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DNA Replication

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DNA. We talk about it so much---it is the ultimate director for cells and it codes for
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your traits. It’s a major component of what makes you, you. When you have a really important
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molecule like DNA that is ultimately responsible for controlling the cell…it would make sense
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that when you make another cell (like in mitosis), you would also have to get more DNA into that
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cell. And that introduces our topic of DNA replication, which means, making more DNA.
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First let’s talk about where and when.First where---it occurs in the nucleus. If the cell
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has a nucleus. Remember, not all cells have a nucleus. This video clip is actually going
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to focus on the types of cells that do have a nucleus though known as eukaryote cells.
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Prokaryotes, which are cells that lack a nucleus, do things a little differently. Next When
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does this happen---this typically happens during a stage known as interphase. Interphase
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is when a cell’s growing, it’s carrying out cell processes, and it’s replicating
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its DNA. You know what it’s not doing at the exact same time? Dividing. You don’t
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want a cell to be replicating DNA and dividing at the exact same time. That’s a little
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bit too much multitasking. So DNA replication does not happen during cell division (aka
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mitosis). In fact, the cell replicates its DNA before division processes like mitosis
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and meiosis. Because once you make that new cell, you better have DNA to put in there.
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I think DNA replication would actually make a great video game. It’s actually quite
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exciting. I’m going to introduce the key players in DNA replication so that you can
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get some background information. The majority of these key players that I’m going to introduce
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are enzymes. In biology, when you see something end in –ase, you might want to check as
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it is very possible that it’s an enzyme. Enzymes have the ability to speed up reactions
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and build up or break down the items that they act on. So here we go with the key players.
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Helicase- the unzipping enzyme. If you recall that DNA has 2 strands, you can think of helicase
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unzipping the two strands of DNA. Helicase doesn’t have a hard time doing that. The
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hydrogen bonds that hold the DNA strands together is pretty weak compared to other kinds of
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bonds. DNA Polymerase- the builder. This enzyme replicates DNA molecules to actually build
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a new strand of DNA. Primase- The initializer. With as great as DNA polymerase is, poor DNA
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polymerase can’t figure out where to get started without something called a primer.
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Primase makes the primer so that DNA polymerase can figure out where to go to start to work.
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You know what’s kind of interesting about the primer it makes? It’s actually a piece
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of RNA. Ligase- the gluer. It helps glue DNA fragments together. More about why you would
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need that later. Don’t feel overwhelmed. We’ll go over the sequence in order. Please
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keep in mind, that like all of our videos, we tend to give the big picture but there
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are always more details to every biological process. There is more involved than what
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we cover. DNA replication starts at a certain part called the origin. Usually this part
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is identified by certain DNA sequences. There can be multiple origins within the DNA strand.
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At the origin, helicase (the unzipping enzyme) comes in and unwinds the DNA.
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SSB proteins (which stands for single stranded binding proteins) bind to the DNA strands to keep
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them separated. Primase comes in and makes RNA primers on both strands. This is really
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important because otherwise DNA polymerase won’t know where to start.
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Now comes DNA Polymerase. Remember, it’s the important enzyme that adds DNA bases.
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Now you have 2 strands right? But they’re not identical.Remember they complement each other. They
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also are anti-parallel so they don’t really go in the same direction.
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With DNA, we don't say it goes North or South. The directions for the DNA strands are a little different.
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We say that DNA either goes 5’ to 3’ or 3’ to 5’. What in the world does that
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mean? Well the sugar of DNA is part of the backbone of DNA. It has carbons. The carbons
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on the sugar are numbered right after the oxygen in a clockwise direction. 1’, 2’
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3’, 4’ and 5.’ The 5’ carbon is actually outside of this ring structure. Now you do
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the same thing for the other side but keep in mind this strand is flipped just because
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DNA strands are anti-parallel to each other. So let’s count these---again, clockwise
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after the oxygen. 1’, 2’ 3’, 4’ 5’. And the 5’ is out of this ring. This strand
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on the left runs 5’ to 3’ and the strand on the right here runs 3’ to 5’. Well,
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it turns out that DNA polymerase can only works in the 5’ to 3’ direction. So…the
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strand that runs 5’ to 3’ is fine. It is called the leading strand. But the other
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strand will make it a little tricky. DNA polymerase can only go in the 5’ to 3’ direction.
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(NOTE: Reads in 3' to 5' direction). Primase has to set a lot of extra primers down to
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do that as shown here. It takes longer too. This strand is called the lagging strand which
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is pretty fitting.On the lagging strand, you tend to get little fragments of synthesized
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DNA. These are called Okazaki fragments. Okazaki. What an amazing name. The primers have to
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get replaced with DNA bases since the primers were made of RNA. Ligase, the gluing enzyme
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as I like to nickname it, has to take care of the gaps in the Okazaki fragments.Now at
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the end, you have two identical double helix DNA molecules from your one original double
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helix DNA molecule. We call it semi-conservative because the two copies each contain one old
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original strand and one newly made one. One last thing. Surely you have had to proofread
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your work before to catch errors? Well, you definitely don’t want DNA polymerase to
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make errors. If it matches the wrong DNA bases, then you could have an incorrectly coded gene…which
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could ultimately end up in an incorrect protein---or no protein. DNA polymerase is just awesome…it
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has proofreading ability. Meaning, it so rarely makes a mistake. Which is very good. That’s
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it for the amoeba sisters and we remind you to stay curious!

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