Discovering the Structure of DNA

1. Discovering the Structureof DNA

2. What is DNA? • DNA = deoxyribonucleic acid • Holds all our cell’s information • Located in the cell’s nucleus

3. What we already know about DNA • Codes for proteins essential to life • A nucleic acid macromolecule • Monomer of a nucleic acid is a nucleotide • The three parts of a nucleotide: • 1. Phosphate group • 2. Sugar (deoxyribose) • 3. Nitrogen base

4. Nitrogen bases • The nitrogen base can either be a purine or a pyrimidine. • How many carbon rings does each have? • Purines have 2 • Pyrimidines have 1

5. More about nitrogen bases • DNA has 4 nitrogen bases: • Thymine (T) • Adenine (A) • Cytosine (C) • Guanine (G) • Adenine and Guanine are purines • Cytosine and Thymine are pyrimidines.

6. You could draw this in your notes... YouTube: DNA Structure of DNA

7. A collaborative effort! • Early 1900s • known: information is passed from cell to cell. • Unknown: what carried the information? • Some scientists thought a protein was responsible, others that it was a nucleic acid. • Three major experiments helped show that a nucleic acid carried cell information: • Griffith • Avery-MacLeod-McCarty • Hershey-Chase

8. Frederick Griffith • Griffith was working on a vaccine for Streptococcus pneumoniae • He worked with 2 strains of the bacteria: • Smooth capsule that causes disease • Rough capsule that does not cause disease

9. Frederick Griffith • He added these bacteria strains, some dead and some living, to lab rats. • These are his results:

10. Live Smooth Bacteria Live Rough Bacteria Dead Smooth Bacteria Live R+ dead S Bacteria Frederick Griffith

11. Frederick Griffith got lucky? • Griffith studied pneumonia bacteria • In 1928, he isolated two strains of bacteria, and injected them into mice

12. Griffith’s experiments • Griffith’s findings: • Injection of live R strain was harmless (mice lived) • Injection of live S strain caused pneumonia (mice died) • Injection of heat-killed S Strain was harmless (mice lived) • BUT....Injection of mixture of live R strain with the heat-killed S strain caused pneumonia (mice died) What happened to the bacteria? • Griffith’s conclusions: • Something transferred from heat-killed bacteria to live harmless bacteria, making them deadly • Transformation = process by which one strain of bacteria changes the gene(s) of another bacteria

13. Avery-MacLeod-McCarty • Following Griffith (1943), scientists heat killed the virulent S strain and then selectively destroyed parts of the bacteria before combining with R strain • Destroyed proteins, lipids, carbs = mice died something different was transforming bacteria • Destroyed nucleic acids = mice lived! DNA was transforming bacteria • Demonstrated that DNA was the transforming agent

14. Hershey and Chase • Experimented (1950) with bacteriophages to see if information is carried on proteins or DNA • Used radioactive elements to “mark” DNA and protein • Only the radioactive DNA was found in bacteria cells (not proteins) • Further supported Avery’s experiment that genetic material is DNA http://www.accessexcellence.org/RC/VL/GG/images/HERSHEY.gif

15. Discovery of the structure of DNA • Many scientists contributed to determining the structure of DNA • Erwin Chargaff • Rosalind Franklin • James Watson & Francis Crick

16. A T A A T A T A A A T T T T C C G C G G Erwin Chargaff • Chargaff discovered that DNA has the same amount of adenosine as thymine and the same amount of cytosine as guanine. A = T and G = C

17. Erwin Chargaff • Worked with DNA nitrogen bases, discovered (1950): • In any sample of DNA, • # adenines (A) = # thymines (T)‏ • # cytosines (C) = # guanines (G)‏ • Therefore, in DNA, the bases are always paired: A with T, and C with G. • This is Chargaff’s Rule!

18. Rosalind Franklin • Worked with x-ray photography to try to find DNA structure • Her “Photo 51” revealed DNA’s structure (1952) • Died of cancer in 1958

19. Watson and Crick http://teachers.sduhsd.k12.ca.us/lolson/images/watson_crick.jpg • Credited with finding the structure of DNA (1953) • Watson got a sneak peak at Franklin’s x-ray photos and used them with other evidence • They described DNA as a double helix, with the strands held together by weak hydrogen bonds formed between the bases A-T and C-G.

20. DNA structure • Looks like a twisted ladder made of nucleotides • The nucleotide: • Phosphate group • Sugar (deoxyribose) • Nitrogen base • Sugars and phosphates make the sides of the ladder, nitrogen bases are the rungs • The atoms within the two strands are held together by strong covalent bonds • The two strands are held together by weak hydrogen bonds between the nitrogenous bases.

21. What bonds with what? • A bond between two purines would be too wide. • A bond between two pyrimidines would be too narrow. • THUS, a purine always bonds with a pyrimidine. • A bonds with T • G bonds with C

22. We love DNA Made of nucleotides Sugar, phosphate, and a base Bonded down one side Adenine and thymine Make a lovely pair Cytosine without guanine Would feel very bare D, D, DNA Different set of genes Look inside the nucleus For instructions to make proteins We love DNA It’s the code of life And the genetic makeup of All you have inside To the tune of: Row, Row, Row Your Boat COPY DOWN ON A SHEET OF PAPER

23. Your turn...the structure of DNA Sugar /P backbone Sugar /P backbone Base pair • On the diagram: • Circle and label a nucleotide. • Label the sugar and phosphate molecules. • Label the bases that are not already labelled • Label a base pair. • Label the sugar-phosphate backbones. • Label the hydrogen bonds. A Hydrogen bonds C G P A T S T A C G G nucleotide