DNA • Contained in chromosomes containing DNA and protein • Nucleic acid is made up of nucleotides • Nitrogenous base • Deoxyribose sugar • Phosphate
Important Scientists in the Discovery of DNA • Frederick Griffith • Oswald Avery • Alfred Hershey and Martha Chase • Rosalind Franklin • Francis Crick and James Watson
Fredrick Griffith • 1928 • Studied Streptococcus pneumoniae • 2 strains • One pathogenic • One harmless
Frederick Griffith • Hypothesized that when the live harmless bacteria was mixed with heat-killed disease causing, some “factor” was transferred from the heat-killed bacteria into the live bacteria • Transforming factor might be a gene
Fredrick Griffith • Transformation: • Definition: change • Harmless bacteria became harmful when mixed with heat killed bacteria
Oswald Avery • Repeated Griffith’s work (1944) • Made extract from the heat-killed bacteria • Tested three possible factors for transformation • DNA • RNA • Protein • What was his conclusion? What factor allowed transformation to occur?
Alfred Hershey and Martha Chase • 1952 • The Hershey-Chase Experiment • Studied viruses (bacteriophages) • Viruses: non-living particles smaller than a cell that can invade living cells
Fig. 16-3 Phage head Tail sheath Tail fiber DNA 100 nm Bacterial cell
Hershey and Chase • Specifically looked at T2 • T2 invades Escherichia coli bacteria • Radioactive isotope of sulfur marked protein coat • Radioactive isotope of phosphorus marked DNA • What did they find?
Fig. 16-4-1 EXPERIMENT Radioactive protein Phage Bacterial cell DNA Batch 1: radioactive sulfur (35S) Radioactive DNA Batch 2: radioactive phosphorus (32P)
Fig. 16-4-2 EXPERIMENT Empty protein shell Radioactive protein Phage Bacterial cell DNA Batch 1: radioactive sulfur (35S) Phage DNA Radioactive DNA Batch 2: radioactive phosphorus (32P)
Fig. 16-4-3 EXPERIMENT Empty protein shell Radioactivity (phage protein) in liquid Radioactive protein Phage Bacterial cell DNA Batch 1: radioactive sulfur (35S) Phage DNA Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Batch 2: radioactive phosphorus (32P) Centrifuge Radioactivity (phage DNA) in pellet Pellet
X-Ray Evidence • Rosalind Franklin • Used X-Ray crystallography to find out structure of DNA molecules • Diffracts light to reveal image
X-Ray Evidence • X near center shows DNA twists around center • Angle of the X suggests two strands and the nitrogenous bases (hydrophobic) are near the center of the molecule • Shows diameter of the double helix
James Watson and Francis Crick Built three-dimensional models of DNA Used Rosalind Franklin’s x-ray pictures of DNA to assist in the model The Double Helix The Double Helix
The Double Helix • Two complementary strands of DNA wrapped around each other • Will have a uniform diameter • Realized that hydrogen bonds held the two strands together (A with T; C with G) • Two hydrogen bonds between A and T • Three hydrogen bonds between C and G
Nitrogenous Bases Make up DNA molecules Two Types Purines – two rings in the structure Adenine (A) Guanine (G) Pyrimidines – one ring in the structure Cytosine (C) Thymine (T)
Chargaff’s Rule Chargaff studied percentages of nitrogenous bases (1950) Percentage of guanine and cytosine are almost equal Percentages of adenine and thymine are almost equal Chargaff’s Rule supports idea that Adenine (A) bonds to Thymine (T) and Cytosine (C) bonds to Guanine (G)
Fig. 16-5 Nitrogenous bases Sugar–phosphate backbone 5 end Thymine (T) Adenine (A) Cytosine (C) DNA nucleotide Phosphate Sugar (deoxyribose) 3 end Guanine (G)
p. 310 1-3 • A fly has the following percentages of nucleotides in it’s DNA • 27.3% A • 27.6% T • 22.5% G • 22.5% C How do these numbers demonstrate Chargaff’s rule?
p. 310 1-3 • How did Watson and Crick’s model explain the basis for Chargaff’s rule? • If transformation had not occurred in Griffith’s experiment, how would the results have differed? Explain.