Transcript File - Mrs Jones A

Protein synthesis
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Gene
DNA
Triplet code
Degenerate
Amino acid
Polypeptide
Transcription
mRNA
Template strand
Hydrogen bonds
Complementary
Phosphoryl groups (energy)
Coding strand
Nuclear pore
Ribosome
O Translation
O Codon
O Anticodon
O tRNA
O AUG=start mRNA
O Amino acid
O Peptide bond
O Stop codon:
UAA,UAG,UGA
How is protein synthesis
controlled?
O So you should already know the mechanism of
protein synthesis……
O How is the process controlled??
O Learning outcome:
O Explain genetic control of protein production in
a prokaryote using the lac operon
O Pages 112-113
Genetic Control of Enzyme
Production in Prokaryotes
O Genes control protein synthesis. Some proteins
are required all the time by cells whilst others
are only needed in specific circumstances and
so are not made all the time.
O Such genes must be capable of being switched
on or off – it would be wasteful if they were
made and not required! Switching on is known
as induction, whilst switching off is known as
repression.
Gene control in Prokaryotes
O Research was carried out using bacteria, when it was
noticed that bacteria could vary the synthesis of enzymes
in response to environmental change such as food
O This operon mechanism was proposed by Jacob and
Monod to account for the regulation of gene activity in
response to the needs of a cell.
O (Genes are not found in operons in Eukaryotes, so does
not apply)
O Operons consist of a group of closely linked genes that
act together and code for enzymes that control a
particular metabolic pathway. An operon consists of at least
one structural gene coding for the primary structure of an
enzyme and 2 regulatory elements, the operator and
promotor
AS revision……… first!
Example of the control of gene
expression: switching genes on!
O A bacterium called Escherichia coli has the
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gene to make the enzyme β-galactocidase
Breaks down lactose to glucose and galactose
Only produced when lactose is present!
In it’s normal environment lactose is not
present to be used as a food source
So, since enzymes are ‘expensive’ to make
(need energy, amino acids etc)
the enzyme is not required if no lactose is
present
SO: When there is NO LACTOSE
promoter
In normal circumstances when lactose is not
present
a repressor protein binds to the DNA close to the
promoter region (where RNA polymerase has to
bind to start transcribing the DNA strand)
so RNA polymerase cannot bind so the gene is not
transcribed into mRNA so no enzyme produced
When there is LACTOSE PRESENT..........
mRNA bound to promoter
•If lactose is present it gets into the cell and binds with
the repressor protein
•this now cannot bind to the DNA
•so RNA polymerase can bind to the promoter
•so the gene is transcribed into mRNA
•and the enzyme is produced to break down the lactose
as a food source
i.e. lactose is the signal to switch on the gene
Lac system genes form an operon
O An operon: is a section of DNA, made up of structural
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genes and control sites
The structural genes code for proteins, such as enzymes
The control sites are the operator region and a promotor
region.
The operator and promotor are both genes, as they are
lengths of DNA but they do not code for polypeptides
The operator region: Non-coding sequence of DNA that is
the binding site for the repressor molecule
A promoter region where the RNA polymerase attaches to,
so allowing transcription
There is also regulator gene (not actually part of the
operon) on another part of DNA molecule, that codes for a
repressor protein. Behaviour of the repressor protein is
determined by whether the gene is induced or repressed.
Whiteboards
1. Draw and label an operon
2. Define: an operon, structural gene, operator,
promotor, regulatory gene.
O Exam question:
O All human babies produce the enzyme lactase. The
genetic change that allows adults to produce this
enzyme is thought to involve a mutation in a
regulatory gene. This mutation causes the structural
gene to be expressed in adults.
O Distinguish between the terms ‘regulatory gene’ and
‘structural gene’ 2 marks
2 marks
O Regulatory:
O Idea that makes regulatory/repressor protein
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O Idea that it switches another/structural gene on/off
O Structural:
O Idea that it makes enzyme/polypeptide/protein
O Relationship between the 2:
O Idea that the regulatory gene controls/affects the
expression of the structural gene
Your specification requires only that you
know about the lac operon of Escherichia
coli.(E-coli)
It is however possible that you may be
provided with detail of another system and
asked to compare it with the lac operon.
E-coli lac operon and its
regulator gene
The lac operon is a section of DNA within the
bacterium's DNA
Structural genes: Z=enzyme beta-galactosidase
Y= lactose permease
Operator region=O, next to structural genes, can
switch genes on/off
The promotor region=P, length of DNA to which the
enzyme RNA polymerase binds to begin the
transcription of Z and Y structural genes
Regulatory gene is not part of the operon
SO: When there is NO LACTOSE
Regulatory gene is expressed
A repressor protein synthesised, with 2 binding sites
(lactose+operator)
a) Binds to operator region
b) Covers the promotor region where RNA polymerase
normally attaches
So structural genes cannot be transcribed into mRNA
So no enzymes produced
Whiteboards!
