2. DNA Replication

3. Watson and Crick 1953 article in Nature

4. 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

5. 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 will be IMPORTANT!!

6. The DNA backbone Putting the DNA backbone together – refer to the 3 and 5 ends of the DNA the last trailing carbon 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 Sounds trivial, but… this will be IMPORTANT!!

7. 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 3 5 5 3

8. 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

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

10. Copying DNA 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 semi-conservative copy process

11. DNA Replication Large team of enzymes coordinates replication Let’s meet the team…

12. Replication: 1st step Unwind DNA – helicase enzyme unwinds part of DNA helix replication fork helicase I’d love to be helicase & unzip your genes…

13. DNA Polymerase III Replication: 2nd step  Build daughter DNA strand  add new complementary bases  DNA polymerase III

14. 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

15. DNA polymerase III Replication fork / Replication bubble 5 3 5 3 leading strand lagging strand leading strand lagging strand leading strand 5 3 3 5 5 3 5 3 5 3 5 3 growing replication fork growing replication fork 5 5 5 5 5 3 3 5 5 lagging strand 5 3

16. Replication fork 3’ 5’ 3’ 5’ 3’ 5’ helicase direction of replication primase DNA polymerase III DNA polymerase I ligase Okazaki fragments leading strand lagging strand SSB

17. Fast & accurate! It takes E. coli <1 hour to copy 5 million base pairs in its single chromosome – divide to form 2 identical daughter cells Human cell copies 6 billion bases & divide into daughter cells in only few hours – remarkably accurate – only ~1 error per 100 million bases – ~30 errors per cell cycle

18. 1 2 3 4 What does it really look like?

19. What Does It Really Look Like, (3D Animated Movie Version)

21. 2007-2008 Any Questions??