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1) Griffith’s Transformation Experiments• What two strains of bacteria did Griffith use? – S (smooth) strain –cause pneumonia – R (rough) strain –harmless• What happened in the fourth experiment? – The mice died. There were live S strains found in the mice body.• What can be concluded from the experiment? – Cell components in the dead S strains transformed the live R strains to live S strains
2) Oswald Avery experiment• The transforming factor is DNA
3) The Hershey-Chase’s bacteriophages experiment• Bacteriophages – viruses that infect bacteria – consist of protein coat and a DNA or RNA core – inject their hereditary material into bacteria
3) The Hershey-Chase’s bacteriophages experiment
virus particle virus particle labeled with 35S labeled with 32P• Radioactive Markers – Radioactive bacterial cell (cutaway view) isotope – 32P could only mark DNA – 35S could only mark protein coat label outside cell label inside cell
DNA Structure• Chargaff’s Rule : – Amount of adenine = amount of thymine, amount of guanine = amount of cytosine – A=T and G=C
DNA Structure• Rosalind Franklin – Used x-ray diffraction to examine DNA fibers – Concluded that DNA was helix Rosalind Franklins X-ray diffraction photograph of DNA, 1953 Photo: courtesy HarperCollins
DNA Structure• Watson and Crick’s model – DNA have two strands run in opposite directions – Strands are held together by hydrogen bonds – A binds with T and C with G – Molecule is a double helix Hydrogen bond
James D. Watson Francis H. C. Crick Wilkins Rosalind Franklin
Questions:• 20% A in DNA strands, how many C ?• A=T=20%,• A+T+G+C=100%• G=C,• C=30%
The drawing below shows a short section of a DNAmolecule. What is labelled by I, II and III? I II III I II III A. 3′ end purine hydrogen bond B. 5′ end pyrimidine covalent bond C. 3′ end pyrimidine hydrogen bond D. 5′ end purine covalent bond