2. 2007-2008 DNA Replication

3. Double helix structure of DNA “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

4. Directionality of DNA  You need to number the carbons!  it matters! OH CH 2 O 4 5 3 2 1 PO 4 N base ribose nucleotide This is IMPORTANT!!

5. DNA replicates 5’ to 3’ OH O 3 PO 4 base CH 2 O base O P O C O –O–O CH 2 1 2 4 5 1 2 3 3 4 5 5

6.  Opposite DNA strands are antiparallel  One strands runs 5’ to 3’ while the other is 3’ to 5’

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

8. Base pairing in DNA  Purines  adenine (A)  guanine (G)  Pyrimidines  thymine (T)  cytosine (C)  Pairing  A : T  2 bonds  C : G  3 bonds

9. Copying DNA  Replication of DNA  base pairing allows each strand to serve as a template for a new strand  DNA replication is semi- conservative = new strand is ½ parent strand and ½ new DNA

10. DNA Replication  Large team of enzymes coordinates replication

11. DNA Replication 1. Unwind DNA  helicase enzyme  unwinds part of DNA helix  stabilized by single-stranded binding proteins single-stranded binding proteins replication fork helicase

12. DNA Polymerase III DNA Replication 2. Build daughter DNA strand  add new complementary bases  DNA polymerase enzyme

13.  Bases can only add nucleotides to 3 end of a growing DNA strand therefore new strand only grows 5  3 direction DNA Replication

14. Limits of DNA polymerase enzyme  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 Okazaki Leading strand  continuous synthesis Lagging strand  Okazaki fragments DNA polymerase  3 5 growing replication fork

15. 3’ 5’ 3’ 5’ 3’ 5’ helicase direction of replication DNA polymerase Okazaki fragments leading strand lagging strand SSB DNA polymerase

16. DNA polymerases  What we need to know is that a group of enzymes known as DNA polymerases catalyze the reaction that joins nucleotides together with covalent bonds