When there is LACTOSE PRESENT..........
Lactose added
Lactose inducer molecules attach to other site on repressor protein
Repressor protein changes shape so its other binding site cannot bind to
operator region Inducer-repressor complex
The repressor breaks away from operator region
So RNA polymerase can bind to unblocked promotor region and initiate
transcription of genes for Z and Y
So E coli can use lactose permease to take up lactose from the cells
The enzyme beta galactosidase can convert lactose to glucose and
galactose…respiration!!!
O Explain the advantage of having an inducible
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enzyme system that is regulated by the presence
of a substrate 2 marks
Extension: Suggest how gene control in a noninducible system may be achieved? 2 marks
In an inducible system the enzymes required for the
metabolism of a particular substrate are only
produced when the substrate is present.(1)
This saves the cell valuable energy in not producing
enzymes that have no immediate use.(1)
Gene repression in Prokaryotes, structural genes
are transcribed all the time,(1) and when the end
product is present in excess to requirements the
gene is switched off.(1)
Watch
O http://www.sumanasinc.com/webcontent/anim
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ations/content/lacoperon.html
Summarise the structure of a Lac system in E
coli
How can this system control protein
production in a Prokaryote!
Use diagrams to explain to each other
O PLENARY:
Exam question: ‘The control of the
expression.’ (3 marks)
http://www.sumanasinc.com/webcontent/anim
ations/content/lacoperon.html Whiteboards
quiz!
Learning outcome
O State that cyclic AMP activates proteins
by altering their 3 dimensional structure
O Importance of referring to the spec!!
O NB How do you make your notes??
O You probably covered this in
Homeostasis/ endocrine system??
O We have looked at the genetic
involvement in development..other
molecules involved..
O As we go through jot key words onto
your whiteboard
Enzymes activating proteins
O Enzymes are responsible for the metabolism
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of cells.
Some function continuously; some are
present all the time, but are ‘idle’, only working
when given the appropriate signal;
Some are only needed if particular substrates
are present
Some may be needed for particular aspects of
development. Clearly their action is regulated.
Such regulation can be via chemical signals
such as hormones or under genetic
control.
Protein activation by cyclic AMP
O Cyclic adenosine monophosphate (cAMP)
is a second messenger which targets
specific intracellular enzymes to induce a
cellular response.
O Its actions allows hormones that cannot
pass across the plasma membrane to still
bring about an effect inside the target cell.
O cAMP acts via other enzymes to initiate a
response
O What do you think cAMP is a derivative
of?
Remember? Mechanism of action of
peptide hormones:
peptide hormone: first messenger
receptor
cell surface membrane
Inactive secondary
messenger
Secondary messenger
activated: activates
enzymes or
transcription factors
Can act as a
transcription factor
which switches on
gene for a specific
protein
O Cyclic AMP is a second messenger, allowing for transmission
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of messages across the cell membrane.
Hormones such as insulin and glucagon cannot pass
through cell membranes.
Instead, there are receptors on the outer surface of the cell to
which the hormone binds.
This activates a system that produces a second messenger –
in the case of glucagon, adenylate cyclase brings about the
production of cyclic AMP from ATP.
The cyclic AMP then activates intracellular enzymes like
protein kinase A (PKA) by altering their three dimensional
structure.
The activated enzyme then catalyses the required reaction in
muscle cells which then switches on glycogen
phosphorylase and switches off glycogen synthase.
In short, the required enzyme action is switched on.
Protein kinases
O Are an important family of target enzymes for cAMP.
O Kinase activation modifies the function of other proteins
by adding phosphate groups to them
O About 30% of human proteins can be modified by
protein kinases.
O Protein kinase A is involved in controlling the transcription
of many genes.
O It has an important role in regulating glycogen
metabolism
O It adds a phosphate group to enzyme phosphorylase
kinase to activate it, this then phosphorylates and activates
glycogen phosphorylase which catalyses the breakdown
of glycogen to glucose 1 phosphate.
Review: Learning outcome
O State that cyclic AMP activates proteins
by altering their 3 dimensional structure
O WATCH??
https://www.youtube.com/watch?v=Y2er_Df
gg44
https://www.youtube.com/watch?v=NaOBRv
AFiJQ
Make a simple flow diagram to explain
the LO
O State that cyclic AMP activates proteins by altering
their 3 dimensional structure
O Glucagon binds to receptor on cell surface membrane
O Stimulates production of adenylate cyclase
O Stimulates production of second messenger cAMP
O This activates intracellular enzymes: protein kinase A
by altering 3D structure
O It adds a phosphate group to enzyme phosphorylase
kinase to activate it
O This then phosphorylates and activates glycogen
phosphorylase
O Which catalyses the breakdown of glycogen to
glucose 1 phosphate.