DNA

1. Chapter 12-1 DNA
(deoxyribonucleic acid)

2. DNA function
• Nucleic acid
• Store and transmit
hereditary information

3. Experimental Proof of DNA as
Genetic Material...

4. 1) Griffith’s Transformation Experiments

5. 1) Griffith’s Transformation Experiments
• What two strains of bacteria did Griffith use?
– S (smooth) strain –cause pneumonia
– R (rough) strain –harmless
• What happened in the fourth experiment?
– The mice died. There were live S strains found in the mice
body.
• What can be concluded from the experiment?
– Cell components in the dead S strains transformed the live R
strains to live S strains

6. 2) Oswald Avery experiment
• The transforming factor is DNA

8. 3) The Hershey-Chase’s bacteriophages
experiment
• Bacteriophages
– viruses that infect bacteria
– consist of protein coat and a
DNA or RNA core
– inject their hereditary
material into bacteria

9. 3) The Hershey-Chase’s bacteriophages
experiment

10. virus particle virus particle
labeled with 35S labeled with 32P
• Radioactive
Markers
– Radioactive bacterial cell (cutaway view)
isotope
– 32P could only
mark DNA
– 35S could only
mark protein
coat label outside
cell
label inside cell

15. DNA
• Made by nucleotide
– 5-carbon sugar
(deoxyribose) 5
– phosphate group
4 1
– nitrogenous bases
3 2
• purine (2 rings):
adenine , guanine;
• pyrimidine (1 ring):
cytosine , thymine

16. 5
4 1
3 2

17. Purines Pyrimidines
Adenine Guanine Cytosine Thymine
1 1 1 1
2 2 2 2
3 3 3 3
5 4 5 4 5 4 5 4
Phosphate group Deoxyribose

18. DNA Structure
• Chargaff’s Rule :
– Amount of adenine = amount of thymine, amount of
guanine = amount of cytosine
– A=T and G=C

19. DNA Structure
• Rosalind Franklin
– Used x-ray diffraction to
examine DNA fibers
– Concluded that DNA was
helix
Rosalind Franklin's X-ray diffraction
photograph of DNA, 1953
Photo: courtesy HarperCollins

20. DNA Structure
• Watson and Crick’s model
– DNA have two strands run in
opposite directions
– Strands are held together by
hydrogen bonds
– A binds with T and C with G
– Molecule is a double helix
Hydrogen bond

21. DNA Structure

22. Nucleotide
Hydrogen bonds
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)

23. James D. Watson Francis H. C. Crick Wilkins
Rosalind Franklin

28. Questions:
• 20% A in DNA strands, how many C ?
• A=T=20%,
• A+T+G+C=100%
• G=C,
• C=30%

29. The drawing below shows a short section of a DNA
molecule. What is labelled by I, II and III?
I
II
III
I II III
A. 3′ end purine hydrogen bond
B. 5′ end pyrimidine covalent bond
C. 3′ end pyrimidine hydrogen bond
D. 5′ end purine covalent bond