DNA end-replication problem

[Monkeymaniac]

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I think the crux of the end replication problem is that at the end of DNA replication, the lagging strand, when the last primer is removed, will be left in that state without DNA Pol III filling the empty space because the DNA Pol can add only to the existing sequence.

But wouldn't it be also true for leading strand? I mean there won't be any sequence upstream of 5' end of replicated starnd once the primer is removed. But it seems that DNA Pol III can remove the primer while replacing the removed position with DNA nucleotides.

My question is, why isn't DNA Pol III, then, also used to solve the end-replication problem in the lagging strand? Thanks in advance.

 

[lorenzomicron]

yes, but in the leading strand, it does so for the 5' to 3' direction.

this can not be done in the lagging strand, because its primer is a 5' end; DNA pol can not replace the 5' primer. thus, telomeres.

 

[Monkeymaniac]

But then, wouldn't both leading and lagging strands have primers at the 5' end? like one below?

Bloded strands are parent strands, and = are primer sequence and - are nucleotides

Leading strand

3'------------------5'
5'=----------------3'


Lagging strand

5'------------------3'
3'--=--=--=--=--=5'

Why can't Pol III repalce this 5' primer with DNA in 5'->3' direction as well?

 

[Monkeymaniac]

Anyone please?

 

[Handinhand]

You're talking about the right most 5' primer on the lagging strand? It can't synthesize DNA at this point because it has no primer to the right of it to associate to and start synthesis on. Primase cannot synthesize a primer here either because there is no DNA to the right of it to synthesize on. That's where Telomerase comes it. Telomerase synthesizes a junk strand of DNA on the lagging template strand so Primase can synthesize a primer, and allow DN Pol III to finish replicating the small amount of genetic material to the right of that 5' Primer.

 

[soby10]

DNA pol III can't add nucleotides in the 5'-3' direction only in the 3'-5' direction. Therefore the leading strand is continuous in the 3'-5' direction but the lagging strand is discontinuous so it requires an RNA primer upstream from dna pol III. There are no okazaki fragments in the 3'-5' direction bc that's the direction that dna pol III can work.
The purpose of RNA primer on the lagging strand is to have a template to which DNA pol can work in the 3'-5' direction the leading strand has a template the lagging strand doesn't have one.

 

[Handinhand]

Please refrain from answering people's questions when you don't know what you're talking about. You just got one of the fundamental rules of DNA Synthesis wrong and that is synthesis occurs 5' to 3', not 3' to 5' like you said in your bolded statement.

 

[Monkeymaniac]

Diggidy, thank you very much. That helped a lot. Now for the leading strand, wouldn't it have the same restriction that it doesn't have nucleotides before 5' end (from picture above left of 5' end)? Is telomerase involved in replacing the primer of the leading strand as well?

 

[Handinhand]

You're on the right track. To really understand it you need to draw a replication bubble, and see that to left of the leading strand would actually be a lagging strand. Telomerase is also only involved in synthesizing "junk" DNA at the telomeres, or the ends of the DNA.

Here's a picture that hopefully explain it:

 

[Monkeymaniac]

Wow, that was exactly what I was looking for. Thank you so much!

 

[sylhet]

......Now for the leading strand, wouldn't it have the same restriction that it doesn't have nucleotides before 5' end (from picture above left of 5' end)? Is telomerase involved in replacing the primer of the leading strand as well?

You're on the right track. To really understand it you need to draw a replication bubble, and see that to left of the leading strand would actually be a lagging strand. Telomerase is also only involved in synthesizing "junk" DNA at the telomeres, or the ends of the DNA. ......
Here's a picture that hopefully explain it:......

many thanks Monkeymaniac coz i had the same question. and special thanks to Diggidy for his patience & ofcourse nice illustrated explanation..

be blessed.

 

[Morsetlis]

I believe one of the confusions is on how to read DNA. In this instance, you're reading the direction of the new strand being synthesized. Of course it would flip if you were reading the template strand's direction.

Be sure to know both.

 

[DrRichand1]

thanks diggidty, i was trying to figure that out to, but once you said replication bubble and lagging strand it all made sence, sweet.

 

[Dasypus]

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