2. 1 BIOLOGY PRESENTATIONS FOR A-LEVEL, IN MULTIMEDIA JAMES BOWLES, 2001 BIOLOGY PRESENTATIONS FOR A-LEVEL, IN MULTIMEDIA JAMES BOWLES, 2001 LEFT CLICK OR PRESS SPACE BAR TO ADVANCE, PRESS P BUTTON TO GO BACK, PRESS ESC BUTTON TO END LEFT CLICK OR PRESS SPACE BAR TO ADVANCE, PRESS P BUTTON TO GO BACK, PRESS ESC BUTTON TO END

3. 2 ‘A’ Level Biology DNA Replication

4. 3 Introduction Theoretically there are two possible ways in which DNA could replicate

5. 4 This would be CONSERVATIVE replication

6. 5 This would be SEMI- CONSERVATIVE replication

7. 6 Experiments in 1958 by Meselsohn and Stahl showed that DNA replication is semi-conservative

8. 7 Meselsohn and Stahl Culture of E.coli E.coli grown for many generations with heavy isotope of nitrogen ( 15 N)

9. 8 Meselsohn and Stahl Extracted DNA was centrifuged and a ‘heavy’ band was established

10. 9 The E.coli were transferred to a culture containing 14 N, for one generation

11. 10 Meselsohn and Stahl A lighter band for the DNA was established when it was centrifuged

12. 11 Meselsohn and Stahl The E.coli E.coli were grown for successive generations on the medium containing the ‘light’ isotope

13. 12 Meselsohn and Stahl Two bands now appear, one is an intermediate and one represents the ‘light’ isotope

14. 13 DNA Replication

15. 14 DNA Replication DNA strands run anti- parallel to one another 5’3’ 3’5’

16. 15 DNA Replication The enzyme Helicase unwinds the DNA double helix 5’3’ 3’5’

17. 16 DNA Replication DNA polymerase attaches to the ‘leading’ strand 5’3’ 3’5’

18. 17 DNA Replication DNA polymerase synthesises a new DNA strand in the 3’ to 5’ direction 5’3’ 3’5’

19. 18 DNA Replication Another DNA polymerase molecule attaches to the ‘lagging’ strand 5’3’ 3’5’

20. 19 DNA Replication DNA is synthesised in the 3’ to 5’ direction 5’3’ 3’5’

21. 20 DNA Replication Helicase continues to unwind the DNA double helix5’3’3’5’

22. 21 DNA Replication Continuous synthesis proceeds on the ‘leading’ strand5’3’3’5’

23. 225’3’3’5’ DNA Replication DNA polymerase detaches from the ‘lagging’ strand, and rejoins further down

24. 23 DNA Replication DNA polymerase detaches from the ‘lagging’ strand, and rejoins further down 5’3’ 3’5’

25. 24 DNA Replication DNA polymerase detaches from the ‘lagging’ strand, and rejoins further down 5’3’ 3’5’

26. 25 DNA Replication DNA polymerase detaches from the ‘lagging’ strand, and rejoins further down 5’3’ 3’5’

27. 26 DNA Replication DNA polymerase detaches from the ‘lagging’ strand, and rejoins further down 5’3’ 3’5’

28. 27 DNA Replication DNA polymerase detaches from the ‘lagging’ strand, and rejoins further down 5’3’ 3’5’

29. 28 DNA Replication Synthesis of DNA continues on the ‘lagging strand’5’3’3’5’

30. 29 DNA Replication DNA polymerase is unable to join up the fragments on the lagging strand5’3’3’5’

31. 30 DNA Replication This is done by the enzyme Ligase5’3’3’5’

32. 31 DNA Replication This is done by the enzyme Ligase 5’3’ 3’5’

33. 32