2. AP Biology 2007-2008 DNA Replication Ch.12.2

3. AP Biology DNA Replication  Purpose: cells need to make a copy of DNA before dividing so each daughter cell has a complete copy of genetic information  3 proposed Models of Replication

4. AP Biology Meselson and Stahl Experiment

5. AP Biology Semi-Conservative Model  Replication of DNA  base pairing allows each strand to serve as a template for a new strand  new strand is 1/2 parent template & 1/2 new DNA

6. AP Biology Anti-parallel strands  Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons  DNA molecule has “direction”  complementary strand runs in opposite direction THIS WILL CAUSE A PROBLEM FOR REPLICATION 3 5 5 3

7. AP Biology Bonding in DNA ….strong or weak bonds? How do the bonds fit the mechanism for copying DNA? 3 5 3 5 covalent phosphodiester bonds hydrogen bonds

8. AP Biology DNA Replication  Large team of enzymes coordinates replication

9. AP Biology Replication: 1st step  Unwind DNA  helicase enzyme  unwinds part of DNA helix  stabilized by single-stranded binding proteins  PREVENTS DNA MOLECULE FROM CLOSING!  DNA gyrase  Enzyme that prevents tangling upstream from the replication fork single-stranded binding proteins replication fork helicase gyrase

10. AP Biology Replication: 2nd step  RNA Primase  Adds small section of RNA (RNA primer) to the 3’ end of template DNA  Why must this be done? DNA polymerase 3 (enzyme that builds new DNA strand) can only add nucleotides to existing strands of DNA

11. AP Biology DNA Polymerase III Replication: 3rd step  Build daughter DNA strand  add new complementary bases  With the help of the enzyme DNA polymerase III

12. AP Biology Replication: 4 th step  Replacement of RNA primer by DNA  Done by DNA polymerase I

13. AP Biology Limits of DNA polymerase III  can only build onto 3 end of an existing DNA strand Leading & Lagging strands 5 5 5 5 3 3 3 5 3 5 3 3 Leading strand Lagging strand Okazaki fragments ligase Okazaki Leading strand  continuous synthesis Lagging strand  Okazaki fragments  joined by ligase  “spot welder” enzyme DNA polymerase III  3 5 growing replication fork

14. AP Biology DNA polymerase III RNA primer is added  built by primase  serves as starter sequence for DNA polymerase III HOWEVER short segments called Okazaki fragments are made because it can only go in a 5  3 direction DNA replication on the lagging strand 5 5 5 3 3 3 5 3 5 3 5 3 growing replication fork primase RNA

15. AP Biology NEXT DNA polymerase I  removes sections of RNA primer and replaces with DNA nucleotides STRANDS ARE GLUED TOGETHER BY DNA LIGASE Replacing RNA primers with DNA 5 5 5 5 3 3 3 3 growing replication fork DNA polymerase I RNA ligase

16. AP Biology Replication fork 3’ 5’ 3’ 5’ 3’ 5’ helicase direction of replication SSB = single-stranded binding proteins primase DNA polymerase III DNA polymerase I ligase Okazaki fragments leading strand lagging strand SSB