Discussion

Radioactive sulfur was found almost exclusively in the supernatant fraction, while the radioactive phosphate was found in the pellet fraction. This indicated:
(A)phage protein entered the cell, but the DNA did not.
(B)...
(C)...
(D)...
(E)...
(F)...
*This question is included in 05. DNA Structure and Function, question #1

The solution is

Posted: 04/05/2013 18:00
The fact that radioactive sulfur was ALMOST exclusively found in the supernatant, indicates DNA and protein entered the cell. So, I think answer B is wrong because it says no protein entered the cell, which from the statement is incorrect. I think the correct answer should be C.
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Posted: 04/17/2013 20:57
I agree with Dani. Radioactive sulfur was found to be ALMOST exclusively in the supernatant solution, but it was not about 100%. This indicates that some proteins entered the bacterial cell. The pellet contained the radioactive phosphate fraction indicating that DNA of the bacteriophage entered the bacterial cell. Thus, the correct answer is C and not B.
Posted: 04/17/2013 21:09
Thank you Dani for the comment and thank you Lavinia for the additional point. I will ask the test publisher about this disputed problem.
Posted: 04/17/2013 23:17
I diagrammed the experimental process and I find myself agreeing with Lavinia and Dani. Unless the wording of the problem is changed, the answer should be that both protein and DNA entered the bacteria in the pellet.
Posted: 07/22/2013 22:43
The bacteria cells were found in the pellet. Extra phages were in the supernant. So the virus proteins that were tagged with radio active sulphur never make it into the bacteria cell. The bacteriophage just injects its genome into the bacteria.... The actual virus never enters the bacteria. So virus or phage protein never enters the bacteria cell. B is the correct answer and it is correct.
Posted: 07/22/2013 22:47
Centrifuging the mix separated it into bacteria pellet and phage supernat. It comes down to understanding how viruses operate. No radioactive sulphur never enters the bacteria cell because the virus stays outside the cell, attaches to the cell and injects its DNA into the bacteria. Blending the mix causes the phages to be knocked off, then centrifuging it separates bacteria from virus.
Posted: 12/16/2013 17:20
Thank you for chiming in, Leann.
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Posted: 01/15/2014 19:23
Some additional explanation from a researcher friend of mine at Stanford: After reading the evidence presented in the passage I would conclude that the correct answer is (B). The presence of radioactive phosphate in the cell pellet clearly suggests that phage DNA entered the bacterial cells. However, it is unlikely, given the experimental design and reported results, that phage protein was also injected into the cells. Here’s why: having done thousands of centrifugations, no separation/fractionation/centrifugation step you ever do in the lab will have 100% efficiency. During the blending step, for example, a small percentage of the T2 virus or viral protein will remain adhered to the outside of the bacterial cells. Similarly, even a small amount of completely unbound virus can contaminate the cell pellet after centrifugation. Thus, there are two steps in this protocol in which you will introduce a small amount of experimental noise. Given that the radioactive sulfur was found ‘almost exclusively’ in the supernatant, I find it highly unlikely that any was actually injected into the cells. From the presented evidence, I would not be able to conclude that phage protein ever entered the cells, and believe (B) is the most suitable answer.
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