1. The Discovery of DNA

2. ORIGINAL QUESTION: Which molecule contains genetic information?

3. Experiments Frederick 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. 327

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. Conclusion = Genes are made of DNA
 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. 12.3

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 Fig. 12.4

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. 12.6

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. 12.8

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. 12.9

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