1. Discovering DNA
SBI4U

2. 1869 – Friedrich Miescher
Used pus cells from infections (before antiseptics were widely used) to investigate chemical structure of hereditary material (thought to be protein at the time)

3. Found a large quantity of non-protein substance in nucleus that was slightly acidic and contained large amounts of phosphorus and nitrogen and called it NUCLEIN (due to location) – renamed DNA where chemical composition discovered

4. 1930s – Joachim Hammerling
Used a large (5 cm) single-celled algae (Acetabularia) with 3 distinct parts (foot, containing nucleus, stalk and cap)
Cut off cap and new cap
grew; cut off foot and NO
new foot grew

5. Grafted stalk of species A to foot of species B and new cap was an intermediate, cut off this cap and a second cap was that of species B
Conclusion = hereditary material in foot, likely nucleus

6. 1920s
Determined that DNA has 3 main components: deoxyribose sugar, a phosphate group, and a nitrogen base
4 types of nitrogenous bases – A, C, T and G; 2 of which are larger (A & G) and 2 of which are smaller (pyrimidines – C & T)

7. 1920s to 1944
Frederick Griffith (then Oswald Avery, Colin MacLeod (both Canadian!) and Maclyn McCarty)
While trying to develop a vaccine against pneumonia bacteria, Griffith discovered the process of transformation
Transformation = introduction of foreign DNA into a bacterium

9. Later, Avery, MacLeod and McCarty isolated various components of the pneumonia bacteria and identified DNA as the transforming principle

10. 1952 – Alfred Hershey and Martha Chase
Used viruses that infect bacteria (bacteriophages) because they are only composed of DNA and a protein coat

11. This protein coat contains sulfur (which DNA does not) and DNA contains phosphorus (which protein does not)
They labelled the protein with a radioisotope of sulphur (S-35) and labelled the DNA with a radioisotope of phosphorus (P-32) and subjected each to separate bacterial cultures
Bacteria exposed to P-32 showed radioactivity, while those exposed to S-35 did not
viruses, which inject their hereditary material into bacteria for reproduction were injecting DNA
Conclusion = hereditary material was DNA

13. 1949 – Erwin Chargaff
Chemically analyzed DNA to determine that the proportion of adenine (A) is equal to that of thyamine (T) and the proportion of (G) is equal to that of cytosine (C)
Also, total amount of purines equals total amount of pyrimidines

14. 1953 – Rosalind Franklin & Maurice Wilkins
Each used x-ray diffraction analysis of DNA to determine its structure (crystallography)

15. 1953 – James Watson & Francis Crick
Observed patterns of Wilkins’ and Franklin’s diffraction patterns (Franklin’s were key) which revealed a helix (corkscrew) shape about 2 nm (1 x 10-9 m) in diameter with a complete helical turn every 3.4 nm
Also used Chargaff’s results to build the double-helix structure of DNA

16. Watson and Crick got the credit for the structure of DNA. Franklin got
Watson and Crick got the credit for the structure of DNA...Franklin got...nothing.