What is DNA Replication?
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Introduction
In a eukaryotic DNA replication creates problem at the end which we call at the termination problem of eukaryotic replication and the reason for that is in eukaryotic replication. It's like it's a linear DNA most of the time .
So if you have a linear DNA the replication of the linear DNA in the terminal part which is known as a telomere is really really difficult.
We are going to know how the termination of DNA replication occur in case of eukaryotic linear DNA and to understand that we need to know three separate things we need to know about this replication termination .
But before knowing replication termination we need to know the detailed structure of telonium and also the responsible enzyme which helps the process to proceed telomerase .
DNA replication:
It will be helpful also to understand the mechanism of telomerase and how telomerase protects the telomere from being clipped so let us talk about it now to understand the problem associated with the linear DNA at the end of DNA replication.
So if you have a linear DNA the replication of the linear DNA in the terminal part which is known as a telomere is really really difficult.
We are going to know how the termination of DNA replication occur in case of eukaryotic linear DNA and to understand that we need to know three separate things we need to know about this replication termination .
But before knowing replication termination we need to know the detailed structure of telonium and also the responsible enzyme which helps the process to proceed telomerase .
DNA replication:
It will be helpful also to understand the mechanism of telomerase and how telomerase protects the telomere from being clipped so let us talk about it now to understand the problem associated with the linear DNA at the end of DNA replication.
You need to understand how the DNA replication take place so let me draw it in a very schematic drawing in is very linear drawing what we know is that there are two separate strands both the strands serving as a template DNA .
For the replication of a new DNA strands new daughter DNA strand since for both of them we have a specific direction of the marking for that for example this one is five prime three prime let's say this is this is five prime three prime and this is three prime five prime , so it differ the two separate template DNA strands or parental DNA strands.
Primer:
What we all know is that for the start of the DNA replication DNA polymerase cannot initiate it on its own they need a primer which is a RNA section or RNA fragment .
Procedure
Separation of Parent DNA Strands: For the replication of a new DNA strands new daughter DNA strand since for both of them we have a specific direction of the marking for that for example this one is five prime three prime let's say this is this is five prime three prime and this is three prime five prime , so it differ the two separate template DNA strands or parental DNA strands.
Primer:
What we all know is that for the start of the DNA replication DNA polymerase cannot initiate it on its own they need a primer which is a RNA section or RNA fragment .
So the RNA fragment is produced from five prime so generally from five prime end which provides a free three prime hydroxyl this free three prime hydroxyl is a must requirement for the DNA polymerase to elongate.
The Strand to polymerize nucleotides there right so between this two parental DNA in one of the parental DNA they this DNA polymerase get a free three prime hydroxyl.
Leading Strand:
So if they can easily extend it till the end and that strand as it can extend for a long stretch is known as the leading strand of the DNA .
Leading Strand:
So if they can easily extend it till the end and that strand as it can extend for a long stretch is known as the leading strand of the DNA .
On the other hand the other parent of strength cannot provide that same kind of situation because of the complementary nature of the DNA strands so in this other strand the mechanism of DNA replication occurs in a back stretch way that means you need to have a primer here then only you will get free 3 prime hydroxyl in the opposite end .
Formation of loop:
So this process cannot continue into opposite direction so what you studied earlier is that we know that Dean is start forming a loop so that they can continue the DNA replication in both the strands simultaneously .
That's the idea so what we see here is there will be primers in the middle so let's take it there so there will be primers in the middle and the stage of new DNA will be synthesized from those primers so this is the end product of the DNA replication.
Formation of loop:
So this process cannot continue into opposite direction so what you studied earlier is that we know that Dean is start forming a loop so that they can continue the DNA replication in both the strands simultaneously .
That's the idea so what we see here is there will be primers in the middle so let's take it there so there will be primers in the middle and the stage of new DNA will be synthesized from those primers so this is the end product of the DNA replication.
If you want to know how exactly the coiling and looping the DNA takes place and how looping is helping the DNA to be replicated .
Okazaki Fragments and lagging strand :
What we can study is very important between these two strands there is absolutely no problem with this trend because it's a continuous synthesis of the new DNA known as a leading strand of the DNA .
On the other hand the synthesis is not continuous like small fragments of DNA is produced so these fragments are known as Okazaki fragments and this particular strand is known as lagging strand of the DNA because it's it's lagging in the rate of synthesis .
