The History of DNA Structure Discovery

1869 – Johann MiescherStudied the nuclei of white

blood cells Isolated the material using

HCl(aq) and digestive proteins

Named the substance nuclein

Found the material was rich in nitrogen and phosphorus

1919 – Pheobus LeveneDiscovered that DNA

was made up of chains of nucleotides

ACID

RIBOSE

NITROGENRICH

1920 – DNA vs Protein

thought that 4 nucleotides were connected in the same repeated pattern

protein have 20 amino acids which could be combined in many combinations

RNA DNA

sugar

location

bases

RNA DNA

sugar ribose deoxyribose (one less oxygen)

location

bases

RNA DNA

sugar ribose deoxyribose (one less oxygen)

location mainly outside nucleus mainly inside nucleus

bases

RNA DNA

sugar ribose deoxyribose (one less oxygen)

location mainly outside nucleus mainly inside nucleus

bases AGCU AGCT

Levene’s Nucleotide Pattern

1928 – Frederick Griffithstudied two strains of

pneumococcus bacteria

rough strain = nonvirulentinjection into mouse did not

result in death

smooth strain = virulentinjection caused mouse to diehttp://

www.youtube.com/watch?v=vQOdDGM5vSg

Griffith’s Experiment

Griffith’s Conclusionssome “factor” from the dead, virulent smooth strain

“transformed” the living, non-virulent rough strain

non-virulent rough strain picked up DNA to become virulent

1930 – Joachim Hammerling

nucleus at bottom of stalk

Acetabularia – type of alga

Hammerling’s Experiment

Hereditary information is stored in the nucleus.

http://www.youtube.com/watch?v=tl5KkUnH6y0

no regrowth

1944 – Avery, McCarty & MacLeod

continued Griffith’s work with pneumococcus

Condition Tested Resultpneumococcus & proteasepneumococcus & DNAse

Condition Tested Resultpneumococcus & protease

non-virulent bacteria virulent bacteria

pneumococcus & DNAse non-virulent bacteria remained non-virulent

1940s – Joshua Lederbergdemonstrated

bacterial conjugationbacteria can exchange

DNA

bacteria have no nucleus or chromosomes

http://www.youtube.com/watch?v=7stZk6TesKk

1940s – Edwin Chargafffor all organisms

A = T and G = C

Chargaff’s Rule

organisms with more Gs and Cs tend to be more complex

1952 – Hershey & Chaseconducted

experiments to definitively show that DNA is the hereditary material

bacteriophage used to infect bacteriabacterial virushttp://

www.youtube.com/watch?v=3QJ4CjFsflA

1950s – Maurice Wilkins & Rosalind Franklin

X-ray Crystallographyphysics approach to examining biological molecules https://www.youtube.com/watch?v=0tmNf6ec2kU Start at 14:35

Rosalind Franklin’s X-raysThe photo indicated:1. Backbone of alternating phosphate and sugars

2. Backbone is a helical structure

3. Double helix structure (molecule is a uniform helix)

4. Nitrogenous bases are in the middle of the molecule

5. Bases are at right angles to the backbone

Base Pairing knew that each base could pair with itself

1953 – James Watson & Francis Crickinspired by alpha-helix model of proteins

determined how A + T and G + C bonded together

width of purine + pyrimidine bonds fit perfectly between the sugar-phosphate backbone

the double helix model offered an easy method for replication