Sunday, 6 December 2009

Biochemistry Exams - 2008/2009 3rd year Undenominated Science Semester I - Question 3

Whatsuuuup ya all??

Right here right now question 3:)

To see sample exam paper go to: 2008/2009 3rd year Undenominated Science Exam Paper Semester I

Question 3
Gene expression can be regulated by activating transcription factor activity.
Explain with reference to hormone-inducible gene expression in eukaryotic cells.

First short background:
transcription factor - is a protein that bind to specific DNA sequences in order to control production of mRNA from DNA. Transcription factors act either alone or with other proteins to promote (as an activator), or block (as a repressor) recruitment of RNA polymerase (enzyme which produce mRNA out of DNA) to specific genes.

Picture taken from

Transcription factors enhance or silence gene transcription in response to different stimuli. On the figure above you can see a transcription factor (blue) bound to DNA (red). Notice that DNA bends upon transcription factor binding. This conformational change of DNA interfere with RNA polymerase binding, what result in either shutting down or enhancment of gene transcription.
Remember that there are transcription factors for different sets of genes.
For example hormones can induce gene transcription through binding to hormone-inducible transcription factors. Before we go further please have a look at this animation McGraw Hill - Mechanism of steroid hormone action.

As the animation explain, when hormone molecules enters the cell they bind to the transcription factors. In the nucleus hormone-transcription factor complex binds to specific DNA sites. That interaction is responsible for inducing or shutting down gene transcription. Without the hormone molecule, transcription factor does not bind to DNA and because of that it does not induce gene transcription.
Similar events occure when other types of transcription factors are activiated but mechanism might be different.

Be aware that activation of transcripation factors is not the only way of regulating gene transcription. For more details please see those animations:
McGraw Hill - Control of gene expression in eukaryotes
McGraw Hill - Transcription complex and enhencers

To sum up, what you have to remember is:
- transcription factors - bind to DNA and enhances or silences gene expression,
- transcription factors are activated by different stimuli and either activate or silence gene transcription,
- gene expression might be as well regulated in different manner (mRNA modification and translation, etc),

This is it. Question 4 soon:)


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