theonlytycrane

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Screen Shot 2016-07-30 at 6.58.56 PM.png

Doesn't chargaff's second parity rule support (A)? If not, I guess A and D are both possible.
 

laczlacylaci

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From what I understand, the second parity rule actually says the %A~%T & %G~%C formula does not apply to ssDNA or any RNA.

I would agree that only A would be the correct answer.
BTW, what company is this question from?
 

wizzed101

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Classic representativeness heuristic. Whoever wrote that question must've thought that D was more likely than A.
 

bobeanie95

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"A" could be true but since the % base pairs match up exactly it would most likely represent a double stranded oligonucleotide. On the other hand, "D" has uneven % bp so it definitely has to be a single stranded oligonucleotide. Part of the trick is just finding the best answer
 

aldol16

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Doesn't chargaff's second parity rule support (A)? If not, I guess A and D are both possible.
The idea is that you're looking for the "most likely" choice. And since A has equal amounts of A&T and G&C, it's more likely that it's part of a double-strand than a single-strand. That's because with single-stranded DNA, there's no actual reason for A and G base concentration to be similar to T and C base concentrations respectively. Is it possible that, by chance, you could encounter a single-stranded DNA like (A)? Yes. Is it very likely given vastly larger sequence space? No.
 

wizzed101

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May 20, 2016
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The idea is that you're looking for the "most likely" choice. And since A has equal amounts of A&T and G&C, it's more likely that it's part of a double-strand than a single-strand. That's because with single-stranded DNA, there's no actual reason for A and G base concentration to be similar to T and C base concentrations respectively. Is it possible that, by chance, you could encounter a single-stranded DNA like (A)? Yes. Is it very likely given vastly larger sequence space? No.
But there is no reason for them to not be in that proportion either. It's like asking if you flip a coin 6 times, whether HHHHHH is less likely than HTHHTT.
 

aldol16

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But there is no reason for them to not be in that proportion either. It's like asking if you flip a coin 6 times, whether HHHHHH is less likely than HTHHTT.
Correct, but people here are reading too much into the question. There will be questions on the MCAT that do not appear to go too in-depth into the concept and a chemist or physicist might find problems with many of the C/P questions because they read as though they're written by biologists (many are). It's obvious that most of the people on here understand the idea and we are only discussing the semantics of the question now. Neither (A) nor (D) are excluded in this question - the converse would have been a better question: "Which one of these is most likely to be a part of a double-stranded DNA?"