Discovery of DNA Structure
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The structure of DNA was published in 1953.
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Rosalind Franklin: the unsung hero
- Took the x-ray crystallography pictures of the DNA.
- Without the pictures, the structure would never have been determined!
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Related books
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What Is Life? by Erwin Schrödinger
- Caused a paradigm shift in the way biologists thought about life.
- Probably paved the way for this discovery.
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The Double Helix by James Watson
- Published in 1968, it consisted of Watson's memoirs regarding the discovery.
- It was politically incorrect, and upset many people.
- The opening line: “I have never seen Francis Crick in a modest mood.”
Chargaff's First Rule
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Chargaff analyzed the percentage of each base in many organisms and found that:
- The amount of adenine equals the amount of thymine.
- The amount of guanine equals the amount of cytosine.
- This rule was an important piece of data for determining the structure of DNA: it allowed Watson and Crick to discover base pairing.
Percentages of Bases in the DNA of Some Well-Studied Species
| DNA Origin |
Amount of Base (Percent of Total DNA) |
| A |
T |
G |
C |
| Human (Homo sapiens) |
31.0 |
31.5 |
19.1 |
18.4 |
| Corn (Zea mays) |
25.6 |
25.3 |
24.5 |
24.6 |
| Fruit fly (Drosophila melanogaster) |
27.3 |
27.6 |
22.5 |
22.5 |
| Bacterium (Escherichia coli) |
26.1 |
23.9 |
24.9 |
25.1 |
Rosalind Franklin's X-ray Photos
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A diffraction pattern is created by sending X-rays through a crystallized form of DNA.
- From this diffraction pattern, a scientist can infer the structure of the crystal.
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This is not an easy task:
- Getting the photos is hard.
- Interpreting them is tricky.
- Rosalind Franklin took the photos; Watson and Crick interpreted them.
Nitrogenous Bases
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Pyrimidines
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Purines
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Pairing
- A—T
- C—G
- Hydrogen bonds hold them together.
DNA Structure
DNA is a double helix; its key features are its:
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Sugar-phosphate backbone
- Each sugar is linked to its neighboring phosphates at its 5' and 3' carbons.
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Antiparallel strands
- The two strands run in opposite directions.
- One runs in the 5' → 3' direction; the other runs in the 3' → 5' direction.
