namf
Forum Elite
Topics: 80
Posts: 312
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Hi!
Just read a Qbank q with electropherisis strip having several bands from top to bottom like this and asking which is likely tRNA:
A
B
C
D
E
F
Anything wrong with my reasoning (which follows)?
I remembered FA's mnemonic "tRNA=tiny", so I wanted to look for the band that had electrophoresed the farthest--the smaller, the easier for it to move across the strip on electrophoresis I reasoned. I chose A, thinking it had gone from top to bottom, as shown above. The Qbank ans was actually F, and they said that it was based on the molec that had gone furthest.
So how can you know which way the chemicals are "phoresing"?? Is there a convention to look from the top down--top being the starting point? Maybe if someone could explain the mechanism--I did adsorption [sic] in orgo lab, and remember the travelling was against gravity, bottom to top. Is electrophoresis done on a horizontal surface, or against gravity anyway??
Your help is appreciated!
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| Ahab
Forum Elite
Topics: 9
Posts: 228
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I remember that question also. Got caught out by the fact that I assumed that the top was furthest away and the bottom was closest, that will teach me to read the question 
As far as electrophoresis goes it is carried out on the flat, picture standing on the touch line of a football field and you have several balls of different weight (=size) Apply the same strength throwing each ball and the heavier (larger) ones will fall closer to you while the lighter (smaller) one will go father from the line. tRNA being smallest will go the farthest.
As for locating the origin of electrophoresis on the question, go back to the question, its marked  thats what I missed also when I got that question wrong
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| namf
Forum Elite
Topics: 80
Posts: 312
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Thanks dude! "origin"...says it all, huh!? :lol: :lol: :lol:
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| black_tulip
Forum Senior
Topics: 14
Posts: 95
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I have run a lot of DNA or RNA electropherisis gels. Usually the molecules run from top to bottom, so it is an unofficial rule that every time the gel should be presented with the highest moclecular weight on top. you should also read the gel and consider that the toppest one has the highest molecular weight.
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| kalsam
Forum Elite
Topics: 31
Posts: 361
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Yeah..even I saw that question..but some how I guessed it..but the real facts behind that r very interesting..Thanks everybody!!
It seems our forum gives good explanation than Kaplan itself!!! 
___________________
"Read Repeat Recall Remember Recognise & Reproduce"
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| sudipto1973
Forum Newbie
Topics: 4
Posts: 31
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Hi guys,
I run gels almost everyday..so just a word of caution..gels can be run either vertically or horizontally...so you have to be careful which way the gel was run...usually you can make it out by seeing the picture given or better if they have mentioned the molecular standards or weights...for vertical gel electrophoresis the top portion of the gel is where you load your samples to be run and so farther your sample runs to the bottom of the gel the lighter it is.
Another word of caution: Samples are run on the gel not just on the basis of molecular weights but also on the basis of charge they possess.
Hope this helps
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| namf
Forum Elite
Topics: 80
Posts: 312
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Thanks a lot everyone--I hope this question shows on the USMLE, because I feel ready for it now!!
:icon_king:
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