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Lecture no.2 Introduction -2 BCH 361/ Section: xxxxx 1 Lecture Outline Lecture no.2 Terminologies. Early experiments that led to the discover of DNA as the genetic material. cellular components. Next time lecture. 2 3 Characteristics of “The Genetic Material”: Lecture no.2 Must be a stable form containing information about cell form and function. Must replicate accurately. Must be able to change/evolve. Until 1944 it was not known which component of chromosomes was the genetic material. Until 1953 it was not known how DNA could encode genetic information. 3 Early Studies Lecture no.2 Beginning with the earliest observations concerning heredity, genetic material was assumed to exist. Until the 1940s proteins were considered by geneticists to be the best candidates: Very abundant in cells and did nifty things. Nucleic acids were similar, and just a couple of nucleotides connected to each other… Phoebus Levene proposed a tetranucleotide for !!structure DNA Lecture no.2 He though that a DNA molecule contained only four units, each unit contain phosphate-sugar-base -in order- linked together in a repeated manner, i.e. a tetranucleotide. Furthermore, he considered such a simple sequence could not allow DNA any role in coding for anything. This was later to be proved wrong by Chargaff. 5 So… Lecture no.2 It was widely thought that DNA was organized into repeating "tetranucleotides" in a way that could not carry genetic information. Proteins, on the other hand, had 20 different amino acids and could have lots of variation. Most geneticists focused on “transmission genetics” and passively accepted proteins as being the likely genetic material Lecture no.2 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 7 Lecture no.2 First Real BreakFrederick Griffith’s Transformation Experiment. 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.2 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 9 Lecture no.2 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 Lecture no.2 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 RNA as Genetic Material Lecture no.2 - 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.2 DNA (not RNA) is transforming agent DNA (not protein) is the genetic material RNA (not protein) is genetic material of some viruses 13 Lecture no.2 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 14 15