Exam 1 Experiments

Griffith

• Main idea: bacterial transformation is caused by a certain biomolecule

• Hypothesis: Material in dead bacterial cells can genetically transform living bacterial cells

• Used pneumococcus: S strain (smooth) and R strain (rough)

• S strain is virulent and kills the mouse, R strain doesn’t kill mouse, Dead S bacteria doesn’t kill the mouse, A mix of living R and dead S kill the mouse

• Some substance in S cells can transform R cells into virulent

• Conclusion: A chemical substance from one cell is capable of genetically transforming another cell

Avery

• Main idea: bacterial transformation is caused by DNA

• Hypothesis: The chemical nature of the transforming substance from pneumococcus is DNA

• Filtrated dead S cells treated with diff enzymes then added to R cells:

  • RNase: destroys RNA

  • Protease: destroys proteins

  • DNase: destroys DNA

• R strain became virulent with RNase and protease, but not DNase

• DNA is transforming substance

Hershey and Chase

• Main idea: DNA, not protein, is associated with genetic material

• Hypothesis: DNA or protein might be the hereditary material that enters a bacterial cell to direct the assembly of new viruses

• Used bacteriophage T2, which infects the bacterium E. coli

• Virus injects “certain” materials into the bacterial cell to replicate itself

• Viral DNA labeled ³²P and viral proteins labeled ³⁵S

• Bacterial cells were separated from the rest of the culture

  • Blending removes parts of viruses on outside of bacteria

  • Centrifuge separates bacteria (pellet) from the rest of the sample (supernatant)

• ³²P was found in pellet, ³⁵S was found in supernatant

• DNA is associated with genetic material

Meselson and Stahl

• Hypothesis: DNA replicates semiconservatively

• Labelled DNA strands ¹⁵N and ¹⁴N

  • ¹⁵N is heavier

• After one round of replication, all strands are intermediate in density (half ¹⁵N and half ¹⁴N)

• After two rounds of replication, half of DNA is intermediate and half light (¹⁴N)

• This pattern can only be explained by semiconservative replication

Garrod

• Hypothesis: one gene to one enzyme

• He observed black urine in children, especially in those whose parents were first cousins

• Do genes determine enzymes?

Beadle and Tatum

• Confirmed Garrod’s theory

• Grew wild-type Neurospora fungus on a minimal medium b/c it’s a haploid organism which means all alleles are expressed phenotypically and they can easily see if there’s a mutation

• Exposed wild-type Neurospora to X rays and found that some strains could no longer grow on the minimal medium

• Each mutated strain need specific vitamins in the medium to grow

  • One mutated gene impacted one specific enzyme

Srb and Horowitz

• Described the genetic control of a biochemical pathway

• Hypothesis: each gene determines an enzyme in a biochemical pathway

• Used mutant strains of Neurospora and found out that they couldn’t independently synthesize arginine meaning there were mutations throughout the arginine synthesis pathway

• Added certain supplements to growing medium to identify steps in biosynthetic pathway

• Precursor → Ornithine → Citrulline → Arginine

• Wild Type Strain: has no mutations meaning it can grow on all supplements

• Mutant Strain 1: can only grow on arginine meaning it can’t break down any supplements and has mutations in all 3 genes

• Mutant Strain 2: can grow on arginine and citrulline meaning it can only breakdown citrulline into arginine, mutation in gene b

• Mutant strain 3: can grow on all 3 compounds meaning it has a mutation in gene A and can’t breakdown the precursor into ornithine

• One gene, one polypeptide

Nirenberg and Matthaei

• Synthesized artificial mRNA with just one repeating base, added all 20 amino acids, and obtained polypeptides with just one amino acid