Identifying the Molecule of Inheritance Predicting the Results of the Griffith Experiment and (Re)-Designing the Avery, MacLeod and McCarty Experiment

Objectives Hypothesize outcomes for a given experiment given the data collected. Design an experiment to conclude whether protein, DNA or RNA is the genetic material. Explain why DNA makes a better genetic material than protein.

Griffith Experiment: Results

Griffith Experiment: What’s Going On? What are TWO hypotheses that might explain the unexpected results of the experiment? 8000 BCE Selective Breeding 400 BCE Tiny particles from every part of the body mix from the “fluids” of each parent 1700s Preformation An organism contains all of its future descendants encased in miniature form Homunculus 1800s Heredity is a blending process

Bacterial Phenotypes S (smooth) strain R (rough) strain The pneumococcus bacterium was a pathogenic bacterium that causes pneumonia in animals and humans. A polysaccharide capsule surrounds some strains of S. pneumonia is essential for its pathogenicity Layer of polysaccharides that lies outside the cell wall of some bacteria It enhances the ability of bacteria to cause disease because it prevents phagocytosis of the bacterium by a white blood cell (macrophages) The capsule can also exclude bacterial viruses, protect against desiccation, and prevent uptake of antibiotics S (smooth) strain R (rough) strain

Transformation Transformation: A change in phenotype. Includes “transformation” in the context of cancer – a normal cell taking on a cancerous phenotype, including uncontrolled rate of growth, ability to metastasize (spread from primary site), etc. In this case, “transformation” refers to non-pathogenic (non-infectious) bacteria taking on the properties / phenotype of pathogenic bacteria. The R strain was transformed into the pathogenic S strain when mixed together, and pneumonia and death ensued in infected mice This new “S” bacteria replicated like S strain bacteria, and possessed a capsule like the S strain Some thing in the heat-killed bacteria could still transmit disease causing properties to the living “rough” bacteria Transforming Principle In 1928, when Frederick Griffith discovered the phenomenon of transformation in the non-pathogenic, “rough” bacteria. He proposed the concept of the “transforming principle.” Some part of the dead S strain bacteria was able to transform the R strain to make it lethal Some part of the dead S strain had survived the heating process and was taken up by the R strain bacteria These S strain parts must contain the “genetic material” to build a new polysaccharide capsule

Discovery of “Nuclein” In 1869, Freidrich Miescher was working with white blood cells from pus, derived from dirty bandages He would then use techniques to isolate the nuclei from each cell on the bandage Purified “nuclein” from isolated nuclei from every cell that he tested This nuclein was known to be high in phosphorus (unlike proteins, carbohydrates and lipids) The significance of the difference in this composition remained unknown in Miescher’s lifetime (it wasn’t until the discovery of the structure of DNA by Franklin, Watson and Crick that the relevance of PO4 in the backbone of DNA was realized).

Transforming Principle Genes are on Chromosomes T.H. Morgan discovered sex-linked traits, and defined the chromosomal theory of inhertiance, this being that genes lie on chromosomes. BUT: Is it the proteins or the DNA of the chromosomes that are the genes? Basic Requirements for the Genetic Material Coding capacity A large molecule Enough structural variation to encode the huge variety of phenotypes Self Replicable Copying of the genetic material Most people thought that protein was the genetic material. Why? Mostly abundant, central molecules of life Long enough Versatile enough – 20 different amino acids Thought that DNA couldn’t be the genetic material. Why? Structure of DNA is too simple to encode the genetic materail – only 4 different nucleotides

Enzyme Toolbox Protease: Any enzyme that digests proteins by the hydrolysis of peptide bonds that link amino acids together in a polypeptide chain. DNase: Any enzyme that digests DNA by the hydrolysis of phosphodiester bonds in the DNA backbone. RNase: Any enzyme that degrades RNA into smaller components.

Objectives Hypothesize outcomes for a given experiment given the data collected. Design an experiment to conclude whether protein, DNA or RNA is the genetic material. Explain why DNA makes a better genetic material than protein.

Avery, MacLeod and McCarty Conclusions

Avery, MacLeod and McCarty Conclusions