PigeonParliament
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The question says:
During replication, the two DNA strands separate and each acts as a template for the production of a new strand. As new DNA strands are produced, nucleotides can only be added in the 5’ to 3’ direction.
Use your knowledge of enzyme action and DNA replication to explain
why new nucleotides can only be added in a 5’ to 3’ direction.

I think I understand the answer- its that DNA polymerase can only attach at the OH at the 3' end of the nucleotide- but the mark scheme says that you get 1 mark for saying that it attaches to the 3' end and then 1 mark for saying it attaches to the 5' end! How could it attach to both ends?! I thought it might mean the 5' end of the template strand, but I didn't think that DNA polymerase attached to the template strand?
I'm going to attach the mark scheme below because I don't think I've explained what it says very well in this paragraph.
Thanks for any help!
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PigeonParliament
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here is the question and mark scheme:
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Megan_90
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(Original post by PigeonParliament)
The question says:
During replication, the two DNA strands separate and each acts as a template for the production of a new strand. As new DNA strands are produced, nucleotides can only be added in the 5’ to 3’ direction.
Use your knowledge of enzyme action and DNA replication to explain
why new nucleotides can only be added in a 5’ to 3’ direction.

I think I understand the answer- its that DNA polymerase can only attach at the OH at the 3' end of the nucleotide- but the mark scheme says that you get 1 mark for saying that it attaches to the 3' end and then 1 mark for saying it attaches to the 5' end! How could it attach to both ends?! I thought it might mean the 5' end of the template strand, but I didn't think that DNA polymerase attached to the template strand?
I'm going to attach the mark scheme below because I don't think I've explained what it says very well in this paragraph.
Thanks for any help!
There are 2 strands to begin with, one is called the leading strand that runs in 5' to 3'. Then there is the other strand which runs from 3' to 5' is called the lagging strand. DNA polymerase can only add nucleotides to 3' end. So, DNA pol can easily add nucleotides to the strand that runs from 5' to 3'. But DNA pol cannot continuously add nucleotides to the strand that runs from 3' to 5'. why? because they can't add nucleotides from 5'. So DNA pol forms Okazaki fragments.

have a look at this image: https://www.researchgate.net/profile...-g001-1024.jpg
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PigeonParliament
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(Original post by Megan_90)
There are 2 strands to begin with, one is called the leading strand that runs in 5' to 3'. Then there is the other strand which runs from 3' to 5' is called the lagging strand. DNA polymerase can only add nucleotides to 3' end. So, DNA pol can easily add nucleotides to the strand that runs from 5' to 3'. But DNA pol cannot continuously add nucleotides to the strand that runs from 3' to 5'. why? because they can't add nucleotides from 5'. So DNA pol forms Okazaki fragments.

have a look at this image: https://www.researchgate.net/profile...-g001-1024.jpg
Thanks so much for replying!
That's sort of the answer I wrote on my paper when I did it, but the mark scheme seems to give 1 mark for 'only complementary to phosphate end' and then 1 mark for 'only complementary to the 5' end' and you can't get the full four marks unless you put both in your answer. Do you know why it would say that you get a mark for saying 'DNA polymerase is only complementary to 5' end' when it attaches as the 3' end of the developing strand?
Thanks again!
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Megan_90
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DNA polymerase is complimentary to the 3’ of template strand. It is also complimentary to the 5’ of the new strand. This video explains: https://youtu.be/wCKF-2nqaOc
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PigeonParliament
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(Original post by Megan_90)
DNA polymerase is complimentary to the 3’ of template strand. It is also complimentary to the 5’ of the new strand. This video explains: https://youtu.be/wCKF-2nqaOc
Ah! thank you so much!!! This has been bugging me for so long!
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