📖 Notes
LESSON 12 OF 16
DNA Structure & Replication
The Molecule That Copies Itself
Watson and Crick's double helix isn't just a shape — its antiparallel strands and base-pairing rules are what make semi-conservative replication possible.
Structure
Double Helix, 1953
1953
Watson & Crick publish the double helix structure of DNA, built on Chargaff's base-pairing data and Franklin's X-ray images.
- Antiparallel strands: one runs 3'→5', the other 5'→3'
- Nucleotide = deoxyribose sugar + phosphate + nitrogenous base
Base Pairing (Chargaff's Rule)
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A = T
Adenine pairs with thymine via 2 hydrogen bonds.
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G ≡ C
Guanine pairs with cytosine via 3 hydrogen bonds — a stronger bond.
Semi-Conservative Replication
1
Helicase unwinds
Breaks hydrogen bonds, separating the double helix.
2
Primase adds RNA primer
Gives DNA polymerase a starting point.
3
DNA polymerase III synthesises
Adds nucleotides in the 5'→3' direction only.
4
Leading vs lagging strand
Leading strand is continuous; lagging strand forms Okazaki fragments.
5
Ligase joins fragments
Result: 2 identical DNA molecules, each with one old and one new strand.
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Central Dogma
DNA → mRNA (transcription) → Protein (translation) — the one-way flow of genetic information that everything in this lesson ultimately serves.