hello so hi and welcome to chapter 6 part 1b on dna replication now um i found out there's something very fascinating about the current app i'm using i can change my boring background i'm clearly trying to entertain myself because you guys are not here to bug me anyways hi today is about dna replication you can see this dna molecule behind me um sort of replicating um and you can see these two strands are separated and it's all like funky looking and really cool and really sci-fi and i am a huge nut i'll stop talking now anyways hi um let's go back let's start at the beginning of this wonderful process shall we okay so step by step now dna replication is semi-conservative so info we say it is semi-conservative dna replication and this particular process as we already know occurs in the s phase of interface we know this from last chapter didn't we last chapter when we said that during s phase one sister chromatid i mean one chromatid becomes two sister chromatids okay and what's happening is actually one dna molecule becoming two dna molecule that's why the number of chromatids always equal the number of dna molecules in that particular cell this process requires atp and before we start our process let's gather our resources let's see what enzymes we need there are three enzymes we need number one is helicase and helicase um functions to break the hitch bonds break the hydrogen bonds to separate the two dna strands sort of like what like um like what is occurring behind me here the two strands are separating in this fashion and that is by an enzyme kiwi case number two is dna polymerase nearly polymerase uh synthesizes a new strand of dna so dna polymerase is really what is building that new strand and joining things together by catalyzing the formation of phosphodiester but it also proofreads dna proof which dna means to um look through the dna sequence and see if there's anything wrong with it um and to repair that before it's too late excuse me number three is dna like case meaning like is joins dna fragments together and sometimes you have structures like this um fragments between between uh different parts of the strand and we'll see why fragments form later on and yeast also catalyze the formation of sputiester bonds polymerase and ligase do catalyze the same bonds but they are different we will see how and why step by step okay so enzymes needed again is helicase dna polymerase and dna like okay don't freak out they all end with ase they are all enzymes anything you know about enzymes apply to these treating so they all have active sites um they all um have active sites they are all popular and soluble um everything you know about enzymes applies here okay but don't worry about it let's go through the process now step by step now the first part of the process is the unwinding of a double helix so the double helix is a quilt sort of structure isn't it so when it's a bit quilt um it requires some turning to get it to be too straight light so that's unwinding for you okay the whole daily molecule is unwell eventually but this occurs stage by stage step by step okay the next thing that happens is helicase so that's very important to remember the enzymes that involve here helicase what does it do it breaks the hydrogen bonds it makes sense cause helicase double helix it breaks the hydrogen bonds it separates the two dna strands from each other like so and these two strands will be used as templates to form the new dna molecule now this process here in a very casual language we can say is unzipping of the dna but this term is however not allowed in the exam cannot use unzipping but you can really say helicase breaks those hydrogen bonds and the two dna strands are separated so after they're separated what we're going to do we have to build our new strengths so first of all we need some bricks right need some monomers in order to make a polymer so here we are having monomers free activated dna nucleotides in the nucleus now they are free means they are free in the nucleus synthesis of new strands okay they are in the nucleus already there are many many of them um atcg all different types and they are made out of um the same components as a classic nucleotide except that they have two additional phosphate so we have three components the deoxyribose sugar the nitrogenous base and three phosphate groups instead of one that you saw earlier last video now these two additional phosphates is what makes it activated okay um and these two faucet groups get removed later on so don't worry about it too much now um there's three activated nucleotides what they do is they are actually binding forming hydrogen bonds with bases on each exposed dna strands so again the two strands act as the template and there's three activated nucleotides from hydrogen bonds with the complementary base pair which means a minus g and c binds with g now it's important to note here that not no enzymes are required here it is a spontaneous process it doesn't require a reaction it doesn't require energy in the form of atp yeah okay it just happens but these are still monomers they are free activated dna nucleotides they are in no way joined together yet so you can see this g and t they are about to be side by side but they are not joined together they are still separate nucleotides so we need dna polymerase to catalyze the formation of a new strand what it does is that one each one strand one right sometimes more but attached to each of the two separated parental strands okay and these dna polymerase actually joins links the activated nucleotides together and how does it do so it basically removes two phosphate groups and you can see the illustration here and these two phosphate groups can be recycled no problem join to another nucleotide and activate it okay and these zinc polymerase also catalyzes the phosphodiester bond formation between adjacent nucleotides so here we can see the result of this reaction right here in this picture right c is bound to heat this here between c and t is a phosphodiester bond and then this continues um step by step as another nucleotide here you can see this free activity is going to complement your base pair and then dna polymerase is going to join g to t now this occurs step by step also in the sense of um different regions of dna so um it's not the whole dna gets unwind and the whole dna does it at the same time it is actually um section by section so this section gets unwind as forms first new transform first and then the next section gets unwind so step by step process but this process continues along the whole dna molecule now let me start online here dna polymerase also proof fits the dna so other than catalyzing the formation of a new dna strand dna polymerase also checks if there's any errors introduced and corrects it before it's too late okay so i think that should be okay so far right we have heat case that separates the two strands these activate the nucleotides the complementary base pair with the