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Stem Cells
Some definitions…
Totipotent cells can mature into any type of cell.
Found in early embryos and plants.
Pluripotent cells can form all the cell types in
the body (embryonic stem cells).
Multipotent cells can form a number of
different cell types, e.g. adult stem cells/cord
blood stem cells.
Uses of stem cells
Medical research
Medical treatments
e.g. growth of neurones to treat spinal injuries
growth of organs for transplants
Reasons For/Against
Your turn!
Debate – Should the UK government
fund stem cell research?
In pairs:
- Read the statements on the cards and
discuss what that person might think.
iPS = induced pluripotent stem cells (scientists have a
method to turn normal adult cells back into stem cells).
What makes a cell change?
How do we get from stem cells to fully
differentiated, specialised cells?
Controlling development
All organisms begin life as a
single cell. This cell divides and
the new cells produced start to
differentiate and specialize.
‘Switching on’ the expression of
a gene or keeping it switched off
determines the development of
features.
Many organisms contain similar genes that control
development of body plans. For example groups of genes
called the homeobox genes play an important role in the
development of many multicellular organisms.
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© Boardworks Ltd 2009
Homeobox genes
The genome of the fruit fly contains one ‘set’ or cluster of
homeobox genes. These control development, including the
polarity of the embryo, polarity of each segment and the
identity of each segment.
Homeobox genes code for
transcriptional factors.
These regulate the expression
of other genes important in
development.
Mutations in homeobox genes can cause changes in the
body plan. For example a mutation in the gene controlling
leg placement can cause legs to grow where the antennae
are normally found.
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© Boardworks Ltd 2009
Homeobox genes
Homeobox genes are present in
the genomes of most organisms.
They control development of body
parts in similar ways.
There is little variation in many
regions of the homeobox genes
in different organisms. This
suggests that these have been
highly conserved throughout
evolutionary history. They are
thought to be especially
important to the basic
development of organisms.
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© Boardworks Ltd 2009
How is transcription initiated?
In eukaryotic cells, before transcription can begin a gene
needs to be stimulated by a regulatory protein, called
transcriptional factor.
Each transcriptional
factor contains sites that
can bind to a specific
region of the DNA.
They cannot initiate
transcription alone, but
form a pre-initiation
complex with RNA
polymerase.
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© Boardworks Ltd 2009
Function of transcriptional factors
Transcriptional factors function in different ways. Some
transcriptional factors recognize parts of the promoter
sequence at the start of a gene and bind to them. They can
either promote or block the functioning of RNA polymerase.
inhibitor molecule
The action of a transcriptional
factor can be switched off by an
inhibitor molecule. This can
bind to the transcriptional factor,
preventing it from attaching to
DNA. Without the transcriptional
factor the gene cannot be
transcribed.
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DNA
binding
site
transcriptional
factor
© Boardworks Ltd 2009
Oestrogen
Some hormones, e.g. oestrogen, have an effect on specific
cells due to their ability to influence transcriptional factors, and
therefore gene expression in the cell.
Oestrogen diffuses across the cell membrane. Once inside the
cytoplasm it combines with a site on a transcriptional factor.
The hormone changes the shape of the transcriptional factor
causing the inhibitor molecule to be released.
inhibitor molecule
DNA binding site
transcriptional factor
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transcription activated
oestrogen
© Boardworks Ltd 2009
Small interfering RNA (siRNA)
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© Boardworks Ltd 2009
Small interfering
RNA
Petunias
Chalcone
synthase
White
pigment
Purple pigment
Producing a deep purple
petunia
Chalcone
synthase
White
pigment
Insert gene
encoding
chalcone
synthase
Purple pigment
More mRNA
synthesised
More enzyme
produced and
more pigment
formed
Producing a deep purple
petunia
Genetically
engineered
plant
White plant
Deep purple
plant
Instead of deep
purple plants, many
of the plants
produced were
white
Making double-stranded RNA
mRNA is
single
stranded
RNAdependent
RNA
polymerase
A U C
C G
A G U
A C C C A G U
A U
Uses mRNA as a template to produce
a complementary RNA strand
U A G U
G C
C A U G G G U C A U
Two RNA strands held together by
hydrogen bonds
Doublestranded RNA
(dsRNA)
A
What happens to double-stranded
RNA?
Doublestranded RNA
is cut by Dicer
enzyme
Small interfering RNA (siRNA)
• Usually 21 base pairs long
• Two base overhang at each end
Stopping protein synthesis
We will start by
simplifying the
diagram of the
siRNA molecule
Stopping protein synthesis
We will start by
simplifying the
diagram of the
siRNA molecule
Stopping protein synthesis
siRNA forms a
complex (RISC) with
protein
One of the siRNA
strands is destroyed
The siRNA–protein
complex binds to
mRNA
Stopping protein synthesis
The mRNA is cut by
the siRNA–protein
complex
The mRNA is then
broken down. This
prevents further
protein synthesis
So why were white
plants produced
instead of
deep purple plants?
Genetically
engineered
plant
White plant
Deep purple
plant
Use the information
about making doublestranded RNA and small
interfering RNA to
explain why.
The genetically engineered petunia plants had a
higher concentration of mRNA
This resulted in RNA-dependent RNA polymerase
producing double-stranded RNA from this mRNA
More siRNA molecules were formed that would
bind to the mRNA coding for chalcone synthase
Less chalcone synthase was produced so flowers
were white, not deep purple