokaryote using the lac operon
Control of protein synthesis and
activation
• Protein synthesis and activation is tightly
controlled - ensuring body development, cellular
processes and reactions all take place at
appropriate times and rates.
• In eukaryotes:
• Protein synthesis can be controlled at the
transcriptional and translational levels.
• Genes that aren’t transcribed are ‘switched off’
• Genes that are transcribed, but not translated are
‘silenced’
Gene regulation in Prokaryotes
• Lactose: can be an excellent meal for E.
coli bacteria. However, they'll only gobble up
lactose when other, better sugars – like glucose –
are unavailable.
• With that for context, what exactly is
the lac operon? The lac operon is an operon, or
group of genes with a single promoter
(transcribed as a single mRNA). The genes in the
operon encode proteins that allow the bacteria to
use lactose as an energy source.
What is the lac operon?
• The lac operon of E. coli contains genes involved in
lactose metabolism. It's expressed only when
lactose is present and glucose is absent.
• Two regulators turn the operon "on" and "off" in
response to lactose and glucose levels:
the lac repressor and catabolite activator protein
(CAP).
• The lac repressor acts as a lactose sensor. It
normally blocks transcription of the operon, but
stops acting as a repressor when lactose is present.
Genetic control in prokaryotes
• Differential gene expression due to environment,
eg. nutrient availability.
• Involves ‘operons’ - a section of DNA containing
several genes:
– Eg Structural genes (protein products), control
elements (P,O)
– regulatory gene is not strictly part of the operon, but
its product has an important role.
Regulatory gene
Control sites
Structural genes
A
Complete the table
Part of operon
Letter
I
P
O
Z
Function
name
Regulatory
Gene
Structural
gene
Product/detail
of function
Repressor
protein
bgalactosidase
Y
Lactose
• Bacteria produce enzymes to metabolise certain
nutrients only when they are present in their
environment.
• E.coli normally respires glucose, but can also use
lactose as its source of carbon.
• The essential enzyme to allow the metabolism of
lactose is b-galactosidase.
• If lactose is not present, there will be very few bgalactosidase molecules in the cell.
• The presence of lactose in the environment causes a
large increase in the production of b-galactosidase
Lactose not present
A
I- Regulator gene – produces a repressor protein
which binds to operator (O)
P- Promoter region – binding site for RNA polymerase.
Repressor at O prevents it binding.
Z is b-galactosidase, Y is lactase permease. A is
transacetylase
Z, Y, A are not transcribed when R is at O
Lactose present
A
I- Regulator gene – produces a repressor protein which
is prevented from binding to O by lactose binding and
changing the shape of the repressor.
P- Promoter region – binding site for RNA polymerase.
Z, Y, A are transcribed when RNA polymerase binds at P
Plenary
Component
Function
A. structural genes
1. produces repressor protein
B. regulator gene
2. binds to repressor
C. promoter
3. codes for lac enzymes
D. operator
4. binds to RNA polymerase
A–3
B–1
C–4
D-2
Activators
• Proteins which activate genes.
• Activators and repressors are transcription
factors.
• Eg: cAMP can bind to protein kinase A and cause
the protein to change shape, thus activating the
protein.
cAMP activates PKA
which phosphorylates
other proteins and
enzymes
Plenary
• The bacterium Escherichia coli (E. coli) uses
glucose as a respiratory substrate. In the
absence of glucose, E. coli can use lactose. The
use of a different substrate is determined by the
interaction between genes and the environment.
Describe how this happens in this example.
• (5 marks)
Answer
• 1. ref to operon;
2 normally repressor substance bound to
operator;
3 prevents RNA polymerase binding (at
promoter) / prevents transcription;
4 lactose binds to repressor;
5 changes shape of protein molecule;
6 unable to bind (to operator);
7 RNA polymerase binds (at promoter) /
transcription occurs / genes switched on;
8 AVP; e.g. production of lactose permease /
production of beta-galactosidase;
Homework
• Gene control in prokaryotes sheet
• Q p103 green book
• Q4 p149 purple book
Learning Outcomes
• Explain genetic control of protein production in a
prokaryote using the lac operon