DNA replication occurs semi-conservatively, with each parental strand serving as a template for synthesis of a new complementary strand. This results in two identical DNA molecules, each with one original parental strand and one newly synthesized strand. Replication is initiated at the origin of replication and proceeds bidirectionally around the circular bacterial chromosome. Enzymes such as helicase unwind the parental DNA, topoisomerases relieve supercoiling, and DNA polymerase adds complementary nucleotides using the parental strands as templates.

1. 2007-2008 Synthesis of DNA June.21.2010

2. DNA synthesis occurs by the process of replication. During replication, each of the two parental strands of DNA serves as a template for the synthesis of a Complementary strand.

3. Each molecule generated by the replication process contains one intact parental strand and one newly synthesized strand .

4. In eukaryotes, DNA replication occurs during the S phase of the cell cycle The cell divides during the next phase ( M ), and each daughter cell receives an exact copy of the DNA of the parent cells.

5. Cultured MDCK cells Day 1 Day 3

6. Watson and Crick 1953 article in Nature

7. 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

11. 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

20. energy 3  5  5  5  3  need “primer” bases to add on to energy energy energy 3  no energy to bond energy energy energy 3  5   ligase

22. Replication fork / Replication bubble leading strand lagging strand leading strand lagging strand leading strand lagging strand DNA polymerase III 5  3  5  3  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  5  3 

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

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

32. 2007-2008 Any Questions??

35. 5  3  3  5  3  5  3  5  no energy to bond 

36. energy 5  3  3  5  3  5  3  5  ligase

38. Replication fork 3’ 5’ 3’ 5’ 5’ 3’ 3’ 5’ direction of replication

40. Bidirectional replication of a circular chromosome Replication begins at the point of origin ( oriC ) and proceeds in both directions at the same time.

41. Unwinding of Parental Strands Topoisomerases: can break phosphodiester bonds and rejoin them relieve the supercoiling of the parental duplex caused by unwinding . DNA gyrase is a major topoisomerase in bacterial cells.

42. 以上です。