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Junk DNA

G

GoodKat

New Member
arg-fallbackName="GoodKat"/>
What exactly switches genes "on" and "off", I've heard this referred to several times, but I've never seen an explanation.
 
arg-fallbackName="Aught3"/>
Transcription factors are the most likely to switch a gene on. I don't see how this relates to junk DNA though. Junk DNA is both the 'rubbish' that we don't need in our genomes (e.g., transposons) and the sequence which has no known function (yet).
 
arg-fallbackName="GoodKat"/>
Aught3 said:
Transcription factors are the most likely to switch a gene on. I don't see how this relates to junk DNA though. Junk DNA is both the 'rubbish' that we don't need in our genomes (e.g., transposons) and the sequence which has no known function (yet).
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Oh I thought it was DNA that was switched "off".
 
arg-fallbackName="Aught3"/>
Well transposons are quite cool, they're known colloquially as 'jumping genes'. They can copy themselves or cut themselves out of the DNA and will re-insert in a(n essentially) random location somewhere else in the genome - this can include in the middle of an actual gene. As you can imagine having a transposon sequence in the middle of a gene is not good news. It can screw up expression of that gene or the functionality of the protein product. Fortunately, transposons can be silenced or turned off by methylation (and other processes) so that they don't cause any harm to the host.
 
arg-fallbackName="GoodKat"/>
I guess what I'm asking is what aspect of the DNA itself determines which genes are active. Is it the order they are in? Some chemical bonded to them I don't know about?
 
arg-fallbackName="Aught3"/>
fig18_47.jpg


This is a pretty good picture.

What it means to express a gene (initially) is transcription. This is where the polymerase (red) makes mRNA from the DNA. In order for the polymerase to start transcribing the DNA, a whole host of other 'helper' proteins must bind first and form a complex. You can see in the picture activators binding to enhancers, the TATA protein (very important), transcription factors, and coactivators. All have to bind and arrange the DNA in a particular way so the polymerase can get to work making the mRNA copy. The default state of a gene is essentially off, but repressors can make it even harder to turn on.
 
arg-fallbackName="Ozymandyus"/>
Random throwing of transposons in there. =) But they are indeed interesting.

Anyway, what we call junk DNA is largely never transcribed into RNA, and never translated into proteins by default. Each gene is preceded by a promoter sequence that RNA polymerase binds to in order for transcription to begin. DNA that lies before such a promoter sequence is 'inactive' in that it will never produce proteins that will affect the cell.

In addition, there are proteins that bind to promoter sites to prevent transcription of particular genes(repressors), increase transcription of particular genes (activators or enhancers). Many of these proteins are always there, but are only activated when a cell is deficient in whatever protein the gene expresses (by being activated or inactivated by a coenzyme that is more present in such a condition). It's all very cool. DNA can be expressed very differently in different cells mostly by the operation of these enzymes.

Aw someone else posted something cooler with pictures...
 
arg-fallbackName="Spase"/>
When I saw this I had to link this article:

http://www.nytimes.com/2009/02/24/science/24chromatin.html?_r=1&scp=3&sq=epigenome&st=cse

It's about what's now being called the epigenome.. which is a pretty new concept. The idea being that entire stretches of DNA are turned on and off based on signaling which is passed to the proteins that are involved in packing the DNA.

I was writing something.. then... realized how much better this does than I would:

http://www.youtube.com/watch?v=h4aJzcoIhfQ

and.. I'm not sure if I like this second vid as much. More detail but .. I hate everything about it's audio.

http://www.youtube.com/watch?v=yQ-GvPJH8T4&feature=related
 
arg-fallbackName="Zylstra"/>
Spase said:
When I saw this I had to link this article:

http://www.nytimes.com/2009/02/24/science/24chromatin.html?_r=1&scp=3&sq=epigenome&st=cse

It's about what's now being called the epigenome.. which is a pretty new concept. The idea being that entire stretches of DNA are turned on and off based on signaling which is passed to the proteins that are involved in packing the DNA.
Click to expand...
soon as I saw the title, i wondered 'Is anyone familiar w/ epigenetics here'

:)
 
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