On November 25, 1952, three months after returning from England, Pauling finally made a serious stab at a structure for DNA. The immediate spur was a Caltech biology seminar given by Robley Williams, a Berkeley professor who had done some amazing work with an electron microscope. Through a complicated technique he was able to get images of incredibly small biological structures. Pauling was spellbound. One of Williams's photos showed long, tangled strands of sodium ribonucleate, the salt of a form of nucleic acid, shaded so that three-dimensional details could be seen. To Pauling the strands appeared cylindrical. He guessed then, looking at these black-and-white slides in the darkened seminar room, that DNA was likely to be a helix. No other conformation would fit both Astbury's x-ray patterns of the molecule and the photos he was seeing.

Even better, Williams was able to estimate the sizes of structures on his photos, and his work showed that each strand was about 15 angstroms across. Pauling was interested enough to ask him to repeat the figure, which Williams qualified by noting the difficulty he had in making precise measurements.

The next day, Pauling sat at his desk with a pencil, a sheaf of paper, and a slide rule. New data that summer from Alexander Todd's laboratory had confirmed the linkage points between the sugars and phosphates in DNA; other work showed where they connected to the bases. Pauling was already convinced from his earlier work that the various-sized bases had to be on the outside of the molecule; the phosphates, on the inside. Now he knew that the molecule was probably helical. These were his starting points for a preliminary look at DNA. He still lacked critical data - he had no decent x-ray images, for instance, and no firm structural data on the precise sizes and bonding angles of the base-sugar-phosphate building blocks of DNA - but he went with what he had.

It was a mistake. After a few pages of theorizing, using sketchy and sometimes incorrect data, Pauling became convinced - as Watson and Crick had been at first - that DNA was a three-stranded structure with the phosphates on the inside. Unfortunately, he had no Rosalind Franklin to set him right.