stem cells. some definitions… totipotent cells can mature into any type of cell. found in early...
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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 researchMedical 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?
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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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Homeobox genes
Homeobox genes code for transcriptional factors. These regulate the expression of other genes important in development.
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.
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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Homeobox genes
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.
Homeobox genes are present in the genomes of most organisms. They control development of body parts in similar ways.
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How is transcription initiated?
In eukaryotic cells, before transcription can begin a gene needs to be stimulated by a regulatory protein, called transcriptional factor.
They cannot initiate transcription alone, but form a pre-initiation complex with RNA polymerase.
Each transcriptional factor contains sites that can bind to a specific region of the DNA.
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Function of transcriptional factors
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.
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
transcriptional factor
DNA binding
site
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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.
oestrogen
inhibitor molecule
transcriptional factor
DNA binding site
transcription activated
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Small interfering RNA (siRNA)
Small interfering RNA
Petunias
White pigment
Purple pigment
Chalcone synthase
Producing a deep purple petunia
White pigment
Purple pigment
Chalcone synthase
Insert gene encoding chalcone synthase
More mRNA synthesised
More enzyme produced and more pigment
formed
Producing a deep purple petunia
White plant Deep purple plant
Instead of deep purple plants, many
of the plants produced were
white
?
Genetically engineered
plant
Making double-stranded RNA
A U C A G U A C C C A G U A U C G
mRNA is single
strandedRNA-
dependent RNA
polymerase U A G U C A U G G G U C A U A G C
Uses mRNA as a template to produce a complementary RNA strand
Two RNA strands held together by hydrogen bonds
Double-stranded RNA
(dsRNA)
What happens to double-stranded RNA?
Small interfering RNA (siRNA)• Usually 21 base pairs long• Two base overhang at each end
Double-stranded RNA is cut by Dicer
enzyme
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 synthesissiRNA forms a
complex (RISC) with protein
One of the siRNA strands is destroyed
The siRNA–protein complex binds to
mRNA
Stopping protein synthesisThe mRNA is cut by the siRNA–protein
complex
The mRNA is then broken down. This
prevents further protein synthesis
White plant Deep purple plant
So why were white plants produced
instead of deep purple plants?
?
Genetically engineered
plant
Use the information about making double-
stranded 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