Before we begin to study in detail the structure and activities of genes, and the experimental evidence underlying those concepts, we need a fuller outline of the adventure that lies before us. Thus, in this chapter and in chapter 3, we will fl esh out the brief history of molecular biology presented in chapter 1. In this chapter we will begin this task by considering the behavior of genes as molecules. | Molecular Biology Fifth Edition Chapter 2 The Molecular Nature of Genes Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Nature of Genetic Material Historical Background Miescher isolated nuclei from pus (white blood cells) in 1869 Found a novel phosphorus-bearing substance = nuclein Nuclein is mostly chromatin, a complex of DNA and chromosomal proteins End of 19th century – DNA and RNA separated from proteins Levene, Jacobs, et al. characterized basic composition of DNA and RNA 2- Molecular Foundation: Early experiments that explored the question: What is the genetic material? Key experiments performed by Frederick Griffith in 1928 Observed change in Streptococcus pneumoniae — from avirulent (R) rough colonies, bacteria without capsules, to virulent (S) smooth colonies, bacteria that had capsules Result: Heat-killed virulent bacteria could transform avirulent bacteria into virulent . | Molecular Biology Fifth Edition Chapter 2 The Molecular Nature of Genes Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Nature of Genetic Material Historical Background Miescher isolated nuclei from pus (white blood cells) in 1869 Found a novel phosphorus-bearing substance = nuclein Nuclein is mostly chromatin, a complex of DNA and chromosomal proteins End of 19th century – DNA and RNA separated from proteins Levene, Jacobs, et al. characterized basic composition of DNA and RNA 2- Molecular Foundation: Early experiments that explored the question: What is the genetic material? Key experiments performed by Frederick Griffith in 1928 Observed change in Streptococcus pneumoniae — from avirulent (R) rough colonies, bacteria without capsules, to virulent (S) smooth colonies, bacteria that had capsules Result: Heat-killed virulent bacteria could transform avirulent bacteria into virulent bacteria 2- Outline of Griffith’s Transformation Experiments 2- DNA: The Transforming Material In 1944 Avery, Macleod and McCarty used a transformation test similar to Griffith’s procedure taking care to define the chemical nature of the transforming substance Techniques used excluded both protein and RNA as the chemical agent of transformation Exclusion of DNA verified that DNA is the chemical agent of transformation of S. pneumoniae from avirulent to virulent 2- Analytical Tools Physical-chemical analysis has often used: Ultracentrifugation Used to estimate size of material Electrophoresis Indicated high charge-to-mass ratio Ultraviolet Absorption Spectrophotometry Absorbance of UV light matched that of DNA Elementary Chemical Analysis Nitrogen-to-phosphorus ratio of , expected for DNA but lower than expected for protein 2- Confirmation for DNA as the genetic material In the 1940s geneticists doubted the use of DNA as the genetic material as it appeared to be .
