DNA replication

okay so the topic of this video today is the process of dna replication so let's get started okay so before we get started on replication you know here's a cell and cells tend to grow and they divide and multiply but oh no look there's a problem the cell on the right doesn't have a piece of dna on the inside so there has to be a solution before cells divide let's make more dna and so here we have our cell again watch what happens to the dna in the cell a copy is created so when the cell then grows and divides the two cells each have a copy of the dna that they can use through their life so dna replication is how cells obtain a copy of their dna needed when we look at dna replication it's the process of copying the piece of dna one piece of dna gets copied into two and this process takes place during the s stage of interphase if you're in my biology class this is something we learned earlier in the school year hey by the way here's five pictures one of these is interface remember which of these is interphase it was uh picture e the cell with the normal-looking cell the nucleus intact so that's an interphase picture and so during the s stage of interphase is where dna replication tends to occur okay i want to mention that dna is a antiparallel molecule well here's one strip of nucleotides that makes up dna on the left i just labeled that five prime on the right i labeled that three prime the apostrophe is a symbol for the word prime and you'll see what this means in a moment but let me add the second strip of nucleotides and on the right hand side i'm going to label this 5 prime and the left hand side i'm going to label this 3 prime so dna is a parallel molecule parallel because the two strands of nucleotides run alongside one another but it's anti-parallel in that they run in opposite directions the bottom strip the five prime is uh on the left and the top strip the five prime is on the right now let's explore what this means okay let's go ahead and make sense out of what the heck is the five prime and three prime that i talked about a moment ago well here are the four nucleotides of dna the circles represent the phosphates the pentagons represents the sugar molecule and then the a and the t and the c and the g represent the nitrogen base now when we find out the five prime and the three prime direction of of dna what we're doing is we're looking at the sugar and we're counting there is carbon one carbon two carbon three carbon four carbon five the fifth carbon that you see right there we call that the five prime carbon and the third carbon right there is the three prime carbon every sugar has a five prime carbon and a three prime carbon every sugar on the right look on the right hand side of the screen there's a five prime carbon and a three prime carbon and look at the last one there's a five prime carbon and a three prime carbon so this is what it means to have the five prime and three prime it's just the fifth carbon that makes up the sugar of the dna so let's go back and look at our molecule here's that same picture from a moment ago i hope it makes a little more sense now when we say the bottom strip is pointed in the five prime to three prime direction but look at this the the top strip of nucleotides it's pointed with the five prime end on the right and the three prime end on the left so again this just goes to clarify what we mean by dna being a anti-parallel molecule the two strips of nucleotides are arranged in different directions okay let's go ahead and get into the act of dna replication let's talk about an enzyme by the name of helicase here is helicase and what it's going to do it's going to break the bonds in between every a and t and c and g watch what happens notice the bond between the c and the g were broken by the enzyme dna helicase as the helicase enzyme continues down the strip of dna it just continues to break apart the nucleotides notice that another g and c were broken apart and now notice that an a and t are going to be broken apart notice how another a and t are broken apart and this creates what's called a replication fork and as dna helicase continues down the replication fork will grow bigger and bigger and bigger notice how a dna helicase has broken apart a c from a g and now an a from a t and now another a from a t and now a c from a g and now another c from a g so notice how the replication fork is growing as dna helicase continues down and in this case a c from a g will be broken so now i want to talk about the next step in this process on the bottom i'm going to label this strand the leading strand and you're going to see that nucleotides are going to be continuously added from the five prime to three prime direction they're added by an enzyme by the name of dna polymerase so like the description says dna polymerase's job is to bond new nucleotides a's with t's and c's with g's look in the animation notice that there's a g dna polymerase is reading a g so in comes a c as dna polymerase works down the the the leading strand notice how a g was bonded now dna polymerase is reading an a so in comes a t notice how the nucleotides are being added 5 prime to 3 prime