1. Replication of DNA Objectives: 1. Summarize the roles of the different enzymes involved in replication of DNA. 2. Explain how the leading and lagging strands are replicated differently

2. Semiconservative Replication When Watson and Crick proposed the model of the structure of DNA they also proposed a method of its replication, Semiconservative.

3. Aspects of Semiconservative Replication 1.Original strands separate, and serve as template strands. 2.These template strands copy themselves forming two molecules of DNA 3.Each DNA molecule has one strand of parental DNA and one strand of new DNA

4. 3 stages of the process of Semiconservative replication Unwinding Base pairing Joining of the strands

5. Unwinding The Double Helix of DNA must be unwound in order to allow base pairing (copying) of the DNA molecule. An enzyme called Helicase unwinds the DNA molecule. Helicase does this by breaking the weak hydrogen bonds between the two strand of DNA.

6. The Enzyme Helicase http://www.youtube.com/watch?v=UWNhwce Mjfk

7. Base Pairing The enzyme DNA polymerase attaches appropriate nucleotides to the new DNA strands The nucleotides are added to the 3’ so we say that the DNA synthesis moves in the 5’ to 3’ direction.

8. Okazaki Fragments and the leading and lagging strand The two strands are made in a different manner. One strand is called the leading strand. This strand is built continuously from the 5’ to 3’ ends. But, since the strands are antiparallel, the lagging strand can not be built continuously. This strand is built into small segments, called Okazaki fragments. The fragments are later connected by the enzyme DNA ligase.

9. http://www.youtube.com/watch?v=teV62zrm2 P0

10. Joining the Strands When the DNA molecule is finished replicating the enzymes release the strands and they reform two double helix’s, Each with a parental template strand, and with a new strand from replication.

11. Comparing DNA Replication between Prokaryotes and Eukaryotes In Prokaryotes the circular DNA strand is opened at one origin of replication. In Eukaryotes, with up to one million bases pairs to copy there are multiple origins of replication. Both replicate in two directions, that is to say they are both semiconservative (one parent strand and one new strand).