DNA Synthesis

[s2thindi]

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From the passage:

After entry, synthesis and integration
occurs, during which viral RNA is transcribed into
double-stranded DNA by reverse transcriptase. Viral DNA
enters the nucleus and integrates into the host genome.

The question:
A researcher wanting to study the process by which
viral mRNA is transcribed in an HIV-infected CD4+
T-cell would add all of the following reagents to her
cell culture EXCEPT:
A. radiolabeled thymine.
B. radiolabeled guanine.
C. radiolabeled uracil.
D. radiolabeled adenine.

The answer is A.
"The transcription of viral mRNA in an HIVinfected
CD4+ T-cell requires the same reagents as does the transcription of eukaryotic mRNA
in a healthy CD4+ T-cell: RNA polymerase, ribose, phosphate, and adenine, cytosine, guanine,
and uracil. Unlike DNA, RNA doesn’t contain thymine. So, if the researcher wanted to study
mRNA synthesis (transcription) then she would add radiolabeled adenine, guanine, cytosine,
and uracil, but not radiolabeled thymine, since it wouldn’t be incorporated into the mRNA transcripts.
Thus, Choice A is correct and Choice B, Choice C, and Choice D are incorrect."

However, I think it should be C. Aren't we going from mRNA (which has U's) to DNA (which has T's) according to the passage... therefore you wouldn't need radiolabeled Uracil????

Thanks!

 

[Tokspor]

From the passage:

After entry, synthesis and integration
occurs, during which viral RNA is transcribed into
double-stranded DNA by reverse transcriptase. Viral DNA
enters the nucleus and integrates into the host genome.

The question:
A researcher wanting to study the process by which
viral mRNA is transcribed in an HIV-infected CD4+
T-cell would add all of the following reagents to her
cell culture EXCEPT:
A. radiolabeled thymine.
B. radiolabeled guanine.
C. radiolabeled uracil.
D. radiolabeled adenine.

The answer is A.
"The transcription of viral mRNA in an HIVinfected
CD4+ T-cell requires the same reagents as does the transcription of eukaryotic mRNA
in a healthy CD4+ T-cell: RNA polymerase, ribose, phosphate, and adenine, cytosine, guanine,
and uracil. Unlike DNA, RNA doesn’t contain thymine. So, if the researcher wanted to study
mRNA synthesis (transcription) then she would add radiolabeled adenine, guanine, cytosine,
and uracil, but not radiolabeled thymine, since it wouldn’t be incorporated into the mRNA transcripts.
Thus, Choice A is correct and Choice B, Choice C, and Choice D are incorrect."

However, I think it should be C. Aren't we going from mRNA (which has U's) to DNA (which has T's) according to the passage... therefore you wouldn't need radiolabeled Uracil????

Thanks!

The question does not seem to be clear on this, but it is likely that it is not asking about the reverse transcription process, but rather what happens after the viral RNA has already been reverse transcribed, integrated into the DNA, and is ready to undergo transcription. At this point, you can reason that you wouldn't need thymine.

 

[raab32]

So the passage says "viral RNA is transcribed into
double-stranded DNA by reverse transcriptase". The question wants to know about this specific transcription process. You're thinking a step ahead... the researcher isn't concerned with the actual DNA, he wants to follow the RNA and see what's going on with it. Because of this, he would need to use labeled uracil.

 

[Isoprop]

I believe Tokspor is correct. The question asked "the process by which viral mRNA is transcribed" which means its asking about the general process of how viral mRNA is created. If it had asked "the process by which viral mRNA is transcribed into cDNA" or "the process by which viral mRNA is transcribed by reverse transcriptase" or even "the process by which viral mRNA is reverse transcribed" then your interpretation would have been correct. But this is just general, run-of-the-mill transcription of mRNA.

I know it's frustrating, but this is known to occur on the actual MCAT where the question is ambiguous. The trick is to interpret the question on face value.

Also, I want to emphasize that I agree that this is a poorly worded question. But like I said, poorly worded questions sometimes appear on the actual MCAT.

 

[zmatrix]

However, I think it should be C. Aren't we going from mRNA (which has U's) to DNA (which has T's) according to the passage... therefore you wouldn't need radiolabeled Uracil????

No, you are going from Viral RNA (already present in the virus) to rtDNA then to mRNA. mRNA is created in the cell, and if you put radiolabelled thiamine and radiolabelled uracil in the cell, you will only see radiolabelled uracil in the mRNA.

Since the question specifically refers to mRNA transcription and not DNA reverse transcription, I would say the solution is written correctly as A.

 

[hyourinmaru]

Actually, after re-reading the question, the question SIMPLY asked about the transcription process in the HUMAN CELL into mRNA. (The infected human cell has the viral genome).

So to track the mRNA, you would want anything but thymine, which is not in RNA.