AQA A2 BIOLOGY UNIT 5: Control of Gene Expression

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Multicellular organisms are made up from many different cell types that are specialised for their
functions.
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 in an organism are called totipotent cells.
Totipotent stem cells in humans are only present in the early life of an embryo.
After this point the embryonic stem cells lose their ability to specialise into all types of cells.
The few stem cells that remain in mature animals are called multipotent stem cells.
They can only develop into a few types of cells.
Totipotent Stem Cells in Plants
Mature plants also have stem cells.
They are found in areas where the plant is growing.
All stem cells in plants are totipotent.
This means they can be used to grow plant organs or who new plants in vitro.
Growing plant tissue artificially is called tissue culture.
Tissue Culture (or micropropagation)
This is a more modern, and very efficient, way of cloning
plants.
Small samples of plant tissue, called an explant, can be grown on agar plates in the laboratory in
much the same way that bacteria can be grown.
Any plant tissue can be used for tissue culture, even differentiated, non-meristematic tissue (e.g. from
a leaf).
The plant tissue can be separated into individual cells, each of which can grow into a mass of
undifferentiated cells called a callus.
If the correct plant growth regulators are added these cells can develop into whole plantlets, which
can eventually be planted outside, where they will grow into normal-sized plants.
Conditions must be kept sterile to prevent infection by microbes.
With micropropagation thousands of clones of a particularly good plant can be made quickly and in a
small space.
Micropropagation is used on a large scale for fruit trees, ornamental plants and plantation
crops such as oil palm, date palm, sugar cane and banana, that cannot be asexually propagated by
other means.
Because whole plants can be grown from single cells, this technique could be combined with
genetic engineering to grow genetically modified plants from a single genetically-modified cell.
All these methods of reproduction using plant stem cells result in clones.
This is useful for exactly reproducing plants with desired characteristics, but the initial variation to
create those desired characteristics can only be produced using sexual reproduction.
Producing plants sexually from seeds is also cheaper, easier and very low tech.

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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
and medicine.
Where do they come from?
Embryo stem cells
are grown in vitro from human embryos called blastocysts.…read more

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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 (e.g. growth hormone, oestrogen).…read more

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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.…read more

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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 controlled.
Gene expression can be prevented by breaking down mRNA before its genetic code can be translated
into a polypeptide.…read more

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