1. DNA

2. DNA Contained in chromosomes containing DNA and protein
Nucleic acid is made up of nucleotides
Nitrogenous base
Deoxyribose sugar
Phosphate

4. Sugar–phosphate backbone 5 end Sugar (deoxyribose) 3 end
Fig. 16-5
Sugar–phosphate backbone  end
Nitrogenous bases
Thymine (T)
Adenine (A)
Figure 16.5 The structure of a DNA strand
Cytosine (C)
Phosphate
DNA nucleotide
Sugar (deoxyribose)  end
Guanine (G)

5. Important Scientists in the Discovery of DNA
Frederick Griffith
Oswald Avery
Alfred Hershey and Martha Chase
Rosalind Franklin
Francis Crick and James Watson

6. Mice and Transformation: Griffith’s Experiment

7. Fredrick Griffith 1928 Studied Streptococcus pneumoniae 2 strains
One pathogenic
One harmless

8. Frederick Griffith
Hypothesized that when the live harmless bacteria was mixed with heat-killed disease causing, some “factor” was transferred from the heat-killed bacteria into the live bacteria
Transforming factor might be a gene

9. Fredrick Griffith Transformation: Definition: change
Harmless bacteria became harmful when mixed with heat killed bacteria

10. Oswald Avery Repeated Griffith’s work (1944)
Made extract from the heat-killed bacteria
Tested three possible factors for transformation
DNA
RNA
Protein
What was his conclusion? What factor allowed transformation to occur?

11. Alfred Hershey and Martha Chase
1952
The Hershey-Chase Experiment
Studied viruses (bacteriophages)
Viruses: non-living particles smaller than a cell that can invade living cells

12. Phage head Tail sheath Tail fiber DNA 100 nm Bacterial cell Fig. 16-3
Figure 16.3 Viruses infecting a bacterial cell
DNA
100 nm
Bacterial
cell

13. Hershey and Chase Specifically looked at T2
T2 invades Escherichia coli bacteria
Radioactive isotope of sulfur marked protein coat
Radioactive isotope of phosphorus marked DNA
What did they find?

14. Fig. 16-4-1 EXPERIMENT Radioactive protein Phage Bacterial cell DNA
Batch 1: radioactive sulfur (35S)
DNA
Radioactive DNA
Figure 16.4 Is protein or DNA the genetic material of phage T2?
Batch 2: radioactive phosphorus (32P)

15. Fig. 16-4-2 EXPERIMENT Empty protein shell Radioactive protein Phage
Bacterial cell
Batch 1: radioactive sulfur (35S)
DNA
Phage DNA
Radioactive DNA
Figure 16.4 Is protein or DNA the genetic material of phage T2?
Batch 2: radioactive phosphorus (32P)

16. Fig. 16-4-3 EXPERIMENT Empty protein shell
Radioactivity (phage protein) in liquid
Radioactive protein
Phage
Bacterial cell
Batch 1: radioactive sulfur (35S)
DNA
Phage DNA
Centrifuge
Radioactive DNA
Pellet (bacterial cells and contents)
Figure 16.4 Is protein or DNA the genetic material of phage T2?
Batch 2: radioactive phosphorus (32P)
Centrifuge
Radioactivity (phage DNA) in pellet
Pellet

18. X-Ray Evidence Rosalind Franklin
Used X-Ray crystallography to find out structure of DNA molecules
Diffracts light to reveal image

19. X-Ray Evidence X near center shows DNA twists around center
Angle of the X suggests two strands and the nitrogenous bases (hydrophobic) are near the center of the molecule
Shows diameter of the double helix

20. The Double Helix Francis Watson and James Crick
Built three-dimensional models of DNA
Used Rosalind Franklin’s x-ray pictures of DNA to assist in the model
The Double Helix

21. The Double Helix
Double helix: two strands of DNA wound around each other
Looks like a twisted ladder
Will have a uniform diameter
Realized that hydrogen bonds held the two strands together (A with T; C with G)
Two hydrogen bonds between A and T
Three hydrogen bonds between C and G

22. Nucleotides Make up DNA molecules Two Types
Purines – two rings in the structure
Adenine (A)
Guanine (G)
Pyrimidines – one ring in the structure
Cytosine (C)
Thymine (T)

23. Chargaff’s Rule
Chargaff studied percentages of nitrogenous bases (1950)
Percentage of guanine and cytosine are almost equal
Percentages of adenine and thymine are almost equal
Chargaff’s Rule supports idea that Adenine (A) bonds to Thymine (T) and Cytosine (C) bonds to Guanine (G)

24. p
A fly has the following percentages of nucleotides in it’s DNA
27.3% A
27.6% T
22.5% G
22.5% C
How do these numbers demonstrate Chargaff’s rule?

25. p
How did Watson and Crick’s model explain the basis for Chargaff’s rule?
If transformation had not occurred in Griffith’s experiment, how would the results have differed? Explain.