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Eukaryotic DNA Replication
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0:08
Hello this is Sehar from EasyPeasy, and the topic we are going to discuss today is called Eukaryotic DNA Replication. OK now before I start discussing the process of replication, we should understand what is the difference between Eukaryotes and Prokaryotes. Now the Eu from Eukaryotes stands for good, well or true, while karyotes come from a Greek Word karutos and it stands for nuts. It means all the cells that have good, well or true nut
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like structures in it are called Eukaryotes, and that nut like structure is basically the nucleus with a nuclear membrane and nucleoli. The genetic material DNA is present inside that nucleus. Now look at the picture, In prokaryotes, this DNA is present within the cytoplasm while in eukaryotes you have a good, well or true rounded structure called nucleus in it. so that's the difference. fine, now if I look at the six kingdoms I can find these
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eukaryotic organisms covering four different kingdoms there. So Eukaryotic organisms are really important to us and to understand the DNA replication in it is really important. So that’s why I took a Eukaryotic cell in front of you. In this Eukaryotic cell, the DNA is present within the nucleus and in mitochondria. Now we know about the nucleus that all the genes that are necessary for our survival are present within the nucleus. But why DNA
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is present in Mitochondria?. The answer of this question is within its function. The function of mitochondria is to generate energy that's why it is called Powerhouse of the cell right so all the enzymes that is necessary for Phosphorylation and Dephosphorylation to make ATP molecules there is coming from the genes that is present within the mitochondria. So mitochondria is basically independent of all the other organelles there and that's
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why they're generating the energy within our cells. Fine,okay now I take the nucleus because DNA is present in it and within the nucleus I find all the different kind of organelles there, now I selected three like in the green boxes there Nuclear Pore, Nucleoplasm and Nuclear Envelope. These three organelles are involved in the process of protection and transportation of molecules and enzymes from cytoplasm to the nucleus and from nucleus
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to cytoplasm.Fine now Nucleolus, this organelle is basically synthesizing ribosomes there and chromatin is the thing that we're looking for and that have the DNA structure and Heredity information in it. Fine now we will take the chromatin material and we can see that they are present in the form of chromosomes. The chromosome structure is in front of you, it has two Small arms called P arms. They are joined at the junction of centromere there
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and the Long arms are called Q arms there. They are also coiled upon protein name as histone proteins there, fine, but these chromosomes are not visible with the naked eye even if we try to look at them in the microscope we cannot find it. Because chromosomes can only be visible to us when the cell is in the dividing state. okay so they are not visible in a normal State but once the cell is gonna divide these chromosomes will be visible to us in the microscope.
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Fine, ok now I take that double stranded DNA and now I need to find out that, how to open it up? So this is the strand for me.fine now just like in Prokaryotes within the DNA helical structure we do have specific locations where we can find a specific kind of sequence. And this sequence is called as Origin of Replication sequence just remember in prokaryotes we just have one origin of replication but as in eukaryotes we have a lot of different chromosomes within
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human body we have 23 pairs of chromosomes right so we have a lot of genetic material so that's why we need a lot of different Origin of Replications to speed up the process. Fine now the sequence we have within this origin of replication consists of repetitive ,common nucleotides and it is called consensus sequence. They are also rich in AT segments or AT nucleotides. why? Because adenine makes two hydrogen bonds with Thymine while Guanine makes three different
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bonds with Cytosine. to break two bonds is a lot easier than to break 3 bonds. That's why the segment that is going to open up have AT-rich segments there. Fine makes sense. Ok next next is I take this DNA strand and the Origin of Replication. now this is a group of Protein and this Protein is basically called as Pre replication complex, these proteins are going to join on the top of the Origin of Replication and they allow
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the double-stranded DNA to open up within the origin of replication so this strand will look like this for us. fine now keep in mind, Pre replication complex just opens up the DNA within the origin of replication, but what about the Strand that is not present within the origin of replication? For that we have another enzyme and that enzyme is called helicases. It's gonna open up the Strand that is present beside the origin of replication.