Problem at the Termination:
So now what is the exact problem at the end or termination of the replication because you know the termination part of the any replication is very important remember at the very beginning to initiate the polymerization they need to add a stretch of ribonucleotide sequence as primer and we know in the DNA we should never keep ribonucleotides.
So we need to keep those ribonucleotide sequence out and fill that gap with deoxyribonucleotides that is the end work to be done for the eukaryotic trans detain a replication so in case of bleeding strand is perfectly fine .
Problem at the Termination:
So now what is the exact problem at the end or termination of the replication because you know the termination part of the any replication is very important remember at the very beginning to initiate the polymerization they need to add a stretch of ribonucleotide sequence as primer and we know in the DNA we should never keep ribonucleotides.
So we need to keep those ribonucleotide sequence out and fill that gap with deoxyribonucleotides that is the end work to be done for the eukaryotic trans detain a replication so in case of bleeding strand is perfectly fine .
You know why it's perfectly fine because the idea is if this is a primer at the 5 prime end of the primer and this is the 3 prime end so simply if we remove this there is a chance of synthesizing it from that end.
Addition of Enzyme Telomerase:
Then we can stretch it but we will provide this 3 prime hydroxyl and even to provided free prime hydroxyl we need to have a template right but there is no template left because this is the end that is known as the end replication problem in eukaryotic DNA replication.
Because at the end there is no room for providing any template and as there is no template no further primer can be added as there is no primer no elongation can be made so this is a big issue and to solve this issue we need to use an enzyme telomerase.
Addition of Enzyme Telomerase:
Then we can stretch it but we will provide this 3 prime hydroxyl and even to provided free prime hydroxyl we need to have a template right but there is no template left because this is the end that is known as the end replication problem in eukaryotic DNA replication.
Because at the end there is no room for providing any template and as there is no template no further primer can be added as there is no primer no elongation can be made so this is a big issue and to solve this issue we need to use an enzyme telomerase.
And that's what eukaryotes use telomerase enzyme we provide that extra template segment and with the help of that template they can make the primer and as well as elongate their own DNA parental strength so that new strands can be synthesized pretty easily .
Example for Justification
How if you look at here to understand the process you need to know the structure of both colonial and telomerase now telomere is the end of eukaryotic DNA linear DNA and the end of Columbia has a sequence a lot of GV sequence like if not always conserved .But it's mostly containing more and more G repeats so you see somewhat like te gggg ttg sequence like that this is a kind of in sequence of the telomere so let me let me draw here as the end sequence t g g g g t TG .
Now as this is a telomere sequence to bind with the sequence PLO marie's serves the template telomerase is an enzyme it carries two separate units one is the protein unit another one is a RNA element RNA element .
Our RNA template the RNA template that is presenting telomerase so you can somewhat draw something like this it's kind of providing an RNA template and the RNA template telomerase provide carries exact complimentary sequence to that of this telomere at the end .
Now as this is a telomere sequence to bind with the sequence PLO marie's serves the template telomerase is an enzyme it carries two separate units one is the protein unit another one is a RNA element RNA element .
Our RNA template the RNA template that is presenting telomerase so you can somewhat draw something like this it's kind of providing an RNA template and the RNA template telomerase provide carries exact complimentary sequence to that of this telomere at the end .
So somewhat like let me write it somewhat like a si si si si a a see this is the sequence that is present in the telomerase so now what will happen here it simply at the end there is no room for any extension .
So let me show you how the extension will be done here simply at the end there is sequence and AC sequence will bind to it and rest of the section of the telomere will be placed something like this s now try to understand a and G will bite with the complementary nature but risk of the sequence that is CCC AAA C .
That is also present in the telomerase serve as a template strand that we glide that DNA the parental strand to be elongated because you know in the parent of strands if it's free prime so parent those chains can be elongated right.
So in this case they can add here g g g t t g and the same process continues so again telomerase we slowly move an bind to this T G and again for the stretch of the DNA will be synthesized .
So let me show you how the extension will be done here simply at the end there is sequence and AC sequence will bind to it and rest of the section of the telomere will be placed something like this s now try to understand a and G will bite with the complementary nature but risk of the sequence that is CCC AAA C .
That is also present in the telomerase serve as a template strand that we glide that DNA the parental strand to be elongated because you know in the parent of strands if it's free prime so parent those chains can be elongated right.
So in this case they can add here g g g t t g and the same process continues so again telomerase we slowly move an bind to this T G and again for the stretch of the DNA will be synthesized .
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