Chapter 16: The Molecular Basis of Inheritance

1. Chapter 16: The Molecular Basis of Inheritance

2. Essential Knowledge • 3.a.1 – DNA, and in some cases RNA, is the primary source of heritable information (16.1 & 16.2). • 3.c.1 – Changes in genotype can result in changes in phenotype (16.2).

3. Question? • Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? • Two possibilities: • Protein • DNA

4. Qualifications • Protein: • Until 1940s, evidence for protein was STRONG! • Very complex structure • High specificity of function • DNA: • Simple structure • Not much known about it (early 1900’s)

5. Griffith - 1928 • Pneumonia in mice • Two strains: • S – pathogenic (caused pneumonia) • R - harmless

6. Griffith’s Experiment

7. Result • Something turned the R cells into S cells (in 4th experiment) • Transformation - the assimilation of external genetic material by a cell • And…the pathogenic trait was inherited by all new offspring!

8. Problem • Griffith used heat • Heat denatures proteins • DNA – heat stable • Then, could proteins still be the genetic material? • Griffith’s results were contrary to accepted views

9. Avery, McCarty and MacLeod - 1944 • Repeated Griffith’s experiments, but added specific fractions of S cells • Result - only DNA transformed R cells into S cells

10. Avery, cont. • Experiment not believed • Why? • Scientists thought bacteria make-up was considerably different from humans/other living organisms

11. Hershey- Chase 1952 • Genetic information of a virus or phage • Phage • Virus that attacks bacteria and reprograms host to produce more viruses (by injecting its own DNA)

12. Virus Intro • DNA and/or RNA core • Enclosed by envelope • Made of protein • To reproduce, a virus must attach to a cell and inject its genetic info (either RNA/DNA) INTO the cell

14. Bacteria with Phages

15. Phage Components • Hershey/Chase knew viruses reproduced, but didn’t know what was injected… • Two main chemicals: • Protein • DNA

16. Hershey/Chase used tracers • Radioactive isotope tracers • Protein - CHONS, can trace with 35S • DNA - CHONP, can trace with 32P

17. Experiment • Used phages labeled with one tracer or the other and looked to see which tracer entered the infected bacteria cells • Hershey - Chase movie

19. Result • DNA enters the host cell, but the protein did not • Therefore, DNA is the genetic material that is passed down

20. Picture Proof

21. Watson and Crick - 1953 • Used X-ray crystallography data • Used model building • Result - Double Helix Model of DNA structure • One page paper, 1953

23. Rosalind Franklin

24. Rosalind Franklin • Also used x-ray crystallography • Determined DNA had two strands • Died in 1958 • Her colleague got Nobel Prize (because Franklin published under his name!)

25. DNA Composition • Made of nucleotides: (3 parts) • Deoxyribose Sugar (5-C ring) • Phosphate (PO4-) • Nitrogen Bases: A,T,C,G • Purines: A,G • Pyrimidines: C,T

26. DNA Backbone • Polymer of sugar - phosphate • 2 backbones present • Phosphate of one nucleotide is attached to sugar of the next • Alternates sugar-phosphate

27. Nitrogen Bases • Bridge the backbones together • Purine + Pyrimidine = 3 rings • Keeps a constant distance between the 2 backbones • Nucleotide held together by H-bonds

30. Chargaff’s Rule • Studied chemical composition of DNA • Found: • the nucleotides were found in certain ratios • % composition differed between species

31. Chargaff’s Rule • A = T • G = C • Example: in humans A = 30.9% T = 29.4% G = 19.9% C = 19.8%

32. Chargaff’s Rule • Explained by double helix model • %A = %T, 3 ring distance • %G = %C, 3 ring distance

34. Pyrimidines Purines

35. Watson and Crick • Published a second paper (1954) that speculated on the way DNA replicates • Proof of replication given by others

36. Replication • The process of making more DNA (from existing DNA) • Completed during S-phase of Interphase • Problem: When cells replicate, the genome must be copied exactly • How is this done?

37. Models for DNA Replication • Conservative – • one old strand, one new strand • Semiconservative – • each strand is 1/2 old, 1/2 new • Dispersive – • strands are mixtures of old and new

38. Replication Models

39. Meselson - Stahl late 1950’s • Grew bacteria on two isotopes of N • Started on 15N, switched to 14N • Looked at weight of DNA after one, then 2 rounds of replication • Results: • Confirmed the Semiconservative Model of DNA replication • Parent strand serves as a template

41. Replication - Preview • DNA splits by breaking the H-bonds between the backbones. • Then DNA builds the missing backbone using the old backbone as a template. • DNA is replicated in only a few hours.

43. Origins of Replication • Specific sites on the DNA molecule that start replication. • Recognized by a specific DNA base sequence. • Proteins/enzymes initiate replication

44. Prokaryotic replication • Ex: bacteria (E. coli) • Circular DNA • 1 origin site • Replication runs in both directions from the origin site

45. Eukaryotic replication • Many origin sites. • 100s/1000s • Replication bubbles fuse to form new DNA strands. • Faster replication (usually) • Replication also runs in both directions from origin site

47. DNA Elongation • Done so by DNA Polymerases • Adds DNA triphosphate monomers to the growing replication strand • These triphosphate contain the complementary nucleotides • Matches A to T and G to C

48. Energy for Replication • Exergonicrxn • Comes from the triphosphate monomers. • Loses two phos as each monomer/nucleotide is added. • Similar to ATP cycle • ATP contains ribose sugar • DNA = deoxyribose

50. Problem of Antiparallel DNA • The two DNA strands run antiparallel to each other • Two “ends” of strand • 3` - sugar/OH end • 5` = phosphate end • New DNA strand can only elongate in the 5` 3` direction • Old DNA strand 3’  5’