exposed template strands um we also call it parental strand or we sometimes call it the original dna strand right either way activated nucleotides you complementary base pairs stem and then dna polymerase links the activated nucleotides with the other activated nucleotide right and this catalyzes the formation of a new strand of dna but to be honest that's a very basic simplistic view and we haven't even talked about dna ligase just yet there is a slight complication though dna polymerases attach to two separate parental strains as we said just now but they move in opposite directions meaning it does not move this way but imagine that it binds here and it binds here and it moves this way and this one moves this way right but the reality is more complicated like usual right it moves in opposite directions which means one moves left and one moves right let's see what that means now there's also complications in the sense of the new dna molecules they are always formed in one direction but as we know the dna strengths are anti-parallel so how does this work out let's look at it in a visual format now dna polymerase will move from 3 prime to 5 prime always from this side to this side let's look strand by strand right let's look at the strength first moves from 3 prime here to 5 prime this entire strand is factored so this is 3 prime with 5 prime as it moves the new dna strand is formed and this is anti-parallel right this is the 5 prime because this is 3 prime so this must be a 5 prime n and it's formed towards the 3 prime n now this one has no problem because the process is able to continue as dna and winds as i said this dna replication happens region by region stage by stage step by step so as if the dna continues unwinding dna will continue moving dna polymerase we're going to be moving and its new strand will be formed but let's look at the other strand now the other strand is quite different the dna polymerase would bind here or somewhere in the middle why because again it moves from the tree prime that's three prime towards the five prime n and in that process it also forms a new dna like i said uv and a always form in the same direction five prime to three prime five prime to three times prime always remember that okay so new dna is from five prime three prime direction but you can see this dna polymerase here it cannot move any further than five prime um it's at the end already so as the dna unwinds another dna polymerase is needed to start the process again so it will bind here will continue and as as it unwinds another dna polymerase will be needed so when we say that the one getting strand is synthesized continuously and this strand up here which is a nicer picture is called the leading strand and the other is called or is synthesized in sections or synthesized discontinuously as okazaki fragments and you end up with these fragments here because each of this fragment is formed by one individual dna polymerase right this fragment is one one dna polymerase that went from that forms the new dna strand from five prime three prime the new energy strand is formed by another dna polymerase here that moves this way another dna polymerase binds here moves this way and therefore this is this confusing this is and this is called the lagging strand which makes sense because it's like like move stop move stuff it lags a little bit like i hope this video is not doing right so it's lagging strand and that continuous synthesis continuously synthesized strand is called the leading strand and this is when the ending ligase comes in a picture because we can't leave these okazaki fragments like this we need dna ligase to come in and mend these strands and make it one long new strength and the result of the is after all of that is two daily molecules two nice wonderful double helical dna molecules each containing one original strand and one newly synthesized strand one original strand here one neody synthesized strand in performia and that's why it's called semi-conservative in one molecule half of the molecule or one strand of the molecule is the original means semi-conservative one strand is conserved out of two now let me show you um let me show you um how it looks like in the form of an animation to get a closer idea about how it works okay so this is a summary this is a repeated thing okay so just reminder first off we have helicase the attaches to double helix and it slowly is moving slowly breaks the hydrogen bonds between those two strands and those two strands separate now you can see the labels one is called the leading strand and one is called the lagging strength okay and you can see that this five three prime n is actually talking about new dna right this new dna strand is actually being formed from the five prime end to the tree prime n this dna polymerase is linking the activated three nucleotides they are coming in this dna polymerase however right is forming is moving in the opposite direction and it's also forming the new dna strands from five prime n okay to the tree prime n you can see how it moves it cannot move all together it has to detach and then it has to bind again you can see this dna ligase sealing the nyx or joining the fragments between these fragments what am i saying join catalyzing the phosphodiester bonds between these fragments to join them up together and we are almost done here almost done here here what this one is synthesized this is a leading strand being synthesized continuously and this is the lagging strand being synthesized discontinuously and therefore i guess it lags behind i guess so yeah you get the idea the result is two dna molecules again each made of one original strand and one newly synthesized strength and this is why it's called semi-conservative dna replication now you may be wondering isn't such thing called fully conservative meaning replication or very liberal dna replication well it's not called that but there is something called conservative and there's something called dispersive these are models that people came up with before we confirmed a semi-conservative um a conservative model says that hey or during after the first replication the dna molecule either has two old strands or two new strands okay it's fully all of them being new this is fully conservative or just conservative dispersive is another model that we know is not true now but it was another model that is saying that after a first replication each dna strand has a mixture of all and new strands in this manner and the second replication will look the same so you get identical dna molecules and a mixture of all the new strands each now now we know um at the moment that semi-conservative dna replication is the true one but how did they find out in the first place how did they know that it's semi-conservative and not conservative and not dispersive well they did the most beautiful experiment in biology well at least it's called that and we will talk about that next video thank you for listening but bye