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Multicellular organisms are made up from many different cell types that are specialised for their
All these specialised cell types originally came from stem cells.
Stem cells are unspecialised cells that can develop into other types of cells.

Stem cells that can mature into any type of body cell…

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Stem cells possess two key properties:

Stem cells are potent ­ they have the potential to differentiate into specialized cell types
Stem cells are immortal ­ they can divide indefinitely

Because of these two properties stem cells can be grown in the lab (in culture) and used for research

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Genes are expressed through transcription and translation into proteins, which give cells their
functions and properties.
But cells don't express all their genes all the time.
Gene expression can be switched on or off by other genes (e.g. during embryo development), by
stimuli (e.g. light, injury, nutrients), or by hormones…

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The effect of oestrogen on gene transcription

Hormones like oestrogen can switch on a gene and thus start transcription by combining with a
receptor on the transcriptional factor.
This releases the inhibitor molecule.

·Oestrogen is a lipid-soluble molecule and therefore diffuses easily through the
phospholipid portion of the cell surface…

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The Effect of SiRNA on gene expression

mRNA molecules have a fairly short lifetime ­ they are degraded by enzymes after they have been used
for translation.
The quicker the mRNA is broken down, the less protein can be made, and this is another point where
gene expression can be…


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