1. THE STRUCTURE OF DNA
Section 4.2
Page 210

2. DNA
Deoxyribonucleic acid
A polymer of many nucleotide subunits

3. Nucleotides Three main components:
Deoxyribose sugar
Phosphate group (negatively charged)
Nitrogenous base
The identity of a nucleotide is determined by its nitrogenous base

4. Purines
Adenine (A)
Guanine (G)
Differences in the sequence of nucleotides are the source of variation between individuals and organisms.
Pyrimidines
Thymine (T)
Cytosine (C)

5. Nucleotide structure Ribose – five carbons Carbon naming: Clockwise
Start at C to the right of O 5’ 4’ 1’ 3’ 2’

6. DNA COMPOSITION: Chargaff Experiment
Erwin Chargaff, 1949
Chemical analysis:
Proportion of adenine in DNA is equal to thymine
Proportion of guanine is equal to cytosine
# of purines = # of pyrimidines

7. DNA STRUCTURE Rosalind Franklin and Maurice Wilkins
X-ray diffraction analysis (imaging technique)
James Watson and Francis Crick
Used X-ray diffraction pattern obtained by Franklin
Combined with information from Chargaff experiment
Developed the double-helical model of DNA

8. ...injustice?
Nobel Prize in Physiology or Medicine, 1962: Watson, Crick and Wilkins
...but what about Rosalind Franklin?
Not many women in science at that time
Hostile working relationship with Maurice Wilkins
Franklin died in 1958 of cancer
Nobel prizes are not award posthumously
Would she have also been awarded the Nobel prize?

9. The Double Helix Two antiparallel strands of nucleotides
Parallel, but running in opposite directions
3`end (OH group)
5`end (phosphate)

10. The two strands wind around each other  helical structure
Clockwise direction
One complete turn of the helix :
10 nucleotides
3.4 nm
Helix is 2 nm in diameter
3.4 nm
0.34 nm
2 mm

11. Sugar-phosphate backbone
Nitrogenous bases facing in
Bases are paired with one another: complementary base pairing

12. Complementary base pairing
hydrogen bonds
purine + pyrimidine
A-T (2 H-bonds)
G-C (3 H-bonds)
this pairing is crucial for the storage and transfer of genetic information

13. Convention: Write only the 5`to 3`strand... why?
Can easily deduce the sequence of the complementary strand
Homework: Pg. 216 Q. 1-3, 5-7