DNA

DNA Contained in chromosomes containing DNA and protein Nucleic acid is made up of nucleotides Nitrogenous base Deoxyribose sugar Phosphate

Sugar–phosphate backbone 5 end Sugar (deoxyribose) 3 end Fig. 16-5 Sugar–phosphate backbone 5 end Nitrogenous bases Thymine (T) Adenine (A) Figure 16.5 The structure of a DNA strand Cytosine (C) Phosphate DNA nucleotide Sugar (deoxyribose) 3 end Guanine (G)

Important Scientists in the Discovery of DNA Frederick Griffith Oswald Avery Alfred Hershey and Martha Chase Rosalind Franklin Francis Crick and James Watson

Mice and Transformation: Griffith’s Experiment

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

Phage head Tail sheath Tail fiber DNA 100 nm Bacterial cell Fig. 16-3 Figure 16.3 Viruses infecting a bacterial cell 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) DNA Radioactive DNA Figure 16.4 Is protein or DNA the genetic material of phage T2? Batch 2: radioactive phosphorus (32P)

Fig. 16-4-2 EXPERIMENT Empty protein shell Radioactive protein Phage Bacterial cell Batch 1: radioactive sulfur (35S) DNA Phage DNA Radioactive DNA Figure 16.4 Is protein or DNA the genetic material of phage T2? Batch 2: radioactive phosphorus (32P)

Fig. 16-4-3 EXPERIMENT Empty protein shell Radioactivity (phage protein) in liquid Radioactive protein Phage Bacterial cell Batch 1: radioactive sulfur (35S) DNA Phage DNA Centrifuge Radioactive DNA Pellet (bacterial cells and contents) Figure 16.4 Is protein or DNA the genetic material of phage T2? 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

The Double Helix Francis Watson and James 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 Double helix: two strands of DNA wound around each other Looks like a twisted ladder 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

Nucleotides 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)

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.