Presentation on theme: "Lecture no. 3 Historical perspectives- Early evidence , DNA is the genetic material-II."— Presentation transcript:
1 Lecture no. 3Historical perspectives- Early evidence , DNA is the genetic material-II
2 Outline How we know DNA is the genetic material? Lecture no. 3How 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 ExperimentAvery’s Transformation ExperimentHershey-Chase Bacteriophage ExperimentTobacco Mosaic Virus (TMV) ExperimentChargaff’s Rules
3 3 Characteristics of “The Genetic Material”: Lecture no. 3Must 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. 3Until the 1940s, the case for protein seemed more likely. Why?
5 Search for genetic material Lecture no. 3Timeline 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. 319281944Griffith’s Transformation ExperimentAvery’s Transformation ExperimentToward The Real Conclusion19531956Tobacco Mosaic Virus (TMV) ExperimentHershey-Chase Bacteriophage Experiment
7 First Real Break- Frederick Griffith’s Transformation Experiment. Lecture no. 3First 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. 3Continue…1928Griffith 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. 3Griffith’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. 31944When 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. 31944When 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. 31953Because 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. 3Where 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. 3RNA as Genetic Material- Gierer & Schramm (TMV) Experiment1956Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957Used 2 viral strains to demonstrate RNA is the genetic material of TMV.
15 In conclusion DNA (not RNA) is transforming agent Lecture no. 3DNA (not RNA) is transforming agentDNA (not protein) is the genetic materialRNA (not protein) is genetic material of some viruses
16 Chargaff’s Rules 1949-1953, evidence for DNA as genetic material. Lecture no. 3Chargaff’s Rules , evidence for DNA as genetic material.Digested many DNAs and subjected products to chromatographic separationResults: A = T, C = GA + G = C + T (purine = pyrimidine)A + T does not equal C + GMembers of a species similar but different species vary in AT/CG ratio
17 What could these observations suggest? Lecture no. 3What could these observations suggest?What are possible relationships between (A) and (T), and between (C) and (G)?
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