Regulation of Transcription and Translation
The general principles involved in preventing expression of
a gene by preventing transcription
1)
For transcription to begin the gene needs to be
developed by a specific molecule that moves from the cytoplasm to nucleus.
2)
Each transcription factor has a site that binds
to specific region of the DNA in the nucleus.
3)
When it binds, it stimulates the region of the
DNA to begin the process if transcription.
4)
Messenger RNA is produced and the genetic code
it carries is translated into a polypeptide.
5)
When a gene is not being expressed (switched
off) the site on the transcription factor that binds to DAN is blocked by an
inhibitor molecule
6)
This inhibitor molecule prevents the
transcriptional factor binding to DNA and so prevents transcription and protein
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.
1)
Oestrogen is a lipid soluble molecule and
therefore easily diffuses through the phospholipid portion of the cell surface
membrane
2)
Once inside the cytoplasm of a cell oestrogen
combines with a site on the receptor molecule of the transcription factor.
The shape of this site and the shape of the oestrogen molecule complement
one another.
3)
By combing with the site, 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.
4)
The transcriptional factor can now enter the
nucleus through a nuclear pore and combine with DNA
5)
The combination of the transcription 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 the process of preventing gene
expression is small double stranded sections of RNA called small interfering
RNA or siRNA
1)
An enzyme cuts down large double-standard molecules
of RNA into smaller sections called small interfering RNA – siRNA
2)
One of the two siRNA strands combine with an
enzyme
3)
The siRNA molecule guides the enzymes to a
messenger RNA molecule by pairing up its base with the complementary ones on a
section of the mRNA molecule.
4)
Once on position the enzymes cuts the mRNA into
smaller sections
5)
The mRNA is no longer capable of being
translated onto a polypeptide.
6)
This means that the gene has not been expressed
or in other words it has been blocked.
siRNA has a number of potential scientific and medical uses .
1)
It could be used to identify the role of genes
in a biological pathway.
Some siRNA that blocks a particular gene could be added to cells
By observing the effects or rather the lack of them we could determine
what the role of the blocked gene is.
2)
As some diseases are genetic (caused by genes )
it may be possible to use siRNA to block these genes and thus prevent the
disease.