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Now we are talking about Eukaryotes right , so in Eukaryotes we have CMG helicase and these again a group of different proteins attached with each other performing the same function. Fine now the DNA is open up so these single stranded DNA is very likely to have the degradation process because they are open, they are spare so nucleases can attack them and they can also coil back again. So to keep them open up, We need a kind of protein that
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needs to maintain the structure so we have tiny different proteins attached to these structures and these proteins are called as single stranded binding proteins. Fine now, when these proteins are attached with single strands there, these DNA single-strands are not going in the process of degradation then. Bingo. ok now I'm taking only this point of Origin of Replication now when the DNA opens up, the strand that is present on the side
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basically got supercoiled like this, on both sides, now there are two conditions if this DNA coils in clockwise direction or anticlockwise Direction. If its clockwise then it is positive supercoiling and if it is anti clockwise or counterclockwise direction then it is a negative supercoil and we can afford that because if the supercoil is there it means that there is a chance that DNA is going to break at some point and create mutations and that is
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really really difficult for us to survive in that situation right. So we can not afford supercoiling. How can we avoid supercoiling? We have another Protein that takes care of this job and the protein that is going to unwind the supercoiling thing is called DNA topoisomerase. Now DNA topoisomerases consist of two different types called type-1 and type-2 and these types are further subdivided into type A and type B. Now why are they subdivided?
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because type A forms a covalent bond with 5’ end of DNA and type B binds with the 3’ end of the DNA. so the binding locations are different that's why they have subtype A&B. Now why do we have two different types; type 1 and type 2 there? because Type 1 only produces 1 breakage in one strand only while Type 2 can break both strands in the DNA replication process. fine, so this is the difference in type 1 and type 2 of a DNA topoisomerases.
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Fine, now the DNA is open up we know that, right and we know that they have 3 Prime end and 5 Prime End on it. The New DNA that's gonna form there can only be formed from the direction of 5’ to 3’ end. Now look at the Strand above, if that's the Strand we have, then there is no problem we can attach a new strand to it from 5’ to 3’ end. right, but if we are making The New Daughter strand from The Strand below we will have
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a problem here because the strand below is already from 5’ to 3’ end.right, in this case a small tiny fragments are generated and the upper strand is called leading Strand and the lower strand is called lagging strand and the small fragments that are generating here is called Okazaki fragments.Fine ok, now I just took the open single stranded DNA on both sides and see how these parental DNA is gonna replicate into the daughter strands?
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fine now the DNA nucleotides cannot attach themselves directly with the parental DNA strand they need a Helper and the helper they have is called RNA primer.now if the DNA strand have the nucleotides A T A and C RNA PRIMER will have complementary nucleotides there so instead of T they will have uracil there for Thymine they will add Adenine,for Adenine again uracil and for Cytosine they will attach Guanine with it. So that’s the RNA primer
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. Now the question is, why are we using RNA primer, why not DNA primer right? because in the RNA primer on the sugar molecule we have OH the carbon number 2, in DNA we don't have that OH over there. So that's the problem, this OH is needed to attach the first nucleotide to the parental DNA and that's why we are using RNA primer not DNA primer. Fine ok now RNA primer is there and it is made by an enzyme called primase. Again the primase is having
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a group of proteins there and these proteins are going to attach to the Strand and made this RNA primer for us fine. If you did remember the prokaryotic DNA replication, that prokaryotic DNA replication primase is called Dna G. and that is only a single protein have three subunits over there ok. but in case of eukaryotic DNA organisms we have a bunch of different proteins there. Okay. Now the RNA primer, in eukaryotes can be extended from 10 to 20 nucleotides
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approximately. Fine , now the orangish protein you can see over there. This is protein polymerase Alpha that is basically the major protein there attaching the RNA nucleotides and make RNA primer for us.fine ok.now the RNA primer is made we need to attach the DNA nucleotides with it right.so, now to attach those DNA nucleotides we are using Polymerase. In Eukaryotes for Leading strand the polymerase we gonna use here is called polymerase Epsilon.fine,
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that is ,in green parrot green colour here that's why I just mentioned it over there ok. It also has another protein attach with it in the colour of red and this protein is called proliferating cell nuclear antigen and this is a clamp kind of structure you can say , that is like going to attach this group of proteins within the nucleotide so that they cannot go away from it.fine now the blue circles you can see in this picture
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is basically the helicase, remember this helicase? We did discuss it. CMG helicases that is opening up the DNA strand for us. Bingo. Ok now these groups of proteins move like this and attached one by one the DNA nucleotides or us . ok so that was the story for leading strand. what about the lagging strand?Right, in lagging strand we need to make the okazaki fragments so again we are going to take the primase and this primase is making the RNA primer
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there. So this is the primer for us. And then the polymerase came here to extend the DNA nucleotides with it. but just remember here okay for lagging strand the polymerase we are using here is called the polymerase Delta. for leading strand it was polymerase Epsilon, for lagging strand we have a different kind of polymerase fine. Again it is attached with an another kind of protein here and that is acting is a clamping Region for us. Fine.
