1. Structure of DNA

2. DNA has four kinds of Nitrogenous Bases
Thymine (T)
Cytosine (C)
Adenine (A)
Guanine (G)
Pyrimidines
Purines
Figure 10.2B

3. Partial chemical structure
Let’s look at DNA?
Hydrogen bond
Ribbon model
Partial chemical structure
Computer model
Figure 10.3D

4. Hydrogen bonds between bases hold the strands together
Each base pairs with a complementary partner
A pairs with T
G pairs with C

5. LE 16-8 Sugar Sugar Adenine (A) Thymine (T) Sugar Sugar Guanine (G)
Cytosine (C)

6. The structure of DNA consists of two polynucleotide strands wrapped around each other in a double helix
1 chocolate coat,
Blind (PRA)
Figure 10.3C
Twist

7. 10.3 DNA is a Double Helix – Who helped to determine this?
James Watson and Francis Crick… Nobel Prize
But who helped them?
Figure 10.3A, B

8. Building a Structural Model of DNA: Scientific Inquiry
After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role
Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure
Franklin produced a picture of the DNA molecule using this technique

9. Rosalind Franklin and Maurice Wilkins

10. Animation: DNA Double Helix
Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical
The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases
The width suggested that the DNA molecule was made up of two strands, forming a double helix
Animation: DNA Double Helix

11. Erwin Chargaff, 1940, Columbia University
Studied samples of DNA taken from different sources and found the amount of adenine was almost equal to the amount of thymine, and that the amount of guanine was almost equal to the amount of cytosine. The discovery of A=T and G=C became known as Chargaff’s Rule. 

12. Purine + purine: too wide
LE 16-UN298
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data