1. Lecture no. 3
Historical perspectives- Early evidence , DNA is the genetic material-II

2. Outline How we know DNA is the genetic material?
Lecture no. 3
How we know DNA is the genetic material?
Search for genetic material---is it composed of protein or nucleic acid /DNA or RNA?
Griffith’s Transformation Experiment
Avery’s Transformation Experiment
Hershey-Chase Bacteriophage Experiment
Tobacco Mosaic Virus (TMV) Experiment
Chargaff’s Rules

3. 3 Characteristics of “The Genetic Material”:
Lecture no. 3
Must be a stable form containing information about cell form and function.
Must replicate accurately.
Capable of change to allow evolution..

4. Until the 1940s, the case for protein seemed more likely. Why?
Lecture no. 3
Until the 1940s, the case for protein seemed more likely. Why?

5. Search for genetic material
Lecture no. 3
Timeline of events:
1890: Weismann - substance in the cell nuclei controls development.
1900: Chromosomes shown to contain hereditary information, later shown to be composed of protein & nucleic acids.
1928: Griffith’s Transformation Experiment.
1944: Avery’s Transformation Experiment.
1953: Hershey-Chase Bacteriophage Experiment.
1956: First demonstration that RNA is viral genetic material.

6. Toward The Real Conclusion
Lecture no. 3
1928
1944
Griffith’s Transformation Experiment
Avery’s Transformation Experiment
Toward The Real Conclusion
1953
1956
Tobacco Mosaic Virus (TMV) Experiment
Hershey-Chase Bacteriophage Experiment

7. First Real Break- Frederick Griffith’s Transformation Experiment.
Lecture no. 3
First Real Break- Frederick Griffith’s Transformation Experiment.
Griffith worked with two strains of bacteria Streptococcus pneumoniae, IIR(non virulent) and IIIS (virulent).
His experiment was one of the first suggesting that bacteria are capable of transferring genetic information through a process known as transformation.

8. Lecture no. 3
Continue…
1928
Griffith concluded that the type II-R had been "transformed" into the lethal III-S strain by a "transforming factor“.
Griffith hypothesized that the transforming factor was a “IIIS” protein.

9. Griffith’s “transforming factor” was different from proteins because:
Lecture no. 3
Griffith’s “transforming factor” was different from proteins because:
The heat treatment did not denature the unidentified factor;
The factor was able to transfer information from one type of bacterial cell to another;
The factor transformed the host cells by giving them new properties.

10. Continue… Avery’s Transformation Experiment
Lecture no. 3
1944
When DNA was destroyed, the transforming activity was lost,
but when DNA was left intact, the transforming activity survived

11. Continue… Avery’s Transformation Experiment
Lecture no. 3
1944
When DNA was destroyed, the transforming activity was lost,
but when DNA was left intact, the transforming activity survived.

12. Continue… Hershey-Chase Bacteriophage Experiment
Lecture no. 3
1953
Because DNA and not protein entered the cell, then DNA must be the heritable material (i.e., responsible for the function and reproduction of phage).
A.H. received Nobel Prize in 1969 for this & other work

13. Where was radioactive sulfur found at the end of experiment 1?
Lecture no. 3
Where was radioactive sulfur found at the end of experiment 1?
Where was it found at the end of experiment 2?
Based on the Hershey–Chase experiments, is it reasonable to assume that Griffith’s “transforming factor” was DNA, not protein? Why or why not? What is the connection between the two experiments?

14. - Gierer & Schramm (TMV) Experiment
Lecture no. 3
RNA as Genetic Material
- Gierer & Schramm (TMV) Experiment
1956
Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957
Used 2 viral strains to demonstrate RNA is the genetic material of TMV.

15. In conclusion DNA (not RNA) is transforming agent
Lecture no. 3
DNA (not RNA) is transforming agent
DNA (not protein) is the genetic material
RNA (not protein) is genetic material of some viruses

16. Chargaff’s Rules 1949-1953, evidence for DNA as genetic material.
Lecture no. 3
Chargaff’s Rules , evidence for DNA as genetic material.
Digested many DNAs and subjected products to chromatographic separation
Results: A = T, C = G
A + G = C + T (purine = pyrimidine)
A + T does not equal C + G
Members of a species similar but different species vary in AT/CG ratio

17. What could these observations suggest?
Lecture no. 3
What could these observations suggest?
What are possible relationships between (A) and (T), and between (C) and (G)?