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DNA Replication• When/why do cells need to replicate (copy) DNA?• Growth• Reproduction• Repair
• Where do we start?• Replication origin – a specific sequence of DNA (or region on a chromosome)
at which DNA synthesis, or replication begins
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Prokaryotes VS Eukaryotes - Replication
• Prokaryotes – only 1 replication origin• Circular DNA, called a plasmid
• Eukaryotes – many replication origins (because they contain so much more DNA; would take too long to replicate)
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Replication enzymes:At the replication origin:• Helicase = enzyme that unwinds & unzips DNA • RNA primase = produces an RNA primer• DNA polymerase = enzyme that makes new DNA • Ligase = joins separate DNA fragments
• This whole combination of the enzymes, proteins, & DNA = replisome
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Steps of DNA Synthesis1. Proteins & enzymes bind at replication origin. Helicase, an enzyme,
unwinds/unzips the DNA molecule.
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Steps of DNA Synthesis2. Another enzyme, RNA primase, lays down an RNA
primer so that the next enzyme knows where to begin DNA synthesis.
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Steps of DNA Synthesis3. The enzyme DNA polymerase adds nucleotides to the pre-
existing DNA strand by matching the correct base pairs.
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Steps of DNA Synthesis4. DNA polymerase replaces the RNA primers with DNA and replication
continues until the entire chromosome has been replicated.
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PROBLEM!• DNA Polymerase is very stubborn… it only likes to work in one
direction.• What do we know about the direction of DNA’s 2 strands?
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Because DNA is antiparallel, we call one strand the leading strand (5’ → 3’) and the other the lagging strand (3’ → 5’).• Leading strand = continuous DNA synthesis• Lagging strand = discontinuous DNA synthesis
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Lagging Strand ReplicationWHY is the lagging discontinuous???• DNA polymerase can only work in one direction (3’ → 5’), so in lagging
strand – DNA synthesis occurs in short, unconnected segments (called Okazaki fragments) that get joined by another enzyme, ligase.
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End result of DNA replication• End result = 2 identical double helices, each with one original strand
& one newly synthesized strand• Called Semi-conservative DNA synthesis b/c each helix has an original
& a new strand
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Amoeba sisters- DNA Replication
•https://www.youtube.com/watch?v=5qSrmeiWsuc