direction this happens in the leading strand so as dna helicase continues to unzip and break apart the dna the dna polymerase and the leading strand will just continue to follow along hence the reason why it's continuous the leading strand is continuous however the strand on top i'm going to label this the lagging strand in this case we're gonna see that nucleotides they're still added in the five prime to three prime direction but you're gonna see it's reversed they're gonna be added in what is called okazaki fragments named after a husband and wife team of scientists from japan who identified the role that that these fragments play so here is the another dna polymerase and notice how a t was added in the five prime to three prime direction we're going to see that an a is added as the dna polymerase works the opposite direction notice how a c is added again but the c is added in the 5 prime to 3 prime direction so on the lagging strand nucleotides are added in reverse direction so this creates what's called a okazaki fragment so as dna helicase continues to unzip we're going to see that in the lagging strand nucleotides are added in the reverse direction in a series of what are known as okazaki fragments so dna helicase continues to unzip our molecule of dna so dna helicase continues to unzip and break apart the dna molecule and so now what we're going to focus on is the leading strand notice dna polymerase is reading a g so in comes a c continuously being added in the five prime t three prime direction dna polymerase is reading a t so n comes in a in comes a t so this is all happening on the leading strand but on the lagging strand notice i just added another dna polymerase it's going to be added in the reverse direction still in the 5 prime to 3 prime direction but notice how the five prime three prime direction is just facing the opposite way so in comes an a and comes a t and comes a g and this is again creates what we call an okazaki fragment now what we need to do is we have these two okazaki fragments we need to bond them together we need another enzyme an enzyme by the name of ligase here comes a ligase and what it does is it bonds the sugar phosphate backbones on the lagging strand and therefore the okazaki fragments are bonded together to make a continuous piece of dna here so let's continue using dna helicase to break apart c's from g's and an a from a t and a c from a g another c from a g so focusing on the bottom or the leading strand dna polymerase continuously adds nucleotides in the five prime to three prime direction again the continuous adding is only on the leading strand above on the lagging strand nucleotides are added still in the five prime to three prime direction but because dna is an antiparallel molecule it's it's a one strip is reversed from the other strip and so these uh this okazaki fragment has to be bonded with the others that we saw earlier and so in comes the enzyme called ligase which helps to bind the sugar phosphate backbone of a dna molecule okay so let's use dna helicase to unzip a few more nucleotides so again focusing on the leading strand on the bottom dna polymerase adds nucleotides continuously the continuous adding of nucleotides is only on the leading strand let's look above on the lagging strand dna polymerase still adds nucleotides in the five prime to three prime direction but because dna is an antiparallel molecule the directions are reversed here we have an okazaki fragment and so we need the enzyme called ligase to again help to bond the sugar phosphate backbone of a dna molecule together okay let's go ahead and finish the separating of the dna molecule by the enzyme dna helicase and now that we've separated the dna fully we can focus on the leading strand where continuously dna polymerase adds nucleotides in the five prime to three prime direction again continuously is the difference between the leading strand on the bottom versus the lagging strand on the top nucleotides are still being added on the lagging strand in the five prime to three prime direction but they're just add because dna is an antiparallel molecule the direction is simply reversed we have again a a little larger of an okazaki fragment than the other ones but nonetheless we need the enzyme called ligase to help bond the sugar phosphate backbone of a dna molecule together okay so now that we're finished we can see that the end result are two identical copies of a dna molecule it's also worth mentioning that dna is an example of a semi-conservative molecule each molecule of dna is made from one strand of old nucleotides the nucleotides that are blinking were the old original nucleotides and also one strand of new nucleotides the nucleotides blink and are the newly added ones this is what we mean by dna being a semi-conservative molecule okay there you guys go the process of dna replication i hope you found this video helpful and thank you for watching

Channel: Beverly Biology

Views: 40,209

Keywords: DNA, RNA, Biology, Science, Replication, Nucleotide, Okazaki, nucleic acid, enzyme, Helicase, Polymerase, Ligase

Id: T1aR77FLdi0

Length: 13min 6sec (786 seconds)

Published: Wed Apr 07 2021