Question 39

[yfangl097543]

If you have colored fluorescent derivatives attached to the terminators, why would you have to label the terminators themselves? Wouldn't the fluorescence be enough for visualization on a gel?

[mcat_premed3832]

These are actually 2 different ways of labelling which can be seen using 2 different techniques (like a dollar bill that has some green ink that you can see with the naked eye and some flourescent ink that you can only see under flourescent light). Thus there is the radiolabel and the flourescent label which will both be helpful in being specific in detection in the gel.

[jgeng03]

I choose C because the method is faster and uses less supplies therefore doesn't that mean its more economical. I wanted to choose D but even with the dye don't we still need electrophoresis to separate the 2 strands before they go on the gel.

[admin]

[sharra.26428]

I am sorry but I am confused as to why you would need radioactive labeling if you are adding flourescent terminators. I can see your point about the simultaneous reactions but I am still not understanding why you would still need radioactivity for visualization. The way the passage presents this it seems to me it would be reasonable to pick B.
thanks

[tnesbit4508229]

You don't need radioactive labeling if you've got fluorescent labeling. However, it was asking what the biggest [i]benefit[/i] would be, and the biggest benefit would be not having to create 4 different reaction mixtures.

[enachbe18198]

Like the older posts say, I don't understand why you need radioactive labeling as well as fluorescent..
However, I didn't pick this choice because I didn't see how fluorescently treating something could be an advantage over radioactively treating it

aside from that, I think the wording of the correct answer choice is a little off..

it says you can do all 4 reactions simultaneously... but you can also do them simultaneously with non-fluorescently labeled ddNTPS... just in separate containers

I picked that the technique would be faster just because all 4 reactions can now be rolled into one.. allowing for faster reading for example because you can just focus on one gel lane, etc. It's a stretch but I didn't see how any of the other answers could be correct

[danielmlan8880]

[bribercar2809]

Why do you say

"Answer choice B. is incorrect since radioactive treatment is necessary in order to read the products of electrophoresis on the radiogram (P3)."

This is not true!

Page 138 Biochemistry, Berg

"Fluorescence detection is attractive because it eliminates the use of radioactive reagents and can be readily automated'.

This also is nice so you don't work around radioactivity,which is good for health and safety.

As far as D, "all four reactions could be carried out simultaneously prior to electrophoresis"

This could also be done with auto-radiography in the old method, this is not an advantage.

In this method you get to separate the products of all 4 dideoxy chain termination reactions, by electrophoresis through the gel or capillary tube, and direct transfer to a computer.

The only answer which makes any sense is B.

[mcat_premed3832]

[bribercar2809]

I suggest you read a little about this technique, because, you do not use radioactivity at all, in this method, so you obviously are wrong. No radiogram, is involved with this at all, so I am not sure where you get this from? And you cannot detect fluorescence, without a radiogram? I am not sure you even have a clue what fluorescence is? It has nothing to do with radioactivity.

[mcat_premed3832]

You misunderstood what I wrote, 100%.

[jjellybean2461]

I think it's clear that answers B,C, and D are all correct.

B) The use of fluorescent markers eliminates the need for radioactive markers. It is irrelevant that an autoradiogram cannot detect fluorescence, because an autoradiogram would no longer be used.

D) No one can argue that it is easier to dump everything in one vial than it is to separate it into four vials, so D is clearly right.

C) depends on which stage(s) you are considering. If you have to make all the fluorescence-labeled terminators yourself, then this method is tedious and expensive. If, however, you're a lab tech that gets a great deal on the 4 labeled terminators, then score! This experiment just got super fast and economical.

Now there's one more curveball: the question asks which is the "main advantage." So the answer is based on your opinion of which advantage is most valuable. For me, eliminating radioactive materials from my lab would be top on the list. However, for the writer of the exam, it was more important to dump everything in one vial. Clearly this is a matter of opinion - hopefully the real MCAT administrators will recognize that a question like this is unfair and throw it out, but if they don't, let's just accept that it's equally unfair to everyone, and stop insulting the moderators that are trying to help us.