DNA AS A GENETIC MATERIAL Experiments for genetic

DNA AS A GENETIC MATERIAL Experiments for genetic material---nucleic acid or protein/DNA or RNA? • • • Griffith’s Transformation Experiment Avery’s Transformation Experiment Hershey-Chase Bacteriophage Experiment

Search for the genetic material: 1. Stable source of information 2. Ability to replicate accurately 3. Capable of change 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 • 1953 Watson & Crick propose double-helix model of DNA

• Fred Griffith (1928) – Experiments with pneumonia and bacterial transformation determined that there is a molecule that controls inheritance. • Oswald T. Avery (1944) - Transformation experiment determined that DNA was the genetic material responsible for Griffith’s results (not RNA). • Hershey-Chase Experiments (1952) – discovered that DNA from viruses can program bacteria to make new viruses.

Frederick Griffith’s Transformation Experiment • The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 • Griffith worked with two strains of a bacterium, a pathogenic “S” strain and a harmless “R” strain • When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic • He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA

Frederick Griffith’s Transformation Experiment - 1928 “transforming principle” demonstrated with Streptococcus pneumoniae Griffith hypothesized that the transforming agent was a “IIIS” protein.

Oswald T. Avery’s Transformation Experiment • In 1944, Oswald Avery, Maclyn Mc. Carty, and Colin Mac. Leod announced that the transforming substance was DNA • Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria • Many biologists remained skeptical, mainly because little was known about DNA

Oswald T. Avery’s Transformation Experiment - 1944 Determined that “IIIS” DNA was the genetic material responsible for Griffith’s results (not RNA). Peter J. Russell, i. Genetics: Copyright © Pearson Education, Inc. , publishing as Benjamin Cummings.

Hershey-Chase Bacteriophage Experiment - 1953 Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery. Structure of T 2 phage DNA & protein

Life cycle of virulent T 2 phage:

Hershey-Chase Bacteriophage Experiment - 1953 1. T 2 bacteriophage is composed of DNA and proteins: 2. Set-up two replicates: • • Label DNA with 32 P Label Protein with 35 S 3. Infected E. coli bacteria with two types of labeled T 2 4. 32 P is discovered within the bacteria and progeny phages, whereas 35 S is not found within the bacteria but released with phage ghosts. 1969: Alfred Hershey