DNA Structure and Function

1. DNA Structure and Function

2. Objectives • Important experiments • Griffith • Hershey and Chase • Franklin and Wilkins • Watson and Crick model • Structure of DNA

3. Griffith: a “transforming principle” • 1928 • Investigated two forms of bacteria by injecting them into mice • “S form” have a smooth coating • “R form” have a rough coating • Only the live S type is virulent (kills mice) • Mice injected with a combination of heat-killed S bacteria and live R bacteria also died

4. Griffith’s Experiment • “Transforming principle”– Griffith found live S bacteria in blood samples of the dead mice • Some material must have transferred from dead S bacteria to live R bacteria

5. Hershey and Chase • 1952 • Conclusive evidence for DNA as the genetic material • Studied bacteriophages (viruses that take over bacteria cells) • Phage structure: • DNA molecule • Protein coat • This could help answer the DNA vs. protein debate!

6. Hershey and Chase • Protein has sulfur but not much phosphorous • DNA has phosphorous but not sulfur • Hershey and Chase grew phages in cultures with radioactive sulfur or phosphorous isotopes • Experiment 1 • Bacteria infected with phages that had radioactive sulfur in their protein • Used a blender to separate bacteria from parts of the phages that remained outside the bacteria • No radioactivity found in the bacteria

7. Hershey and Chase • Experiment 2 • Bacteria infected with phages grown in radioactive phosphorous • Used a blender to separate bacteria from parts of phage that remained outside bacteria • Radioactivity present inside bacteria • These results showed that bacteriophages’ DNA had entered bacteria but protein had not! • Finally convinced scientists that DNA and not protein is the genetic material • Also called the “blender experiment”

8. Hershey and Chase

9. DNA Structure • Franklin and Wilkins • X-ray crystallography photographs of DNA • Watson and Crick • Early 1950s • Described the structure of DNA • Double helix model- two strands twisted together, antiparallel

10. DNA Structure • DNA molecule is a polymer made of many nucleic acid monomers, called nucleotides • Nucleotide has three parts: • Phosphate group • Deoxyribose (sugar) • Nitrogen-containing base • Adenine • Guanine • Cytosine • Thymine

11. One DNA Strand • The backbone of the molecule is alternating phosphates and deoxyribose sugar • The teeth are nitrogenous bases.

12. Four Nitrogenous Bases • Purines (double ring) • Adenine (A) • Guanine (G) • Pyrimidines (single ring) • Thymine (T) • Cytosine (C)

13. Chargaff’s Rule • Ernest Chargaff analyzed DNA of different organisms • Proportions of different bases varied • BUT in ALL organisms, the amount of adenine (A) always equaled the amount of thymine (T) • The amount of cytosine (C) always equaled the amount of guanine (G) • Why would this happen?

14. Base Pairing • A always pairs with T (2 hydrogen bonds) • C always pairs with G (3 hydrogen bonds)

15. Antiparallel strands • DNA strands are complementary • Fit together (one purine and one pyrimidine) • Opposite of each other • For example, if one strand’s bases are ATCTT, what will the other strand be? • TAGAA