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Author Topic: Electro physics of running gels  (Read 5570 times)

Offline MayoFlyFarmer

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Electro physics of running gels
« on: 22/09/2004 01:03:54 »
Ok, I have a question for any of you who are familiar with running common biological gels and are smarter physicists than me (I actually contemplated putting this in the physics section, but I believe this forum is WAY underused, so I guess I am biased).  

Why is it that when you run a DNA (typically agarose) gel you use a constant Voltage from your power source, while when running a protien gel (typically polyacrylamide) you use constant amps?  Both techniques rely on the same principle of atracting charged particles to an oppositly charged pole through a gel matrix to separate by size, so why the difference.  I guess the same question could be asked about RNA gels, gel to membrane protien transfers etc but I'll just stick with teh two that I know are different from eachother.  
I think I remember hearing a reason sometime in college, but i can't find it in my notes at all.  Chances are it was one of those days I was asleep.

Can anyone help me out?

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Offline neilep

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Re: Electro physics of running gels
« Reply #1 on: 22/09/2004 01:42:26 »
I can safely say and this will come as no surprise to you at all Justy, that I have not got a clue what you're talking about !!..but, as you have requested help I can only offer one bit of advice and that is next time you are learning something, stay awake.................Hey !!...this science lark is easy...I've got it sussed ;).......hope you get your answer matey.:)

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Offline chris

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Re: Electro physics of running gels
« Reply #2 on: 22/09/2004 17:42:40 »
Strictly speaking the constant current is the most accurate way to electrophorese something because the charged particles are migrating in a field and hence by keeping the current constant you ensure that the drift rate remains constant.

Sometimes you make staged gels in protein electrophoresis in which the first part of the gel is a different density to subsequent sections. This enables you to separate a much broader range of proteins (a bit like logarithmic graph paper). The problem I can foresee with this is that if the gel heats up (and its resistance changes) the resistance might change by a different amount for each of the different gel densities. This would affect the accuracy of the separation. But if you use constant current the machine will alter the potential difference appropriately to ensure that any change in resistance is compensated for. This is my guess....

Chris

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Offline MayoFlyFarmer

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Re: Electro physics of running gels
« Reply #3 on: 23/09/2004 00:36:05 »
Thanks chris,
you're right protien gels almost always (if not always) have at least two different layers with different gel densities; some of our really fancy ones actually run a smooth gradient throughout the gel from 4-20% poly-acrylimide.  Your answer makes perfect sense to me; so even though I'm not sure if its what i heard before, I'll take it :)

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Offline DrN

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Re: Electro physics of running gels
« Reply #4 on: 25/09/2004 16:36:45 »
that sounds about right. but about the resistance thing, gels will pretty much always heat up while they are running, so the resistance will always change. a lot of people run their gels - especially protein gels - in the cold room at 4 C, so minimising this. it also helps to run your gel as slowly as possible, which helps reduce the heat and also increases the resolution.
 

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Re: Electro physics of running gels
« Reply #4 on: 25/09/2004 16:36:45 »

 

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