1. DNA
The Genetic Material

2. IDENTIFICATION OF DNA AS THE GENETIC MATERIAL
The genetic material must meet 4 criteria
1. Information:
2. Transmission:
3. Replication:
4. Variation:
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3. 1928 - Frederick Griffith’s Transformation Experiments
Two strains of Streptococcus pneumoniae
S  Smooth
Secrete a polysaccharide capsule
Produce smooth colonies on solid media
virulent
R  Rough
Unable to secrete a capsule
Produce colonies with a rough appearance
avirulent
Two coat types II
III
Four possible phenotypes
IIS or IIIS or IIR or IIIR

4. Griffith’s Experiments
Figure 9.1
Rare mutations convert strains
S into R strain & vice versa
But mutations do not change coat types
II into III or vice versa
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5. Figure 9.2
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6. Griffith’s Conclusions
Something from the dead type IIIS transformed type IIR into type IIIS
Called this process transformation
The unknown substance was termed the transforming principle
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7. 1944 - Avery, MacLeod & McCarty Identify the Genetic Material
Griffith’s transforming principle was the genetic material
Transformation assay to identify actual biomolecule
Major constituents - DNA, RNA, proteins, carbohydrates, & lipids
Made cell extracts from type IIIS cells containing each of these macromolecules
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8. Avery’s Experiments
Mixed each extract with type IIR cells to test for transformation
Only extract containing purified DNA transformed type IIR to type IIIS
Verify that DNA, not RNA or protein, is the genetic material
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Figure 9.3

9. 1952- Hershey & Chase Confirm DNA is Genetic Material Using Phage T2

10. Life cycle of phage T2
Figure 9.5

11. Phage Attached to E. coli Cell
Figure: 10-04a
Caption:
EM of E. coli during infection by phage T2.

12. The Hershey and Chase experiment:
Used radioisotopes to distinguish DNA from proteins
32P labels DNA specifically
35S labels protein specifically
Infect non-radioactive E. coli with radioactively-labeled phages
Remove phage coats from cells
Is 32P or 35S inside bacteria?
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13. Hershey & Chase Experiment
Figure: 10-05a
Caption:
Summary of the Hershey–Chase experiment demonstrating that DNA, and not protein, is responsible for directing the reproduction of phage T2 during the infection of E. coli.

14. Hershey & Chase Experiment
Figure: 10-05b
Caption:
Summary of the Hershey–Chase experiment demonstrating that DNA, and not protein, is responsible for directing the reproduction of phage T2 during the infection of E. coli.

15. The Data
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16. RNA is Genetic Material in Some Viruses
In 1956, A. Gierer and G. Schramm isolated RNA from the tobacco mosaic virus (TMV)
Purified TMV RNA caused the same lesions as intact TMV viruses
Therefore, the viral genome is composed of RNA
Since that time, many RNA viruses have been found
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17. 9-21
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