1. THE MOLECULAR BASIS OF INHERITANCE Chapter 16

2. THE SEARCH FOR GENETIC MATERIAL Frederick Griffith (1928) – something changed normal cells into pneumonia causing cells in mice Alfred Hershey & Martha Chase (1952) – DNA of virus injected into bacteria, not protein

3. Figure 16.1 Transformation of bacteria

4. Figure 16.2a The Hershey-Chase experiment: phages

5. Figure 16.2ax Phages

6. Figure 16.2b The Hershey-Chase experiment

7. Erwin Chargaff (1947) – Amounts of G = C and A = T in DNA Rosalind Franklin (1950’s) – Photographed DNA using X-ray crystallography James Watson and Francis Crick (1953) – DNA is a double helix with base pairs as the rungs

8. Figure 16.4 Rosalind Franklin and her X-ray diffraction photo of DNA

9. Figure 16.3 The structure of a DNA stand

10. Figure 16.0 Watson and Crick

11. DNA STRUCTURE 4 Bases – Purines: adenine and guanine – Pyrimidine: cytosine and thymine – A – T and C – G Hydrogen bonds connect the base pairs to make the rungs Deoxyribose and phosphate make up the uprights Nucleotide – a base, a sugar, and a phosphate

12. Purine and pyridimine

13. Figure 16.6 Base pairing in DNA

14. Figure 16.5 The double helix

15. SEMICONSERVATIVE MODEL OF REPLICATION When DNA copies itself, the resulting “copy” is one of the original strands with a new strand

16. Figure 16.7 A model for DNA replication: the basic concept

17. Figure 16.8 Three alternative models of DNA replication

18. Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 4)

19. Figure 16.10 Origins of replication in eukaryotes

20. DNA REPLICATION Why/when does DNA replicate? Every time a cell divides (mitosis or meiosis) the DNA is copied in interphase (S of the cell cycle). Copying the DNA ensures that the cells created will get the right number and kind of chromosomes during cell division (mitosis or meiosis).

21. DNA REPLICATION Origins of replication – specific sequences of nucleotides that initiate replication Replication fork – a Y shaped region where new nucleotides are added DNA polymerases – enzymes that break H bonds (unwind helix) and add complementary nucleotides Reaction driven by nucleoside triphosphates (like ATP) Loose 2 phosphate groups to be added as a nucleotide

22. Figure 16.11 Incorporation of a nucleotide into a DNA strand

23. The 2 strands of a double helix are antiparallel The sugar-phosphate backbones run in opposite directions One is 5’ to 3’ while other is 3’ to 5’ #1 is the C attached to a base and #5 is the C attached to phosphate

24. Figure 16.12 The two strands of DNA are antiparallel

25. DNA polymerases can only attach nucleotides to the free 3’ end of existing an polynucleotide (already paired to complementary strand). DNA polymerases add nucleotides in only the 5’ to 3’ direction Leading strand – made by DNA polymerase following replication fork

26. Figure 16.13 Synthesis of leading and lagging strands during DNA replication

27. The other strand of DNA that is made is called the lagging strand Polymerase makes a short strand As bubble grows, more short strands are made called Okazaki fragments

28. Figure 16.14 Priming DNA synthesis with RNA

29. Okazaki fragments are joined by DNA ligase Priming: polymerase must attach nucleotides to an existing polynucleotide at a 3’ end Primase (an enzyme) adds RNA nucleotides to make a primer ~10 bases long

31. Polymerase can then add its complementary nucleotides Polymerase also replaces certain RNA nucleotides of primer For lagging strand each fragment must be primed, but only one primer is needed for entire leading strand

33. DNA Polymerases DNA polymerase I – removes primers from 5’ end of leading strand and each Okazaki fragment and replaces it with DNA nucleotides DNA polymerase III – adds nucleotides to both leading and lagging strand

34. OTHER ENZYMES… Helicase – untwists helix at replication fork Topoisomerase – relieves the strain that untwisting causes ahead of the replication fork Single-strand binding proteins – hold strands apart while replication occurs

35. Figure 16.15 The main proteins of DNA replication and their functions

36. Figure 16.16 A summary of DNA replication

37. PROOFREADING Mismatch repairing – polymerase checks each addition Excision pair – nuclease cuts out bad section of strand Replication Animation 1 Replication Animation 2 (can see multiple replication origins) Replication Animation 2

38. Figure 16.17 Nucleotide excision repair of DNA damage

39. Figure 16.18 The end-replication problem

40. TELOMERES Impossible on lagging strand to copy the end of 5’ strand This leaves a gap that would shorten DNA every time it replicates Telomeres (TTAGGG) repeated many times protects genes by postponing erosion of genes from this shortening effect Telomerase – lengthens telomeres – Contains an RNA sequence that is the template for a telomere – Present in germ cells (for future gametes) – Increased activity in cancer cells (allows for more cell division)

41. Figure 16.19a Telomeres and telomerase: Telomeres of mouse chromosomes