RNA to DNA 2 TBR CBT 5 questions

[glam407]

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I was given a passage where the (+) RNA via reverse transcriptase converts into (-) DNA which then again reverse trancriptase acts on to make a DNA double helix with (+ & - strands)


1) If a cytosine in the (+) RNA were radiolabeled with 14C at the 3' position, then this radiolabel would not show up in the:
I. (+) DNA strand.
II. (-) DNA strand.
III. viral proteins of the new virus particles.

The answer says all the above. Is it because the C in the + RNA would reverse transcriptase into a G in the DNA - and not be detected, and when that becomes the Double Helix + & - the label is not found( since its made from the un-labelled DNA - ?)

2) Strains of bacteria that can be transformed by foreign DNA are called competent. Which of the following is the LEAST likely explanation for why some bacterial strains are incompetent?

A. Incompetent cells might not take up the foreign DNA.
B. Incompetent cells might not be able to integrate foreign DNA into their genome.
C. Foreign DNA might not be recognized by bacterial polymerases.
D. Bacterial cells might contain active enzymes called deoxyribonucleases.


So A would be wrong because its likely that they might not take it up--which is why they aren't transformed. And B is the same in the sense that they dont transform because they probably cant integrate the DNA. But I was confused on eliminating C and D. It says deoxyribonucleases break down foreign DNA before they are able to transform so D is valid. But I still don't understand why C is right

 

[sp808]

The reason why it's all of the above is just because whatever was in the original RNA strand stays in the RNA strand. In the context of the experiment, the cell/virus/whatever isn't being pulsed with radioactive dNTPs anymore so new nucleic acids will not incorporate any label. The concept being tested was the semi-conversative nature of transcription and replication, it didn't matter if it was C or any other base.

I remember getting the second question wrong as well. I selected B because a competent cell should be able to receive the DNA as a plasmid, so it doesn't matter if they are able to actually integrate it into their genome or not. I'm still not sure why that's not correct as well. I think the explanation for C was that DNA is DNA and there shouldn't be any difference in recognition between foreign and bacterial DNA (if the polymerase can recognize endogenous DNA, it should be able to recognize foreign DNA as well).

 

[sdm33]

1) If a cytosine in the (+) RNA were radiolabeled with 14C at the 3' position, then this radiolabel would not show up in the:
I. (+) DNA strand.
II. (-) DNA strand.
III. viral proteins of the new virus particles.

The answer says all the above. Is it because the C in the + RNA would reverse transcriptase into a G in the DNA - and not be detected, and when that becomes the Double Helix + & - the label is not found( since its made from the un-labelled DNA - ?)

I picked all the above by thinking of + RNA as the semi conservative concept where DNA replication , where the one DNA will not be consumed.

2) Strains of bacteria that can be transformed by foreign DNA are called competent. Which of the following is the LEAST likely explanation for why some bacterial strains are incompetent?

A. Incompetent cells might not take up the foreign DNA.
B. Incompetent cells might not be able to integrate foreign DNA into their genome.
C. Foreign DNA might not be recognized by bacterial polymerases.
D. Bacterial cells might contain active enzymes called deoxyribonucleases.

In this Q I eliminated B and C , and I had to think about A because it sounded too easy, but then I look at D sounded like the least explanation because I did not know much about it and I thought it can break the bacteria’s own DNA. I still think C is good because DNA polymerase need to recognize the DNA so it can replicate it.