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DNA Replication
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“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick (1953)
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Whiteboard Draw a DNA nucleotide. Hint: circle pentagon rectangle
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My image doesn’t have a rectangle as a base.
Oh well.
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Whiteboard Draw a second DNA nucleotide covalently connected to your previous drawing.
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Whiteboard Draw a two additional DNA nucleotides hydrogen bonded to your previous drawing.
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Be Prepared to Answer this Question
How do the bonds found in DNA fit the mechanism for copying DNA?
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How nucleotides are added in DNA replication
Watch and listen to the animation. Write down all key terms related to DNA replication. How nucleotides are added in DNA replication
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Helicase breaks hydrogen bonds between bases, unzips and unwinds the helix
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Whiteboard Draw a replication BUBBLE of DNA.
Do not erase when done... Draw a replication BUBBLE of DNA. Use an arrow to show where the origin of replication is located Label the 5’ and 3’ end of each DNA strand (you can decide which is which, just be sure it’s antiparallel) Add two helicase enzymes, one at each FORK. Usually drawn as ovals
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AKA “topoisomerase” 2. Gyrase Ahead of the replication fork, gyrase unwinds the DNA supercoil Tension in coiling string demonstration
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Whiteboard Add a gyrase to each replication fork.
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3. Single Stranded Binding Proteins
Hold the DNA strands apart (keeps the separated strands apart and stabilize the unwound DNA).
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Whiteboard Add SSBP to each replication fork.
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4. PRIMASE AND PRIMERS
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“Primer”
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DANCE MOVE
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Whiteboard Create a table comparing DNA to RNA
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5. DNA Polymerase III DNA polymerase III adds DNA nucleoside triphosphates to the RNA primer sequence in a 5’ to 3’ direction. Huh? What?
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Where does energy for building DNA come from?
Ponder….. Where does energy for building DNA come from?
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Whiteboard Draw ATP, GTP, TTP, and CTP Hint: circles pentagon
rectangles of different size
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Adding Bases DNA polymerase III can only add nucleotides to 3’ end of a growing DNA strand –needs a “starter” nucleotide to bond to New strand only grows 5’ → 3’
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On Your Notes Label the 3’ and 5’ end of the printed “parent” strand.
Draw in the new “daughter” strand working from 5’ to 3’. Remember the strands are antiparallel!
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Whiteboard Draw this… Label additional 5’ and 3’ ends 3’
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Whiteboard In a different color, add the new DNA strands that are being made and label their 3’ and 5’ ends. 3’
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What is the “problem” or “limitation” with DNA polymerase III only being able to add to the 3’ end of a growing strand? Whiteboard Add in DNA polymerase III to show it building new DNA at the 3’ end of the new strands. 3’
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Leading Strand DNA polymerase III can synthesize a complementary strand on one side of the template in the 5’ to 3’ direction with no problem
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DANCE MOVE
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What about the other strand??
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Lagging Strand DNA polymerase III must work away from the replication fork. Makes a short strand of DNA, called an Okazaki fragment. As the bubble widens, it can make another short strand, and so on.
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DANCE MOVE
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6. DNA Polymerase I RNA primers are removed and replaced with DNA nucleotides by DNA Polymerase I.
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DANCE MOVE
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7. DNA Ligase Along the lagging strand the Okazaki fragments are joined by DNA Ligase to form a single DNA strand.
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DANCE MOVE
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DNA polymerase I 8. Proofreading –proofreads & corrects typos
–repairs mismatched bases –removes abnormal bases –reduces error rate from 1 in 10,000 to 1 in 100 million bases
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DANCE MOVE
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How nucleotides are added in DNA replication
Detailed Animation
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