exonuclease activity: 3'-5 prime application question?

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lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

exonuclease activity: 3'-5 prime application question?

Post by lotus0618 » Thu Jul 02, 2020 3:56 pm

If a polymerase has only 3 prime-5' exonuclease activity, BUT NOT 5'-3' activity, which of the following is correct?
A. excision of thymine dimers at the 5' end of the growing strand
B. replacement of nucleotides at the 3' end of the growing strand. (correct answer)

I initially chose A because I thought that since this polymerase can only proofread from 3' to 5', then it can only start in the direction of 3' start to 5' end of the growing strand. Therefore, B was wrong to me because the 3' end is the start, not the end of the growing strand.

Can you please explain to me why A is correct?
NS_Tutor_Yuqi
Posts: 57
Joined: Fri May 29, 2020 11:43 pm

Re: exonuclease activity: 3'-5 prime application question?

Post by NS_Tutor_Yuqi » Sat Jul 04, 2020 3:26 pm

Hi! Could you clarify for me what the point of confusion was with this question? (I'm assuming you're asking why B is the correct answer). An exonuclease can only remove the last added nucleotide of the growing strand. Since it only has 3'-5' activity, then it is only able to remove something at the 3' end.
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: exonuclease activity: 3'-5 prime application question?

Post by lotus0618 » Mon Jul 06, 2020 6:07 pm

I know this is so basic but i'm having a hard time visualizing it. And yes I'm confused as to why B is the correct answer sorry

Oh, so as I'm writing this reply, I just realized that the RNA pol transcribes only one DNA strand in the 3' -> 5' direction into a complementary RNA sequence that is 5' -> 3' direction. Therefore, here, " the polymerase has only 3 prime-5' exonuclease activity" means that it will read the template strand in 3'-5' but fix the complementary strand in the 5' to 3' direction (which is why B is correct). YEs?
NS_Tutor_Nancy
Posts: 48
Joined: Wed May 27, 2020 7:25 pm

Re: exonuclease activity: 3'-5 prime application question?

Post by NS_Tutor_Nancy » Thu Jul 09, 2020 2:58 pm

Yep! You've got it.
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