If you don’t know the story of how the role and structure of DNA was discovered, you really should read the full article that I quote below, or even better, read The Double Helix by James D Watson (a charmingly personal account, highly recommended). Anyway, what you probably don’t know is the role that prototyping, particularly paper prototyping, played in the discovery:

When Dr. Watson heard details of Dr. Franklin’s work at a seminar she gave in November 1951, he and Mr. Crick felt they possessed enough data to start building a model.

Their model had three helical chains, with the phosphate groups inside and the bases sticking out. Dr. Wilkins and Dr. Franklin were invited to Cambridge to inspect it but ridiculed the result. Dr. Watson had misunderstood one of Dr. Franklin’s measurements. After a conversation between the two laboratory chiefs, Dr. Lawrence Bragg at Cambridge and Dr. John Randall at King’s, the two young scientists were told to return to their official, non-DNA work and desist from model building. They handed over their equipment to King’s, urging it be used before Dr. Pauling solved the problem.

But both Dr. Wilkins and Dr. Franklin ignored the advice. ”Both Jim and I became impatient with their slow progress and pedestrian methods,” Mr. Crick wrote later. And in January 1952, Dr. Pauling indeed announced that he had determined the structure of DNA.

But his structure, which had three chains, was wrong. Dr. Watson seized the chance to persuade Dr. Bragg to let the model building resume at Cambridge before Dr. Pauling saw his error and corrected it.

Mr. Crick had now deduced from Dr. Franklin’s X-ray data a fact she did not, that DNA must consist of two spiral chains, running parallel to each other but in opposite directions. He and Dr. Watson at last realized the bases might be on the inside of the spiral, even though it was hard to see how an irregular composition of bases could form a regular structure. They ordered exact metal cut-outs from their machine shop to make a new model.

But Dr. Watson was so impatient that he made cardboard cut-outs of the four DNA bases, known as adenine, thymine, guanine and cytosine, or A, T, G and C. Playing with them on his cramped desk, he tried to fit like with like, but nothing worked. After an interruption by a colleague, he suddenly noticed that an A-T pair lined up on his desk was identical in shape to a G-C pair. Immediately he saw how these could be the equal steps of DNA’s spiral staircase. Each of the four letters on one chain is matched by its opposite on the other; when the chains separate, each is the template for a new chain of exactly matching sequence.

”That morning,” Mr. Judson wrote in his account, ”Watson and Crick knew, although still in mind only, the entire structure: it had emerged from the shadow of billions of years, absolute and simple, and was seen and understood for the first time.”

Quoted from “A Revolution at 50” (New York Times article) by Nicholas Wade, Feb 2003