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DNA Replication

DNA Replication. AP Biology Unit 3. Hershey & Chase Experiment. Bacteriophages are viruses that infect bacteria Bacteriophages consist of DNA and protein (capsid) What part of the bacteriophage holds the instructions of how to make more viruses? DNA.

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DNA Replication

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  1. DNA Replication AP Biology Unit 3

  2. Hershey & Chase Experiment • Bacteriophages are viruses that infect bacteria • Bacteriophages consist of DNA and protein (capsid) • What part of the bacteriophage holds the instructions of how to make more viruses? • DNA

  3. Hershey & Chase: determined that DNA is the genetic material

  4. DNA Structure • Double Helix (2 strands of DNA) • Complementary strands pair up (A & T, C & G)– hydrogen bonds • Strands are antiparallel (5’ and 3’ ends)

  5. DNA Replication • When DNA is copied during S Phase of Interphase • Basic Concept = create a new strand by matching nucleotides to an existing strand

  6. DNA Replication • Replication is semi-conservative (one strand is old, one strand new)

  7. Origins of Replication • Where DNA Replication starts • Differs between organisms • Prokaryotes = 1 origin of replication • Eukaryotes = many different origins of replication

  8. General Process of DNA Replication • Step 1: Initiator Proteins bind to origins of replication to begin replication • Attracts other enzymes involved in replication process

  9. General Process • Step 2: • DNA Helicase separates the DNA helix • Topoisomerase (Gyrase) prevents “overwinding” of DNA • Single Stranded Binding Proteins (SSBs) prevent double helix from reforming temporarily

  10. Question… • What kind of bonds does Helicase break? • Hydrogen bonds

  11. Primase 5’ 3’ 5’ RNA Primer DNA Polymerase III General Process • Step 3: Primase builds an RNA primer at the starting from the 5’ end of the new DNA strand • Uses the 3’ end of existing DNA strand • Why? Because the DNA Polymerase III (main DNA building enzyme) needs something to build off of

  12. General Process • Step 4: DNA Polymerase III builds the new strand of DNA in a 5’ to 3’ direction • What kinds bonds are being formed to make a new strand of DNA? • Covalent bonds

  13. Question… • What other enzyme builds similarly to DNA Polymerase III? • RNA Polymerase (don’t mix it up with DNA Polymerase III!)

  14. Problem… • How can both strands of DNA be replicated in a 5’- 3’ direction at the same time they are antiparallel? • Answer: leading and lagging strands

  15. Leading and Lagging Strands • Leading strand is synthesized continuously in the direction of replication (goes in the same direction as helicase) • Lagging strand is synthesized in short fragments the opposite direction of replication (opposite direction as helicase)

  16. Questions… • How many primers does the leading strand need? • Only 1– to start replication • How many primers does the lagging strand need? • Many – one for each Okazaki fragment

  17. Lagging Strand

  18. DNA Ligase • DNA Ligase seals Okazaki fragments together • Forms covalent bonds between nucleotides to create a continuous strand of DNA

  19. Finishing DNA Replication • Problem #1: There are still RNA nucleotides in the DNA (primers) • Solution = DNA Polymerase I cuts out the RNA nucleotides and replaces them with DNA

  20. Finishing DNA Replication • Problem #2: Okazaki fragments and bases replaced DNA Polymerase I are not attached to rest of DNA • Solution = DNA Ligase seals everything together

  21. Putting It All Together • Label the diagram on Page 67 with the following terms: • SSBs - DNA Ligase • Leading Strand - Lagging Strand • Helicase - Primase • DNA Polymerase III - Primer • DNA Polymerase I

  22. SSBs DNA Polymerase III Helicase Leading Strand DNA Ligase DNA Polymerase I DNA Pol III Primase Lagging Strand Primer

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