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WEBVTT
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Today we're going to be looking at DNA replication. Hopefully
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try to make it simple for you to do better
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on the test. If you know it pretty well, then
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this is just a refresher. Every time a cell divides,
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it faces an incredible challenge. It must copy billions of
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DNA base pairs while making remarkably few mistakes. How does
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the cell accomplish this? Why are the two strands of
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DNA copied differently? The answer begins with DNA replication. DNA
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replication is described as a semi-conservative, meaning each newly formed
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DNA molecule contains one original strand and one newly synthesized strand.
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In eukaryotic cells, replication occurs during the S-phase of the
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cell cycle. The process begins at specific locations called origins
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of replication. An enzyme called a helicase separates the DNA
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strands by disrupting the hydrogen bonds between complementary bases. This
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creates what they call a replication fork where the DNA
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strands separate. The single-strand binding proteins help stabilize the exposed DNA,
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while topoisomerase relieves twisting stress ahead of the fork. DNA
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polymerase can synthesize DNA only in the 5' to 3' direction,
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and this is important for the MCAT. This means new
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nuclear ties are added to the 3' end of a
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growing DNA strand. But because the two original DNA strands
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run in opposite directions, replication cannot occur identically on both strands.
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One strand is called the leading strand, and that's synthesized
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continuously in the same direction that the replication fork advances.
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The other is called a lagging strand. Because DNA polymerase
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cannot synthesize DNA in the 3' to 5' direction, the
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lagging strand must be constructed in short segments. These segments
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are called Okazaki fragments, named after Japanese molecular biologist Okazaki.
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Each fragment is synthesized in the 5' to 3' direction,
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even though overall lagging strand synthesis proceeds opposite the movement
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of the replication fork. Well, now let's examine the enzymes involved.
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Primase synthesizes short RNA primers that provide the starting point
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for DNA synthesis. DNA polymerase extends these primers by adding
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complementary nucleotides. In eukaryotes, DNA polymerase epsilon primarily synthesizes the
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leading strand, while DNA polymerase delta primarily synthesizes the lagging strand.
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I'll repeat that. DNA polymerase epsilon synthesizes the leading strand,
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while DNA polymerase delta synthesizes the lagging one. The RNA
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primers are eventually removed and replaced with DNA. Finally, DNA
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ligase seals remaining breaks in the sugar phosphate backbone, joining
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the fragments into a continuous strand. Another important concept is proofreading.
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Many replicative DNA polymerases possess 3' to 5' DNA. exonuclease activity.
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This allows them to remove incorrectly incorporated nucleotides and improve
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replication accuracy. And that's it for our episode today.