1. 12.2 Notes The Structure of DNA

2. Watson and Crick
James Watson and Francis Crick determined that DNA is a double helix- 2 strands twisted around each other like a winding staircase.
Each strand is made up of nucleotides- subunits of DNA. A nucleotide is made up of 3 parts.
1. phosphate group
2. 5 Carbon Sugar (deoxyribose)
3. nitrogen-containing base

4. A Nucleotide Base Sugar P O OH HO NH2 N OH O CH2 H OH Phosphate 2’ 3’4’ 5’ 1’ OH

5. Nitrogen Bases
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)

6. Purines Pyrimidines Uracil CH3 N O NH Thymine N Adenine NH2 NH2 O N NH
(RNA)
CH3 N O NH
Thymine
(DNA) N
Adenine
NH2
NH2 O N NH
Guanine N O
NH2
Cytosine

7. Base Pairing Guanine And CytosineH O N
Guanine - N O H
Cytosine + + - - +

8. Base Pairing Adenine And Thymine- +
Adenine
CH3 N O H + -
Thymine

9. Base Pairing Adenine And CytosineH
Cytosine - + N H - +
Adenine

10. Base Pairing Guanine And Thymine
CH3 N O H + -
Thymine H O N
Guanine + -

11. The Watson - Crick Model Of DNA
T A
G C
C G
G C
T A
3.4 nm
1 nm
Minor
groove
Hydrogen bonds connect the bases!
Major
groove
0.34 nm

12. Many Scientists Contributed to the Discovery of DNA’s Structure
1949: Edwin Chargaff at Columbia Univ, N. Y.
Found that the amount of A = T
Found that the amount of G = C
1952: Maurice Wilkins and Rosalind Franklin in London
Developed high quality X ray diffraction photographs of the DNA molecule