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DNA Replication

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Immediately before cell division, DNA copies itself in a process called DNA replication. First, an enzyme called helicase "unzips" the two strands in the double helix. Then DNA polymerase adds new nucleotides to the new strands. Finally, ligase rejoins the strands and they twist into a helix.

DNA replication is one of the basic processes involved in making cells become more cells. It is the process of making a replica or copy of the DNA inside the nucleus or if it's a bacteria the single circular chromosome. This is something that happens just before cell division, it's not a normal process during the rest of the cell cycle during the rest of the interphase so that's a difference that will help you remember when are going to do transcription, translation versus when you're going to do a DNA replication.

The basic process is pretty simple you have to open up the helix so the enzyme that does that is called helicase "ase" meaning enzyme.

Then you start adding in your new DNA nucleotides wherever there is an a you put a t wherever there is a t you put an a, wherever there's a g you put a c wherever there is a c you put a g and that's done the enzyme DNA polymerase because we're making it polymer of DNA using the enzyme. Then once you've copied off the available bases you have to open up more of the helix and restart some of your strands which are called the lagging strands the once that have to be restarted because they are being built in the wrong direction of the opening helix.

And then finally when you're done you join together those lagging strands which I'll discuss more later and you twist up your two new double helixes.

If we take a quick look at this you can see here, we've opened up what's called a replication bubble and a long molecule of DNA will actually generally have multiple replication bubbles. You can see that the red lines here represent the original or parental strands of DNA sometimes that's called template DNA and then the blue lines here represent the daughter strands the new strands of DNA that are being built. Each side of the replication bubble the place where the DNA is being opened up that is called a replication fork so this bubble has a replication fork that's growing in this direction and a replication fork that's growing in that direction. Ultimately when these two forks grow together their efforts of being joined together using that enzyme called ligase and we wind up with two new DNA molecules.

Now let's take a quick look at this YouTube video that I'll go over a lot of these steps and then right at the end I'll discuss how has some problems youtube started up so we'll go ahead and we'll maximize this and as you can see here is the DNA molecule right inside the nucleus and we're going to see the enzyme helicase come along and open up the here you see it now it's unwinding the helix exposing the two original strands of DNA. Here is some free floating DNA nucleotides the sugar the base and the phosphate they're floating out where do they come from? your food remember it's called DNA because what you used to eat what you eat used to be alive and here we see the DNA polymerase enzyme joining together DNA. Now it's important to notice that it only builds in one direction remember that the two DNA strands in a double helix are antiparallel. This enzyme is copying the other side but because of the antiparallel nature, they have to build in opposite directions so here we see one replication fork, here we see another.

Now I'll go ahead and stop this and we'll go back to the power point and I'll show you one of the ways that video has some problems. Here we see that step now they only showed one continuous strand being built here and actuality each fork has a set of enzymes copying in that the replication fork. Now you'll notice DNA can only be built in one direction even though the strands are on opposite directions. What this means is when this green strand it's growing in this direction hits that fork and you open up more no problem it just keeps going but this one is growing in the opposite direction so what you windup having to do is you windup having to start brand new strands right here which will give you a series of smaller fragments. There was a scientist I believe in Japan doctor Okazaki who figured who found this and thus they're named Okazaki fragments. This portion of the DNA where you have to continually restart is called the lagging strand here the portion at this replication fork that is growing in the direction of that growing helix that's called the leading strand. But ultimately you join together all of your efforts using that ligase enzyme and you windup winding your two new molecules of DNA into new DNA helices.