NS Biology Qbank 10 - Molecular Genetics Passages - Question 16

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NS Biology Qbank 10 - Molecular Genetics Passages - Question 16

Post by wolverina4 » Sun Apr 26, 2020 8:40 pm

I chose answer D, which was marked as correct, but when I look at the answer explanations, it seems as if it is explaining that choice B would be correct.
The question asks: "all of the following are ways that the ploidy status diagnosis would fail EXCEPT..." and under the explanation for choice B it says that the test does not have a lack of controls. So does that mean lack of controls is not a way that the ploidy status diagnosis would fail, making choice B correct??

I just want to clear up the confusion since there is no explanation offered for choice D. I am pretty sure choice D would be correct though..since "competition between test and control DNA for array targets" is the basis behind the experiment itself, so why would that be a possible way for the ploidy diagnosis to fail...unless I am misinterpreting the experiment itself? Can anyone clarify what the Array-CGH experiment does and which choice is correct and why?

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Re: NS Biology Qbank 10 - Molecular Genetics Passages - Question 16

Post by NS_Tutor_Mathias » Wed Apr 29, 2020 7:49 pm

To start off, even though this doesn't answer your bolded question:
D is the correct answer because competition between test and control DNA is what produces an accurate signal that represents copy numbers for each chromosome. So it is not a flaw in the method, rather it is the method. Your reasoning is completely correct.

So array-CGH is described as "FISH for chromosomes". Really all you need to get a good mental image of the technique is to know what FISH is, or really only what is meant by hybridization: The binding of a DNA or RNA element to it's complement.

FISH is a technique where artificial complements for particular sequences are made (in this case: most of a chromosome) and these artificial complements are marked with fluorescent labels. The DNA strand to be analyzed is then marked too at somewhat-regular intervals, but in a different color.

In our aCGH setup here, test DNA was labeled green, reference DNA was labeled red. Then the array (which is a fancy term for "little box full of holes, each with a different probe") was incubated with both of test and reference DNA. We can expect that each probe for every target chromosome captures equal amounts of reference DNA and test DNA if equal amounts of those two are present in the sample. This would mean the signal we detect is an equal mix of red and green, and the log ratio of the 2 is 0, because log(1/1)=log(1)=0.

We can see that this is largely how it goes down in figure 1, except for an extra copy of chromosome 21. In figure 2, we see that when performed on a single polar body, we get half as much signal as before log2(1/2)=-1, except for the X chromosome.

B is just saying that controls are described in the passage as part of the technique, and they function perfectly well as we can see by their as-expected copy numbers.
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