Why aren't DNA strands parallel?

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qwert7890
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#1
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I know they are anti-parallel, but what's the purpose of it being this way?
I read on the internet it needs to be this way or else commentary base pairing, and thus replication, cannot occur. I can't think of why that is the case!
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CaptainDuckie
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The importance of an antiparallel DNA double helix structure is because of its hydrogen bonding between the complementary nitrogenous base pairs. If the DNA structure were to be parallel, the hydrogen bonding would not be possible, as the base pairs would not be paired in the known way.[4] The four base pairs are: adenine, guanine, cytosine, and thymine, where adenine complements thymine, and guanine complements cytosine. Transcription would be another problem if the DNA structure were to be parallel, making no sense of the information being read from the DNA. This would further lead to the production of incorrect proteins.
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CaptainDuckie
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That’s what I found on wikipedia lol
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qwert7890
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(Original post by CaptainDuckie)
The importance of an antiparallel DNA double helix structure is because of its hydrogen bonding between the complementary nitrogenous base pairs. If the DNA structure were to be parallel, the hydrogen bonding would not be possible, as the base pairs would not be paired in the known way.[4] The four base pairs are: adenine, guanine, cytosine, and thymine, where adenine complements thymine, and guanine complements cytosine. Transcription would be another problem if the DNA structure were to be parallel, making no sense of the information being read from the DNA. This would further lead to the production of incorrect proteins.
Right, I acknowledged that already but what I don't seem to understand is why can't hydrogen bonding occur when DNA strand is parallel?
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qwert7890
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(Original post by CaptainDuckie)
That’s what I found on wikipedia lol
Oh lol haha
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ChemGirl95
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(Original post by qwert7890)
Right, I acknowledged that already but what I don't seem to understand is why can't hydrogen bonding occur when DNA strand is parallel?
This article covers this topic quite nicely!

https://www.nature.com/articles/s41598-020-66705-3
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susiebe
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I think they cant be parallel because of the orientation of the sugar molecule.I think it becomes more obvious why they have to be anti parallel if you look at the structure of a nucleotide and how they join up.

Does that sound correct?

Good question as its got me thinking!
Last edited by susiebe; 1 year ago
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macpatgh-Sheldon
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(Original post by qwert7890)
Right, I acknowledged that already but what I don't seem to understand is why can't hydrogen bonding occur when DNA strand is parallel?
Hi there Master/Miss Keyboard (sorry your username has confused me!),

If the two strands were parallel i.e. both strands running from 3' end to 5' end, then EACH BASE WOULD TRY TO FORM A HYDROGEN BOND WITH AN IDENTICAL BASE on the other strand e.g. adenine would try to bind another adenine, guanine to another guanine, etc. THIS IS IMPOSSIBLE!

If you look at the structure of a purine, it has one 6 C ring and one 5 C ring, whereas as pyrimidine has only one ring i.e. a purine is longer than a pyrimidine.

A purine ALWAYS binds to a pyrimidine, it cannot bind another purine, including one of its own kind.

I will give you a memory aid - the word purine is a shorter word than the word pyrimidine, yeah? - so remember the length of the chemical structure is opposite - the shorter named one [purine] has a longer structure.

If a purine were to bind another purine [hypothetically] and the converse, then the shape of DNA would be more akin to e.g. Billie Piper or Jennifer Aniston i.e. something like 34-24-35 [sorry if this is getting the guys excited lol!] - whereas the ACTUAL SHAPE of DNA is more like an ugly woman [or man for that matter] with stats like 40-40-40 i.e. uniform diameter along its length, are you with me?

I hope this clarifies things [btw my answer is not intended to insult any unfit person, but follows one of the principles of memory outlined in Tony Buzan's superb book].
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