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Lecture no. 3 Historical perspectivesEarly evidence , DNA is the genetic material-II 1 Outline 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 2 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.. 3 Lecture no. 3 • Until the 1940s, the case for protein seemed more likely. Why? 4 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. 5 Lecture no. 3 1928 1944 Griffith’s Transformation Experiment Avery’s Transformation Experiment Toward The Real Conclusion 1953 Hershey-Chase Bacteriophage Experiment 1956 Tobacco Mosaic Virus (TMV) Experiment 6 Lecture no. 3 First Real BreakFrederick 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 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. 8 Lecture no. 3 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 Continue… Avery’s Transformation Experiment 1944 When DNA was destroyed, the transforming activity was lost, but when DNA was left intact, the transforming activity survived 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 Lecture no. 3 Continue… Hershey-Chase Bacteriophage Experiment 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 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? 13 RNA as Genetic Material Lecture no. 3 - 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. In conclusion 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 15 Lecture no. 3 Chargaff’s Rules 1949-1953, 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 16 Lecture no. 3 What could these observations suggest? What are possible relationships between (A) and (T), and between (C) and (G)? 17