DNA Double Helix
DNA (deoxyribonucleic acid) is the molecule that carries hereditary information in living cells. Its structure — the double helix discovered by Watson, Crick, Franklin, and Wilkins in 1953 — explains both how information is stored and how it is copied.
The molecule consists of two strands wound around each other in a helical shape. Each strand is a polymer (a chain of repeating units) of four types of nucleotides, distinguished by their bases: adenine (A), thymine (T), guanine (G), and cytosine (C). The two strands are held together by hydrogen bonds between paired bases, and the pairing is specific: A always pairs with T, and G always pairs with C. This complementarity is the structural key to everything the molecule does.
How Information Is Stored
The sequence of bases along one strand encodes information. The pairing rules mean the opposite strand carries the complementary sequence — given one strand, the other is determined. This redundancy is not merely backup; it is the copying mechanism.
How Information Is Copied
To replicate, the two strands separate. Each acts as a template: free nucleotides in the cell float in and pair with their complements on the exposed strand. An enzyme (DNA polymerase) stitches the new nucleotides together into a new strand. The result is two identical double helices from one original — the molecule contains its own copying instructions.
Feynman's point: this is not magic. The whole mechanism follows from the shape of the molecules. The bases are shaped so that only the right pairs fit together; the enzyme is shaped to hold the strand and add the correct nucleotide; the hydrogen bonds are strong enough to hold strands together but weak enough to be separated during replication. Biology at this level is atomic engineering.
Errors and Evolution
The copying is nearly but not perfectly accurate. Occasional mismatches occur — a wrong base slips in. These are mutations. Most mutations are neutral or harmful; a few, rarely, confer advantage. Over vast timescales and populations, this error rate drives evolution. The imperfection of the copying mechanism is what allows life to adapt.
Connections
- atomic-hypothesis — DNA works because molecular shapes are atomic structures
- enzyme-catalysis — DNA polymerase is the enzyme that catalyzes replication
- stellar-nucleosynthesis — the carbon, nitrogen, oxygen, and phosphorus in DNA were forged in stars