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We speak student!

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Molecular Genetics

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DNA Replication

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A la Shmoop

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So we have a lot of new people to biology here taking the course.

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When you talk about them transcribing into this

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or helping with that - walk us through the process.

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Tell us the story of an RNA transcribing itself

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- or DNA transcribing... - Sure.

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How does that work?

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Like, we've had two cells, they've been out at a bar on a Saturday night,

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they met, they winked at each other. [cell with mustache and one with feminine hat meet]

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And now they're gonna replicate.

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How does that work?

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[ ooh ]

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How does DNA replication work?

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Okay, so there's a couple different things there.

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So, DNA replication is what would happen

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when a cell wanted to divide into two cells. [cells duplicate]

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So basically, every time a cell divides,

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it needs to duplicate its DNA.

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Why does a cell divide anyway?

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[ laughs ] So, a bunch of reasons.

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So one is just as we grow up, we need more cells to get bigger. [human grows bigger]

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If you hurt yourself, so if you get a cut, [knife cuts human, bandaid gets put on]

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then you need cells to fill in.

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Sort of regeneration and wound repair and things like that.

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So there's all sorts of reasons.

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Our cells are constantly also dying inside us.

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So if something goes wrong with your cell,

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you wouldn't want it to turn into a cancer cell.

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So a cell will die and another cell has to replace it. [healthy cell replaces damaged cell]

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[ woo! ]

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- Fair enough. - So let's say the cell needs to divide for whatever reason.

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So the first thing that happens is, at sites called origins of replication,

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that's basically where the replication will start.

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An enzyme called DNA polymerase will bind.

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So it's an enzyme that makes new DNA, basically. [packman munches down DNA]

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So there's an unwinding step that happens.

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Remember, DNA is double-stranded,

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so you have to unwind it so that, basically, the DNA polymerase can sneak in there.

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There's a helicase enzyme that unwinds it like a zipper. [DNA zips open and closed]

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[ zipper sound ]

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And then the DNA polymerase can come in.

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There's first another enzyme, it's kind of complicated,

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called a primase that has to come in and make a little RNA primer.

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'Cause the DNA polymerase can't start by itself.

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It's basically like a little snippet of RNA that binds

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and helps the DNA polymerase hang on so it can get started.

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So once that's on there, the DNA polymerase can hop on

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and it starts adding nucleotides.

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Basically following -- it's called semi-conservative replication.

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Which means it's using one of the DNA strands as a template

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to make the complementary strand,

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because of the base pairing rule.

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So if it sees an A on one strand, it's gonna add a T to make the next.

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And what makes it semi-conservative?

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Like it's not in the deep south; it's just kind of in Tennessee maybe. [redneck cell vs. Elvis cell]

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[ Elvis impression: "uh-huh" ]

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That's the name. They came up with it because,

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basically you're using both strands to make a copy.

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So it's not like you take two strands

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that make a total two new ones.

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For some reason they call that semi-conservative.

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I don't know what the historical origins were.

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Got it. And the logic of the zipper breaking.

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I mean, that's a really clear image.

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And inside the zipper, you've got... what?

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What are the things dangling off it?

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So you basically got -- like the teeth of the zipper

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are basically the bases that are hanging off

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that we've talked about before.

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And the bases would be like A, T, C, or G.

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So the polymerase would come and would see an A base,

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and it would know, "oh, there's an A there,

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I gotta match it up with a T on the other strand."

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Got it. And this is like, you know, the Sharks and the Jets,

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or the Capulets and the -- like, the A goes with T. Period? [sports and literary figures pop up]

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Yes. Yup.

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And what if an A went with a C or a G?

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It would just reject it?

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So, yeah, because of the shape of the bases,

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they won't really match up well.

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Sometimes you can get a mutation where you accidentally

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incorporate the wrong base. So DNA polymerase can make a mistake.

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So that can result in a mutation.

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But, typically, it's pretty good and it puts in the right base.

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Got it, okay. So that's like, if you've seen the movie X-Men.

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That's probably what happened.

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[ whoop ]

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How does DNA replication work?

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[ Elvis impression: "thank you very much" ]

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