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Cell Specialisation and Totipotency
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Notes covering Totipotency and cell specialisation,
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Chapter 15.1 Totipotency and cell specialisationAll cells contain the same genes. Every cell is capable of making everything that the body can
produce. So why do the cells produce the substances they do rather than anything else? The answer
is that, although all cells contain all genes, only certain genes are expressed in any one cell at any
time.
Some genes are permanently switched on (expressed) in all cells. For example, the genes that code
for enzymes used in respiration are expressed in all cells. Other genes are permanently not
expressed (switched off) for example the gene for insulin in the small intestine. Further genes are
switched on and off as and when they are needed.
Differentiated cells differ from each other, often visibly so. This is mainly because they each produce
different proteins. The proteins that a cell produces are coded for by the genes it possesses, or more
accurately by the genes that are expressed (switched on).
An organism develops from a single fertilised egg. A fertilised egg clearly has the ability to give rise to
all types of cell. Cells such as fertilised eggs, which can mature into any body cell are known as
totipotent cells. The early cells that are derived from the fertilised egg are also totipotent. They later
differentiate and become specialised for a particular function. For example, mesophyll cells become
specialised for photosynthesis and muscle cells become specialised for contraction. This is because,
during the process of cell specialisation, only some of the genes are expressed.
This means that only part of the DNA of a cell is translated into proteins. The cell therefore only
makes the proteins that it requires to carry out its specialised function. Although it is still capable of
making all other proteins, these are not needed and so it would be wasteful to make them.
Therefore, the genes for these other proteins are not expressed. The ways in which genes are
prevented from expressing themselves include:
Preventing transcription and hence preventing the production of mRNA
Breaking down mRNA before its genetic code can be translated
If specialised cells still retain all the genes of an organism, can they still develop into any other cell?
It depends – there are no hard and fast rules. Xylem vessels, which transport water in plants, and red
blood cells which carry oxygen in animals are so specialised that they lose their nuclei once mature.
As the nucleus contains the genes, then clearly these cells cannot develop into other cells. In fact,
specialisation is irreversible in most animal cells. Once cells have matured and specialised they can
no longer develop into other cells, in other words, they lose their totipotency. Only a few totipotent
cells exist in mature animals, they are called adult stem cells.
Stem cells are undifferentiated dividing cells that occur in adult animal tissues and need to be
constantly replaced. They are found in the inner lining of the small intestine, in the skin and also in
bone marrow, which produces both red and white blood cells. Under certain conditions, stem cells
can develop into any other type of cell. As a result they can be used to treat a variety of genetic
disorders, such as the blood diseases thalassaemia and sickle cell anaemia. In addition to the adult
stem cells found in mature organisms, stem cells also occur at the earliest stage of the development
of an embryo, before the cells have differentiated. These are called embryonic stem cells.
The situation in plants is different. Mature plants have many totipotent cells. Under the right
conditions, many plant cells can develop into any other cell. For example, if a cell is taken from the
root of a carrot and placed in a suitable nutrient medium and give it a certain chemical stimuli at the
right time, it can develop into an entire carrot plant. Growing cells outside of a living organism in this
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