 Here everybody, Dr. O here, and this is where we're going to cover DNA replication. So in order for one cell to become two, 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 player is involved. We have a strand of DNA that's going to be copied. The enzyme that does the reading and transcribing and copying here, not replication, not transcription, is DNA polymerase. So it reads DNA and makes chains of DNA. DNA replication always occurs on the three prime end or from the five prime direction to the three 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 a T. So when it reads an A, it lays down a T. 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. The beginning of this process is going to 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 going to unwind the DNA so that the two strands can be broken apart and each strand can become the template for making new DNA. That's going to be initiation. Elongation is going to be DNA polymerase grabbing on here and anywhere at season A, it lays down a T. If it sees a T, it puts down an A. Sees a C, it puts down a G, sees a G, puts down a C. So it's just going to 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 five prime to three 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 of the leading strand very straightforward. Then zip it. DNA polymerase grabs on and starts reading DNA and making a copy pretty straightforward. 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 going to see is DNA replication is semi-conservative process. So that means that there really isn't any brand new DNA. So here's what semi-conservative means as this one piece of DNA on the left is becoming two 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 errors. If I were to give you a page in the textbook and say you have two options, you can rewrite or retype out this page of the textbook or you can make a photocopy of it, right? The photocopy being from a template, which is going to be less likely to have errors. There will still be errors. Maybe there'll be a streak on the copy machine or you'll put it in there wrong. But you're going to see way less errors if you copy DNA using your complementary base pairs than if you were to start from scratch. So that's why this term semi-conservative 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 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 100 miles long, but it needs that little thing at the bottom 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 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 the lagging strand is copied one Okazaki fragment at a time. The next thing that's going to happen, these RNA primers are replaced. They're removed and replaced by DNA. So now you have a long strand of DNA, but there are 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, you look 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.