Explaining DNA

Why a double helix?
Why was the determination of the double helix such an important discovery? In the 1950s, scientists already knew quite a bit about the chemical structure of DNA and what it did. Why worry about how it is arranged in three-dimensional space?

The key point is that structure often points the way to function – and nowhere is this more graphically illustrated than in DNA. One of the most extraordinary features of DNA is its ability to replicate itself. Every time a cell divides, it makes an exact (or very nearly exact) copy of itself, with one copy going into each of the new cells. That means that three billion letters of DNA are copied, with absolute fidelity, billions and billions of times during the life of just one person. How does DNA manage this?

Clues came from the discovery that there was a relationship in the quantities of the chemical letters in DNA. The amounts of G and C were always the same, as were the amounts of A and T. What Watson and Crick realized was that if DNA existed in two chains, the G and C and the A and T might be linking the chains, like rungs of a ladder. The X-ray diffraction results suggested a helix, implying that the ladder was twisted.

Most importantly, the pairing between the two strands provided a mechanism for the copying of DNA: If the strands were unpealed, each individual strand could act as a template for the construction of a new copy. Opposite every G would be a C, and opposite every T an A.

In Watson and Crick’s immortal words from their 1953 paper: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

History has proved them absolutely right.