The Discovery of DNA

1. The Discovery of DNA

2. ORIGINAL QUESTION: Which molecule contains genetic information?

3. ExperimentsFrederick Griffith (1928) - English A. Smooth bacteria → mouse = dies of pneumonia B. Rough bacteria → mouse = lives C. Heat-killed smooth bacteria → mouse = lives D. Heat-killed smooth bacteria + → mouse = dies of pneumonia Live rough bacteria

4. Fig. _____ p. _____

5. Fredrick Griffith

6. Griffith Conclusion: Some molecules changed the harmless rough bacteria into deadly smooth bacteria → transformation!

7. Oswald Avery (1944) - Canadian Question: What molecule was needed for transformation? • Avery examined 4 macromolecules – destroying 1 at a time 1. Lipids 3. Proteins 2. Carbohydrates 4. Nucleic Acids (DNA)

8. Oswald Avery

9.  Transformation was blocked ONLY when DNA was destroyed Conclusion = Genes are made of DNA

10. Alfred Hershey & Martha Chase (1952) - American Bacteriophage = a virus that infects bacteria Made of protein and DNA Question: Which enters a bacterium during infection? They used “tracers” = radioactive isotopes 1. phosphorus-32 → part of DNA (not proteins) 2. sulfur-35 → part of proteins (not DNA)

11. Question: Which enters a bacterium during infection? They used “tracers” = radioactive isotopes • phosphorus-32 → part of DNA (not proteins) • sulfur-35 → part of proteins (not DNA)

13. Fig. _____

14. Hershey & Chase Procedure: A) Let bateriophages infect bacteria B) Analyzed bacteria for radioactivity C) Found phosphorus-32 sample made the bacteria radioactive Conclusion: Genetic material is DNA NOT protein

15. Martha Chase & Alfred Hershey

16. DNA Structure

17. Nucleic acids = chain of nucleotides (polymer) Nucleotides consist of : 1. 5-carbon sugar 2. phosphate group 3. nitrogenous base

18. Nucleotides______

19. DNA: deoxyribonucleic acid is a nucleic acid  Made of 4 different nucleotides: 1. adenine 3. guanine 2. cytosine 4. thymine

21. Chargaff’s Rules - Erwin Chargaff (1950) - American In samples of DNA: • The amounts of adenine (A) and thymine (T) were equal • The amounts of cytosine (C) and guanine (G) were equal Chargaff’s Rule: C = G and T = A

22. Erwin Chargaff

23. Rosalind Franklin &Maurice Wilkins (1952) – British • Used X-Ray diffraction to analyze DNA molecules • These X-Ray patterns helped to solve the structure of DNA

24. Rosalind Franklin

26. Fig. ______

27. Maurice Wilkins

28. James Watson & Francis Crick (1953) – American & English Solved the mystery! • DNA is made of 2 strands twisted in a “double helix” • “Base pairs” are formed between nitrogenous bases (A, T, C, G) • Hydrogen bonds formed between A and T • Hydrogen bonds formed between C and G

29. Frances Crick James Watson

30. Watson & Crick Kind of like a twisted ladder! • Rails of the ladder are strands of alternating deoxyribose and phophate • One rail runs up, the other runs down • The steps or rungs are the pairs of bases (C-G or T-A)

32. DNA Structure: Fig. ____

33. Nucleosomes • Tiny particles that help to fold and package DNA so it can fit inside the nucleus • If all of the DNA in one human cell were laid out end to end, it would be almost 2 meters long!

34. Nucleosomes: Fig. _____

35. DNA Replication - Semiconservative

36. QUESTION: How does DNA replicate?

37. Semiconservative Replication • Enzymes separate the 2 strands • New strands are synthesized • This occurs during the “S” phase of the cell cycle

39. Fig. ____

40. Why must DNA be replicated? • When a cell divides, each new cell needs a copy of the genetic material

41. Semiconservative Replication A DNA strand that looks like… A T A G C G C A T T A C T A T C G C G T A A T G

42. …can be untwisted, broken apart, and have new nucleotides paired with BOTH of the original sides, forming 2 new strands. After the DNA strand is replicated it will look like this: A T A G C G C A T T A C T A T C G C G T A A T G A T A G C G C A T T A C T A T C G C G T A A T G