Continuing with my mini-focus on the models in my new banner, DNA's double helix (here by Andreas) is one of the most widely recognized structures from modern science. This wasn't always true. By the 1940's it was commonly accepted that DNA held the key to genetic information. It was known that DNA was made of four bases - G, C, A, and T - and also sugars and phosphates, but it was not known how these came together. Erwin Gargaff discoverd that each cell had an equal number of G's and C's, and also an equal number of A's and T's. Linus Pauling proposed a structure that involved a triple helix, with the phosphates on the insides. At this time, Rosalind Franklin was doing some ground breaking work on X-ray crystallography, and she determined that the structure involved a double helix, and the phosphates must be on the outside. Watson and Crick put this information together, and they came up with the now famous double helical structure shaped like a twisted ladder, where the outside strands (the blue bricks in this LEGO model) are comprised of repeating sugar-phosphate units, and the rungs of the ladder (the red and yellow here) are pairs made up of either A and T or G and C bases. Their paper describing the structure has one of the best lines ever in science writing: "This structure has novel features which are of considerable biological interest." Indeed, the base-pairing and the double helix are at the heart of explaining how DNA is copied, bringing information to the next generation of cells, and also explaining how DNA is used by the cell to make proteins.
BTW, the elucidation of DNA's structure is a great story of egos and infighting, probably sexism, and accusations of international politics. Franklin didn't get much credit for her discoveries because another scientist, Wilkins (another DNA researcher who eventually shared the Nobel with Watson and Crick) shared her data with Crick without asking her first. After she left Kings College, the research director there refused to let her continue on with her data, so she turned to other subjects, where she made ground-breaking discoveries on tobacco mosaic virus and polio virus. So one of her key papers was not even published until after her death. Another interesting side story is that Linus Pauling reportedly felt he would have come up with the structure first if he hadn't missed a flight due to passport issues, so he didn't see a talk by Franklin where she shared some of her results. The Wikipedia entry disputes the significance of this missed flight, but I first heard the story at a conference in honor of Pauling on the occasion of his 90th birthday, where several of his former students gave talks. BTW, he was there at the start of the conference, but I didn't make it until the afternoon session, so I didn't get to see him.