biology unit 5 (biol5) gene expression
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Unit 5.7
Gene expression is controlled at DNA level during transcription
Know totipotent cells are body cells that can mature into any body cell.
Understand during development some genes are switched off/only some of them are translated resulting in cell specialisation.
Appreciate once this has happened it cannot be reversed.
Understand how in mature plants many cells remain totipotent and be able to interpret data relating to tissue culture of plants from totipotent cells.
Understand transcription only occurs when certain substances move from the cytoplasm into the nucleus.
Appreciate the effect oestrogen has on transcription. Small interfering RNA (SiRNA) is short double stranded RNA that interferes with the expression of a specific gene.
Understand in an animal a few cells remain totipotent (stem cells) and these can be used to treat genetic diseases, as can embryonic stem cells.
Evaluate the use of stem cells in treating human disorders
Interpret data relating to oncogenes/tumour suppressor genes in prevention, treatment and cure of cancer.
Evaluate the effect on diagnosis and treatment of disorders caused by hereditary mutations and acquired mutations
Key words:
Totipotent, in vitro, somatic cell, germ cell, tissue culture, promoter region, transcription
factor, siRNA.
Totipotency and cell specialisation
Totipotent cells can specialise into anything.
But during specialisation/differentiation only some genes are expressed – this means
that only part of the DNA of a cell is translated into proteins. The cell therefore only
makes proteins that it requires to carry out its specialised function. Although it is still
capable of making all the other proteins, but they are not needed so this would be
wasteful.
Therefore the genes for these other proteins are not expressed. The ways in which
genes are prevented from expressing themselves include:
o Preventing transcription and hence preventing the production of mRNA
o Breaking down mRNA before its genetic code can be translated.
Regulation of transcription and translation
The effect of oestrogen on gene transcription
o For transcription to begin the gene needs to be stimulated by specific
molecules that move from the cytoplasm into the nucleus. These molecules
are called transcriptional factors.
o Each transcriptional factor has a site that binds to a specific region of the
DNA in the nucleus.
o When it binds, it stimulates this region of DNA to begin the process of
transcription.
o Messenger RNA (mRNA) is produced and the genetic code it carries is then
translated into a polypeptide.
o When a gene is not being expressed, the site on the transcriptional factor that
binds to DNA is blocked by an inhibitor molecule.
o This inhibitor molecule prevents the transcriptional factor binding to DNA and
so prevents transcription and polypeptide synthesis.
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. The process is
illustrated in the diagram and
operates as follows;
Oestrogen is a lipid-soluble
molecule and therefore diffuses
easily through the phospholipid
portion of cell-surface membranes
Once inside the cytoplasm of
a cell, oestrogen changes the shape
of the receptor molecule of the
transcriptional factor. The shape of
this site and the shape of the
oestrogen molecule complement one
another.
By combining with the site,
the oestrogen changes the shape of
the receptor molecule. This change
of shape releases the inhibitor
molecule from the DNA binding site
on the transcriptional factor
The transcriptional factor can
now enter the nucleus through a
nuclear pore and combine with DNA.
The combination of the
transcriptional factor with DNA
stimulates transcription of the gene
that makes up the portion of DNA.
The effect of siRNA on gene expression
Gene expression can be prevented by breaking down messenger RNA before its
genetic code can be translated into a polypeptide. Essential to this process are small
double-stranded sections of RNA called small interfering RNA (siRNA). The process
operates as follows;
o An enzyme cuts large double-stranded molecules of RNA into smaller
sections called small interfering RNA (siRNA);
o One of the two siRNA strands combines with an enzyme;
o The siRNA molecule guides the enzyme to a messenger RNA molecule by
pairing up its bases with complementary ones on a section of the mRNA
molecule;
o Once in position the enzyme cuts the mRNA into smaller sections;
o The mRNA is no longer capable of being translated into a polypeptide
o This means that the gene has not been expressed, that is, it has been
blocked;
The siRNA has a number of potential scientific and medical uses
o It could be used to identify the role of genes in biological pathway. Some
siRNA that blocks a particular gene could be added to cells. By observing the
effects (or lack of them) we could determine what the role of the blocked gene
is.
o As some diseases are caused by genes, it may be possible to use siRNA to
block these genes and so prevent the disease.