1. 12.2
The Structure of DNA

2. DNA = deoxyribonucleic acid
Made of repeating nucleotides (subunit)
Nucleotide = sugar (deoxyribose), phosphate, base
4 bases – adenine, guanine, cytosine, thymine
A & G = larger purines
C & T = smaller pyrimidines

3. % of each nucleotide base
ANIMALS A T G C
Human
30.4
30.1
19.6
19.9
Sheep
29.3
28.3
21.4
21.0
Sea Urchin
32.8
32.1
17.7
17.3
Salmon
29.7
29.1
20.8
20.4
PLANTS
Wheat
27.3
27.1
22.7
22.8
BACTERIA
E. coli
24.7
23.6
26.0
25.7
VIRUSES
Lambda
21.3
22.9
28.6
27.2

4. Chargaff (1949) – studies 
A always same % as T
C always same % as G
- so, _______________
= Chargaff’s Rules

5. Franklin – (1952) – X-ray diffraction
- showed DNA as a tightly coiled spring = helix
Watson & Crick (1953) – used Franklin’s and Chargaff’s info to determine structure
Double helix held together by weak Hydrogen bonds

6. Two strands run in opposite directions
= antiparallel
Are complementary P
|  covalent bond
S – B ~ B – S
| |
P P |