15:00
if you did remember the prokaryotic DNA replication that I discussed before we had polymerase 3 that was taking care of leading Strand and lagging strand over there and the clamping was done with the sub-unit of that polymerase 3. but that's not the case here, we have a different protein called proliferating cell nuclear antigen to extend or to clamp the polymerase with The Strand. Fineok now this polymerase again gonna move and attach the
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DNA nucleotide or us. okay now. how the RNA primer is gonna get removed from the nucleotides? Fine now this polymerase Delta is moving right, and when it encounters to another primer the RNA primer attaching with another nucleotide strand there. when they come in contact with each other the RNA primer moves up like this. and it is called a single-stranded RNA flap. and when it makes a flap kind of structure another protein called nucleases come to attack
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it and they will degrade this RNA primer. Fine ok now the RNA primer is removed. And now we need to replace this RNA primer position with DNA nucleotides right. To complete this job we have a specific kind of protein called polymerase beta for it. And this polymerase beta is gonna convert the RNA primer nucleotides into the DNA nucleotides there or fill the gap there.Fine ok. Now the DNA nucleotides are made but they are not attached, they are
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in pieces right. Which protein is gonna use here to attach all these fragments together? We have a separate protein for us and this is called DNA Ligase.This DNA ligase attaches itself to all the strands and then join them together. Fine now we have smooth new daughter strand DNA without any RNA primer there. Right ok now here for example if that's a strand we have or the daughter strand. How we know that we need to end the process of replication
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right. At the end of the parental DNA, like at the end of the chromosomes, we have a specific region that have specific repetitive nucleotides there and that nucleotide is called Telomeres for us. this is a really big topic, I will discuss this topic in a separate video, but right now I can just say that this telomere is basically a part of Parental DNA and it is a non-coding sequence and they are repetitive like AGGGTT is gonna repeat itself again and
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again and then they will make a hairpin kind of structure and that's how the DNA replication end itself. but I will discuss this telomeres in my next video in more detail.fine ok so by this process we can end the process of replication and that's it. Let's recap the differences between prokaryotic DNA replication and eukaryotic DNA replication. In prokaryotic DNA replication we just have one origin of replication while in eukaryotic DNA replication
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we have several different points of origin of Replications there fine. Ok the 2nd difference is that we had Dna A protein that was attaching to the original replication and opening it up. In Eukaryotes We have a bunch of different proteins called pre replication complex and these protein is opening up the DNA within the origin of replication. now in prokaryotes the other DNA strand is going to open a by helicases named as Dna B while in Eukaryotes,
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we have CMG helicases and this is again a group of proteins there. OK. In prokaryotic DNA replication we have a primase that is called Dna G and they have three different subunits over there. right while in eukaryotes we have again a group of different proteins attach to it and the major protein here is Polymerase Alpha. Ok for the replication or attaching the nucleotides, the DNA nucleotide with The Strand in prokaryotes we had Ploymerase
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3 while in Eukaryotes for leading strand we have Polymerase Epsilon and for lagging strand we had Polymerase Delta. For the removal of RNA primer we had DNA polymerase 1 in Prokarytoes while in Eukaryotes the RNA primer is gonna remove by making a single stranded RNA flap there. DNA polymerase 1 in Prokaryotes replaces the RNA primer with DNA over there while the DNA form within the Eukaryotes are done by Polymerase Beta.the termination process is
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also different but i didnt discuss the termination process in detail so thats why didnt put that difference here. Ok thankyou very much if you like this video please hit the button like and please subscribe my channel thankyou bye bye.
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