1. 1 Aim #16: How does a DNA molecule replicate itself?

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4. 4 2. Replication Facts DNA has to be copied before a cell dividesDNA has to be copied before a cell divides New cells will need identical DNA strandsNew cells will need identical DNA strands DNA serves only as a template. Enzymes and other proteins do the actual work of replication.

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6. DNA REPLICATION ANIMATION 6

7. 7 3. Starting Replication a) Begins at Origins of Replication b) Two strands open forming Replication Forks (Y-shaped region) ReplicationFork Parental DNA Molecule 3’ 5’ 3’ 5’

8. 8 3. Starting Replication c. Enzyme DNA Helicase “unzips” and separates the 2 DNA strands by breaking hydrogen bonds d.Single-Strand Binding Proteins d.Single-Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted

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10. Each unzipped strand is a template, or pattern, for a new molecule of DNA to form 10

11. 11 4. RNA Primase RNA primers RNA primers (by DNA Polymerase)RNA primers: before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides (by DNA Polymerase). RNA Primase RNA primerRNA Primase: enzyme that synthesizes the RNA primer.

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13. 13 5. DNA Polymerase DNA PolymeraseRNA primer in the 5’ to 3’ directionDNA Polymerase: with a RNA primer in place, DNA Polymerase (enzyme) adds new nucleotides to the DNA template in the 5’ to 3’ direction DNA polymerase also “proofreads” as it adds nucleotides to make sure it is copied correctly – DNA polymerase also “proofreads” as it adds nucleotides to make sure it is copied correctly RNAPrimer DNA Polymerase Nucleotide 5’ 3’ Direction of Replication

14. 6. Leading and Lagging Strands 14

15. 15 Remember the Strands are Antiparallel P P P O O O 1 2 3 4 5 5 3 3 5 P P P O O O 1 2 3 4 5 5 3 5 3 G C TA

16. 16 Leading and Lagging Strands The Leading Strand single strandThe Leading Strand is synthesized as a single strand from the point of origin in a 5’ to 3’ direction RNAPrimer DNA Polymerase Nucleotides 3’5’

17. 17 Leading and Lagging Strands Lagging Strand 5’ to 3’ directiondiscontinuouslyLagging Strand: also synthesized in the 5’ to 3’ direction, but discontinuously against overall direction of replication This strand is made in MANY short segments RNA Primer Leading Strand DNA Polymerase 5’5’ 5’ 3’ Lagging Strand 5’ 3’

18. 18 7. Okazaki Fragments Okazaki Fragments - lagging strandOkazaki Fragments - series of short segments on the lagging strand Must be joined together by an enzymeMust be joined together by an enzyme Lagging Strand RNAPrimerDNAPolymerase 3’ 5’ Okazaki Fragment

19. 19 Joining of Okazaki Fragments The enzyme Ligase joins the Okazaki fragments together to make one strandThe enzyme Ligase joins the Okazaki fragments together to make one strand Lagging Strand Okazaki Fragment 2 DNA ligase DNA ligase Okazaki Fragment 1 5’ 3’

20. 20 Leading and Lagging Strands Replication Fork Point of Origin

21. 8. Ligase Joins Okazaki Fragments Replaces RNA primer with DNA nucleotides copyright cmassengale21

22. 22 9. Proofreading New DNA DNA polymerase initially makes about 1 in 10,000 base pairing errorsDNA polymerase initially makes about 1 in 10,000 base pairing errors Enzymes proofread and correct these mistakesEnzymes proofread and correct these mistakes The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errorsThe new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors

23. 23 10. Semiconservative Model of Replication Idea presented by Watson & CrickIdea presented by Watson & Crick TheThe two strands of the parental molecule separate, and each acts as a template for a new complementary strand New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA

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26. 11. Prokaryote vs Eukaryote 26 Begins at one point- continues in two directions until chromosome is completely copied Begins at multiple points- continues in both directions until chromosome is completely copied (replication bubbles)