1. Mark Mayo Cypress College
The History of DNA
Mark Mayo
Cypress College
Last update 9/16/13

2. Transformation Frederick Griffith (1923)
Used healthy mice
Mice were injected with either R(rough) strain of Streptococcus pneumoniae. The mice live and their immune system kills R bacteria. No live bacteria
Mice injected with the S (smooth) strain of Streptococcus pneumoniae. The mice die. The dead mice have live S bacteria.
Mice injected with heat-killed S strain. Mice live with no live bacteria found in mice.
Mice injected with mixture of live R strain and heat-killed S strain. Mice die and live S strain bacteria are found in the dead mice.
Heat does not destroy the active factor that is responsible for heredity (DNA).
It is said that the bacteria were transformed by this active agent.

3. Transformation Frederick Griffith (1923)

4. Transformation II Avery, McCarty, Macleod (1944)
Think re-mix of Griffith
Repeated Griffith’s work, but knew that DNA was the substance of transformation
Separated classes molecules from s cell debris
Tested each fraction for transforming ability, one at a time
Only DNA transformed r cells into s cells
To provide r cells with s DNA is to provide r cells with s genes

5. Transformation II Avery, McCarty, Macleod (1944)

6. DNA or Protein as Active Agent Alfred Hershey and Martha Chase
DNA or Protein as Active Agent Alfred Hershey and Martha Chase* (1950’s)
Used radioactive labels on bacteriophage components to decide if DNA or protein was the transforming factor
Label viral protein with S35 *
Label viral DNA with P32 *
Allow infection
Wash viral particles (with blender!)
Check for label after subsequent infection into new bacteria
Found only P32
Hence DNA is the transforming factor

7. DNA or Protein as Active Agent Alfred Hershey and Martha Chase

8. Chargaff’s Rules Erwin Chargaff (1949)
He studied the relative amounts of each nucleic acid base in a great variety of plant and animal species
Roughly found that A=T and G=C, but not exactly due to errors in the technology!
Purines are exactly equal to pyrimidines
His methodology for the time was good, but now we get exact amounts
He could not make the connection (Watson and Crick used his data however)

9. Chargaff’s Rules Erwin Chargaff (1949)

10. Alpha Helix Linus Pauling (1948-1950)
Worked with proteins and determined that collagen has a helical arrangement for its polypeptides
He called the helix an alpha helix
He suspected that DNA might also have a helical arrangement, but could not get it to compute with a single strand
Pauling suggested DNA had a triple helix, but had no proof
Watson and Crick heard his idea about a helix…

11. Alpha Helix Linus Pauling (1948-1950)

12. X Ray Diffraction Maurice Wilkins and Rosalyn Franklin
Used Xray diffraction to study proteins and other molecules
DNA was very difficult to crystallize and a tough candidate for Xray diffraction
Rosalyn Franklin was a very talented graduate student in the lab of Wilkins
She was successful at crystallizing DNA in two forms A and B
The forms on an X was seen indicating some kind of helix
She could measure the distances between repeating units on the molecule
She could also measure the diameter of the molecule
Wilkins sent her unpublished data to Watson and Crick without her permission
She died before the Nobel prize or she might have shared it

13. X Ray Diffraction Maurice Wilkins and Rosalyn Franklin

14. Semi-conservative DNA Replication Matthew Meselsohn and Frank Stahl
They used two isotopes of Nitrogen – 14N and 15N
15N is heavier than 14N
They grew bacteria for several generations in heavy 15N (all DNA would be heavy!)
Abruptly changed the medium to lighter 14N for one or two generations
Used density-gradient ultracentrifugation to separate the DNA strands by weight
After one generation all DNA was medium between heavy and light (thus SEMI-CONSERVATIVE)
After two generation DNA had two bands: medium and light.

15. Semi-conservative DNA Replication Matthew Meselsohn and Frank Stahl
This one would be one heavy and one light band
Half light and half heavy = medium weight

16. Discontinuous DNA Replication Reiji Okazaki
He knew about DNA polymerase
It moves from 5’ to 3’ only on the leading strand
He searched for a second polymerase that worked in the reverse direction
After unsuccessfully searching he used his brilliance
Found numerous small fragments and also long segments as DNA was replicated
Finally decided that the short segments were from the lagging strand *
DNA polymerase worked on both sides continuously on the leading strand and in several places on the lagging strand
DNA ligase connects the small portions (now called Okazaki fragments) * on the lagging strand

17. Discontinuous DNA Replication Reiji Okazaki*