DNA Structure & Function 1 Frederick Griffith and Transformation (1928) • Isolated two slightly different strains of pneumonia bacteria from mice • Only one of the strains caused pneumonia = smooth (S) colonies • Harmless strain = colonies with rough (R) edges • Used mice and discovered deadly bacteria could “transform” harmless bacteria 2 3 Transformation • Process: one strain of bacteria is changed into another strain of bacteria • Griffith hypothesized since ability to cause disease was inherited by transformed bacteria's offspring, the transforming factor might be a gene (genetic material) 4 Oswald Avery’s experiment (1944) 1. Made extract (juice) from heat killed bacteria 2. Used enzymes to destroy biomolecules & RNA 1. Transformation still occurred 3. Repeated experiment using DNA destroying enzyme 1. Transformation did NOT occur = DNA must be the genetic material Discovered nucleic acid DNA stores & transmits genetic info from one generation to the next 5 Alfred Hershey & Martha Chase Experiment (1952) • Used bacteriophages (“bacteria eater”) • Goal: find out which part of the phage (DNA or protein) produced new phages • Grew bacteriophages with radioactive markers – phosphorus-32 (32P) only in DNA – sulfur-35 (35S) only in Protein 6 7 DNA • Long molecule of units called nucleotides. • Nucleotides made of three basic components: • 5-carbon sugar (deoxyribose) • a phosphate group • a nitrogenous base • 4 kinds of nitrogen bases found in DNA • adenine (A), guanine (G) = double ringed (purines) • cytosine (C), thymine (T) = single ringed (pyrimidines) 8 9 Erwin Chargaff • American biochemist, 1940s • Discovered percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA – Later found that [A] = [T] • Scientists had NO idea what this was… 10 Chargaff's Rules 11 Rosalind Franklin & Maurice Wilkins (1952) • Used X-ray diffraction to get info about structure of DNA molecule • X-shaped pattern showed strands in DNA are twisted around each other, like coils of a spring – shape known as a helix – X suggests there are two strands in structure – Other clues suggest nitrogenous bases are near center of the molecule 12 James Watson & Francis Crick • Trying to understand structure of DNA – Built 3-D models • 1953: Shown picture of Franklin’s x-ray & immediately knew the structure: • double helix - 2 strands wound around each other (twisted ladder/spiral staircase) • Discovered Hydrogen bonds form between certain nitrogenous bases – base pairing • Provide just enough force to hold the two strands together 13 14 Genetic Code • Genome: organism’s complete set of DNA (genetic code) • All living things have a genetic code – Bacteria – several hundred to several thousand genes – Human – ~ 30,000 genes – Yeast – 6,000 genes *about 1/3 of yeast genes related to human genes • Nitrogenous bases hold key to this code – Order of bases determines what protein is made15 16.