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

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Presentation on theme: "Historical perspectives- Early evidence, DNA is the genetic material-II 1 Lecture no. 3."— Presentation transcript:

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

2 Outline 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 2 Lecture no. 3

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

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

5 Search for genetic material 5 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. Lecture no. 3

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

7 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. Lecture no. 3

8 Continue… 8 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. 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 Lecture no. 3

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

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

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

12 Continue… Continue… Hershey-Chase Bacteriophage Experiment 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 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 1953 Lecture no. 3

13 13 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? Lecture no. 3

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

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

16 16 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 Lecture no. 3

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


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