1. DNA: The Genetic Material
Ag Biology

2. Early 1950’s
Most scientists were convinced that genes were made of DNA
However, they weren’t sure what DNA looked like
2 young researchers at Cambridge University started to piece it together
James Watson
Francis Crick

3. Structure of DNA Double helix – two strands twisted around each other
Each strand is made of linked nucleotides

4. Structure of DNA
Nucleotides: subunits that make up DNA; each one contains 3 parts
a phosphate group
a 5-carbon sugar (called deoxyribose)
a nitrogen containing base

5. Types of Nitrogen Bases
Adenine
Guanine
Thymine
Cytosine
Classified as purines
(bases made of 2 rings of carbon & nitrogen atoms)
Classified as pyrimidines
(bases made of 1 ring of carbon & nitrogen atoms)

6. Structure of DNA
The sugar-phosphate backbones are like the side rails of a ladder
The pairs of nitrogen bases are like the rungs of the ladder
These bases face each other
Double helix shape is held together by the weak hydrogen bonds between the base pairs

7. Erwin Chargaff Biochemist at Columbia University
In 1949, he observed that the amount of adenine always equaled the amount of thymine (A=T)
Also, the amount of guanine always equaled the amount of cytosine (G=C)
However those amounts varied between different organisms

8. Wilkins & Franklin’s Photos
X-ray diffraction used to study the structures of molecules
In 1952, W & F developed high quality X-ray diffraction photographs of fibers from a DNA molecule
It showed that DNA resembled a tightly coiled helix, composed of 2 or 3 chains

9. Watson & Crick’s DNA Model
Based on prior findings, W & C were able to construct a model of DNA
Used tin & wire models of molecules
Used the double helix shape
A purine on 1 strand is always paired with a pyrimidine on the other strand (A=T and G=C)
These are known as the “base pairing rules”
2 strands are complimentary to each other