1. Chapter 16 Molecular Basis of Inheritance

2. DNA genetic material Chromosomes composed of DNA + protein

3. DNA base composition Nucleotide base Guanine Cytosine Thymine Adenine Guanine, C 5 H 5 N 5 O

4. DNA is a polymer of nucleotides

5. Chargaff’s rules (1950) [T] = [A] [G] = [C] A certain chromosome is 19% A. What is the % of C ?

6. DNA structural model Watson, Crick, Franklin 1953 X-ray crystallography DNA is helical Spacing of bases Width of helix suggested 2 strands

7. DNA double helix Sugar-phosphate “backbone” Anti parallel strands

8. Bases face inward Hydrogen bonds connect bases

9. A - T (2 bonds) G - C ( 3 bonds)

10. Original DNA copied to new DNA helix Original DNA broken up and combined in new DNA 1 strand original DNA maintained in new DNA

11. Meselson and Stahl 1950s 1.Label DNA (E. coli) with 15 N in growth media 2. Transfer E. coli to 14 N media for 1 generation (20 min)

12. Results: The density of the DNA is intermediate Cells grown longer 14 N, make lighter DNA

13. What would the DNA density be after 20 more minutes of cell group? 15 N DNA1.724 gm/cm 3 14 N DNA 1.710 gm/cm 3

14. DNA replication is semi-conservative

15. DNA replication: mechanism (E. coli) E. coli genome = 4 X 10 6 bp DNA 1 circular chromosome 1 origin of replication (ori)

16. Ori nucleotides – Replication proteins attach to ori – Forms a replication bubble Two strands of DNA open

17. Replication fork in both directions

18. Proteins in DNA replication Table 16.1 1.DNA polymerase (enzyme) Adds nucleotides 5’  3’ direction only

19. 2. Helicase (enzyme) – unwinds double helix 3. Single stranded binding protein (SSB) binds to DNA strands to stabilize them

20. 4. Topoisomerase (enzyme) – breaks, rejoins DNA to relieve physical stress 5. Primase – synthesizes a primer

21. Each strand is a template for new DNA Leading strand is Lagging strand is

23. DNA replication leading strand: steps 1.Primase (enzyme) – synthesizes primer complementary to leading strand – primer is ~10 bases

24. 2. DNA polymerase (pol III) synthesizes new strand 5’  3’ G, A, T, C nucleotides complementary to template strand 500 nuc/sec Continuous elongation until end of chromosome

25. DNA Synthesis steps: lagging strand 1. Primase makes RNA primer 2. DNA adds nucleotides to primer in 5’  3’ direction only

26. 3. DNA pol III detaches Okazaki fragment ~ 1, 000 nucleotides long

27. 4. Another primer added, another Okazaki fragment formed Many primers needed

28. 5. Gaps between primers filled in 6. Ligase enzyme bonds fragments

29. DNA replication Fig. 16.17

30. Telomeres, the protective ends

31. Linear DNA has telomeres No genes Repetitive DNA TTAGGG up to 1000 times Human chromosomes capped by telomeres 5 '...TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG..3‘ 3'...AATCCC AATCCC AATCCC AATCCC AATCCC AATCCC..5 '

32. Chromosomes shorten with each cell division

33. When telomeres are too short  cell senescence (irreversible) ~ 125 cell divisions (humans)……life span? Telomeres shorten ~100 bp each time cell divides Mouse fibroblasts in culture

34. Cells that do not divide often – Example: heart muscle Telomeres do not shorten with age

35. Lagging strand problem Animation garland

36. Embryonic cells, some wbc, stem cells, cancer cells express telomerase White blood cell cervical cancer cellembryo