replication fork opens 3' to 5'?

[cherr1pop]

During transcription, the replication forks opens _______ and polymerization of the leading strand occurs _______ , which adds bases to the _____ end of the chain.
a) 5' to 3' ; 5'-3'; 5'
b) 3' to 5' ; 5' to 3' ; 3'
c) 5' to 3' ; 3' to 5' ; 3'
d) 3' to 5' ; 3' to 5' ' 3'

this was a question posted by HoangDDS and i got the answers narrowed down to *SORRY I MEANT BETWEEN B AND C* but i can't seem to understand the part about the replication fork 🙁 i think i'm confused cause all i can remember about this is that DNA is antiparallel so it opens both 3' to 5' and 5'to 3'!??! does helicase start working on the 3' end or something...
if someone could please explain this and clarify it for me i would really appreciate it... thanks.

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[Albuterol]

between answer choices A and B...answer B is correct. Remember DNA strands are complementary and antiparallel so when new mrna is synthesized it will only use one of those strands as the template. The anti-sense strand is the template which is used for mrna synthesis and it's orientation is 3'----5' so mrna will be made 5'----3'. The other strand is the sense strand which is complementary to the DNA molecule and its orientation is 5'---3'.

5'-----3' sense strand DNA (complementary to antisense)
3'-----5' anti-sense strand DNA
5'-----3' mrna (same as to sense strand but with U instead of T)

DNA Helicase job is just to open up the DNA molecule and create replication forks. the opening of replication forks creates a gap between the 2 strands of DNA. Now RNA pol must come in and bind to the correct strand. Rna pol will bind to the antisense strand and start synthesizing mrna in the 5'---3' direction.

hope this clears up any confusion.

 

[Mstoothlady2012]

yea that doesn't make sense to me either. I know that helicase works 5' --> 3' meaning it binds to the strand that starts with 5' and goes toward 3' end. So if anything I would say replication fork is from 5---> 3.

Hoang can you explain any further about this?

 

[cherr1pop]

Thanks Albuterol. I'm sorry but I meant I was confused between b and c, not a and b. Anyone else??? Are we supposed to assume that the question is asking on the basis of leading strand or something??

 

[Albuterol]

polymerization of the leading strand during transcription occurs 5'----3' . That makes answer C wrong.

 

[cherr1pop]

oh yea. duh. i must be going blind.. 🙁 thanks!
so i guess you really didn't know which way the rep fork opens for this prob at all.............

 

[utdent20]

so what is the final asnwer? it goes 5-3 or 3-5

 

[HoangDDS]

Mstoothlady2012 : "yea that doesn't make sense to me either. I know that helicase works 5' --> 3' meaning it binds to the strand that starts with 5' and goes toward 3' end. So if anything I would say replication fork is from 5---> 3.

Hoang can you explain any further about this?"

sorry I was away for a while. But I made this question up out of nowhere. So if it doesn't sound right, it's b/c i'm no professional test writer. I was just testing your attention to confusing details.

So basically you gotta remember: POLYMERIZATION ALWAYS OCCURS IN THE 5' to 3' DIRECTION!!! (no exception!)

Template strands are read 3' to 5' while daugther strands are elongated 5' to 3'. This means the existing chain is always lengethened by the addn of a nucleotide to the 3' end. The 3' hydroxyl group acts as a nucleophile in the polymerization rxn to displace the 5' pyrophosphate from the dNTP to be added. The area where the parental double helix continues to unwind are called replication forks.

I wanted to change C to 5'-3', 3' to 5', 5' to make it extra tricky incase if people was unsure that nucleotide are added on the the 3' end. But I think those who answered all got it correct. I know, I'm a jerk =)

 

[PooyaH]

I believe when it says what direction the replication fork opens, it's referring to the strand that does replication continuously which would be the leading strand. And that opens from the 3' to 5'. But the lagging strand opens from the 5' to 3'. The replication occurs from the 5' to the 3' prime and it adds new oligonucleotides to the 3' of the chain!