Molecular mechanism of dna replication khan academy

Video transcript

- [Voiceover] In the earlier video on DNA replication, we go into some detail about leading strands and lagging strands and all of the different actors, all of these different enzymatic actors. But I left out what is probably the most mind-boggling aspect of all of this, and that's the speed and the precision with which this is actually happening. As we talked about in that video, it feels pretty complex. You have this topoisomerase that's unwinding things, the helicase is unzipping it. Then you have the polymerase that can only go from the five prime to three prime direction, and needs a little primer to get started, but then it starts adding the, it starts adding the nucleotides. On the lagging strand, you have to have the R, you get the RNA primer, but then it's going from, once again, from five prime to three prime, so you have these Okazaki fragments. And all of this craziness that's happening, and remember, these things don't have brains. These aren't computers. They don't know exactly where to go. It's all because of the chemistry. They're all bumping into each other and reacting in just the right way to make this incredible thing happen. Now what I'm about to tell you is really going to boggle your mind. Because this is happening incredibly fast. DNA polymerase has been clocked, at least in E. coli, has clocked at approaching 1,000 base pairs per second. I think the number that I saw was 700-something base pairs per second. So polymerase, let me write this down. This is worth writing down, because it's mind-boggling. Gives you a sense of just how amazing what the machinery in your cells are. So it's been as high as, and it can change. It can speed up and slow down, and that's actually been observed. But polymerase as fast as, as fast as 700-plus base pairs per, per second. So if this, on this diagram, it, man, it's just zipping, it's just zipping along, at least from our perceptual frame of reference. A second seems like a very short amount of time to us, but on a molecular scale, these things are just bouncing around and just getting this stuff done. Now the second thing that you might be wondering, okay, this is happening fast, but surely it has a lot of errors. Well, the first thing you might say, well, if it had a lot of errors, that would really not be good for biology, because you always have, you have DNA replicating all throughout our lives. And at some point you just have so many errors that the cells wouldn't function any more. And so lucky for us that this is actually a fairly precise process. Even in the first pass of the polymerase, you have one mistake, you have one mistake, let me write this down, 'cause it's amazing. One mistake for every, for every approximately 10 to the seventh. So this is 10 million, 10 million in nucleotides. Nucleotides. And that might seem pretty accurate, but you gotta remember, we have billions of nucleotides in our DNA. So this would still introduce a lot of errors. But then there's proofreading that goes back and makes sure that those errors don't stick around. And so once all the proofreading takes place, it actually becomes one mistake, one mistake for every approximately 10 to the ninth nucleotides. So approximately, you can, it would do this at an incredibly fast pace, as fast as 700-plus approaching 1,000 base pairs per second. And you have one error every billion nucleotides, especially after you go through these proofreading steps. And so it's incredibly fast, and it's incredibly precise. So hopefully that gives you a better appreciation for just the magic that's literally, I would look at your hand, or just think about, this is happening in all of the cells or most of the cells of your body as we speak.

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