1. DNA Replication 6.4

2. DNA Replication DNA replication is essential for mitotic & meiotic cell division Is DNA replication semi-conservative or conservative?

3. Semi-conservative?

4. Meselson & Stahl (1958) grew Escherichia coli bacteria in medium containing 15 N (heavy isotope of nitrogen) for 17 generations, ensuring that bacterial DNA was labelled with 15 N.

5. Meselson & Stahl Experiment put the 15 N-labelled bacteria into a medium containing only 14 N and allowed it to grow compared centrifuged samples of the 15 N-labelled bacteria

6. Meselson & Stahl Experiment Click on “Meselson and Stahl Experiment” animation here.here Results of Meselson & Stahl experiments: What would the results have been if DNA replication was conservative?

7. DNA Replication Enzymes Watch “How nucleotides are added in DNA replication” here.here DNA helicaseprimase topoisomerasesDNA polymerase I DNA polymerase IIIDNA ligase

8. DNA Replication

10. Replication Enzymes 1 DNA helicase breaks H-bonds between complementary base pairs, unwinding the double helix single-stranded binding proteins (SSBs) bind to the unwound single strands of DNA to prevent the H-bonds from reforming

11. Replication Enzymes 2 topoisomerases relieve tension caused by the unwinding of the double helix they cut both strands and allow them to swivel around each other, then reseal them

12. Replication Enzymes 3 primase synthesizes an RNA primer of 10-60 base pairs to the template strands DNA polymerase III adds complementary nucleoside triphosphates in the 5’ to 3’ direction to the RNA primer

13. Replication Enzymes 4 the leading strand is built continuously toward the replication fork the lagging strand is built away from the replication fork, and is built in short segments called Okazaki fragments

14. Replication Enzymes 5 DNA polymerase I removes RNA primers (from both the leading and lagging strands) and replaces them with deoxyribonucleotides DNA ligase joins Okazaki fragments into one strand, through creation of phosphodiester bonds

15. Replication Enzymes 6 DNA polymerase II and DNA polymerase I check new strands of DNA for errors as they are synthesized act as exonucleases, excising incorrect nucleotides and adding the correct nucleotides to the strands

16. Differences between Prokaryotic & Eukaryotic Cells there is usually only one replication origin in prokaryotic DNA, and more than one replication origin in eukaryotic DNA

17. Differences between Prokaryotic & Eukaryotic Cells in prokaryotic cells, DNA polymerase I, II, and III function in replication and repair; in eukaryotic cells, there are more than 3 different types of DNA polymerase

18. Self-Study This interactive animation with questions is a good self-study tool. Click on “DNA Replication”.interactive animation

19. Resources “The Biology Project” is a fantastic site hosted by the University of Arizona. At this part of the site there are multiple- choice problem sets. Try the problem sets on Meselson-Stahl and DNA replication. this part of the site

20. Resources (cont’d) I’m not a huge fan of this site, but it’s interesting to see the molecular structures of the DNA & enzymes during the animation of DNA replication.this site