hey everybody dr. Rowe here this we're going to cover DNA replication so in order for one cell to become 2 your DNA has to replicate or double so that both of those cells have all the DNA they need so DNA replication here you see it very simply the players involved we have a strand of DNA that's going to be copied the enzyme that does the reading and transcribing and copying here in our replication nut transcription is DNA polymerase so it reads DNA and makes chains of DNA DNA replication always occurs on the 3 prime end or from the 5 prime direction to the 3 prime direction you'll see why that's important in a moment the other key thing to note here is the complementary base pairs so DNA polymerase does its job because anywhere at season a it knows that the complementary base needs to be at e so when it reads an A it lays down at E when it reads a G it lays down a C so remember a and T are complementary base pairs G and C are complementary base pairs let's go ahead and dive in so here you see it happening initiation of this process is gonna start be started there because the double helix has to be unwound or unzipped an enzyme helicase you see right there in the middle it's gonna unwind the DNA so that it can't the two strands can be broken apart and each strand can become the template for making new DNA that's gonna be initiation elongation is gonna be DNA polymerase grabbing on here and then we're at season a it lays down a T if it sees a T it puts down an A ceases C it puts down a G sees a G puts down a seat so that's gonna it's just gonna go and go and go until it runs to the end so this is going to be very straightforward and very simple on the top there and what's called the leading strand because DNA polymerase is actually copying DNA from that 5 prime to 3 prime direction it's not going to be so simple as we'll see in a moment with the other strand called the lagging strand then so this is going to be the entire process of DNA replication and the leading strand very straightforward unzip it DNA polymerase grabs on and starts reading DNA and making a copy pretty straightforward one I wanted to zoom in here to show you one more thing before we get into the lagging strand DNA replication a key word you're gonna see is DNA replication is is semi-conservative process so that means that there really isn't any brand new DNA so here's what semi-conservative means whereas you as this as this one piece of DNA on the left is becoming too brand-new daughter strands on the right each new piece of DNA has an old strand and a new strand that's what semi-conservative means very very important process though because less copying years but if I were to give you a page in the textbook and say you have two options you can rewrite or re-type out this page of the textbook or you can make a photocopy of it right the photocopy being the temp from a template which is gonna be less likely to have errors there will still be errors maybe there'll be a streak on the copy machine or you put it in their wrong but you're gonna see way less errors if you copy DNA using your complementary base pairs then if you were to start from scratch so that's why this term semi conservative is is very important so all new double-stranded DNA has an old strand the template and a new strand that's what semi-conservative means okay leading strand i think is very straightforward things are a lot more complicated here on the bottom and what's called the lagging strand and that's because DNA has to be copied backwards so DNA has to loop around and make make copies of itself backwards so in order to do that the first step is you lay down RNA primers I'll have it circled there the RNA primer so a little chunk of RNA here's how I think about this DNA polymerase can make DNA but it has to have a starting point just like a zipper a zipper can be a hundred miles long but it needs that little thing at the bottom in order in order to get started once you start zipping then you can zip as long as you want but you need that place to start so the lagging strand requires these little RNA primers that's where DNA polymerase will start and make chunks of DNA so so the lagging strand has to be it's going to have a bunch of little RNA primers chunk of DNA RNA primer chunk of DNA so chunks are not a technical term these chunks are called Okazaki fragments so lagging strand the lagging strand is copied one Okazaki fragment at a time the next thing it's gonna happen these RNA primers are replaced there they're removed and replaced by DNA so now you have a long strand of DNA but there's still little gaps everywhere you had one of these RNA primers that need to be sealed together so the last step with the lagging strand is there on the right DNA ligase is the enzyme that comes in and seals all those gaps now once you're done at the top and bottom the leading strand was synthesized easily and continuously the lagging strand had to be synthesized by Okazaki fragments so discontinuously but once you're done with all these steps you cannot tell which strand was the leading strand which strand was the lagging strand so it takes more steps but in the end both new daughter DNA molecules are perfectly fine so that is DNA replication why it's important that it's semi conservative and that is how you synthesize the leading strand and the lagging strand I hope this helps have a wonderful day be blessed