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The structure of DNA
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  The race to discovery
model of DNA
When he returned to Cambridge, Watson shared the new results with Crick and they applied the information to their ball-and-stick models. Watson wanted to try making a model in which just two phosphate-sugar-base chains were linked together. He thought it made sense for genes to come in pairs, partly because most organisms have two parents. Watson and Crick also decided to try orienting the bases towards the center of the pair. Watson later recounted that they tried this approach simply because it was something they hadn't yet tried, though Franklin had previously given them good reason to think that the bases should be on the inside and phosphates on the outside of the molecule where they could attract water. Both of them were surprised by how well the new two-strand, bases-in model fit the clues Watson had scribbled down during his dinner with Wilkins. But Watson and Crick weren't the only ones thinking about a double helix — Rosalind Franklin's notes from February 10th show that she started wondering if DNA B might be a two-chain helix around the same time.

The symmetry of DNA molecules
Of course, because she had produced the results, Franklin was the only one with all the data — and Watson and Crick needed more information to keep working. In science, researchers regularly share their findings with other scientists through journal publications, but Franklin's results were so new that they hadn't been thoroughly peer-reviewed and published. However, Watson and Crick were able to find out more about Franklin's work from another source. Her lab was funded by the Medical Research Council, which required grant recipients to report on their progress at the end of each year. All of the clues that Franklin had uncovered were summarized in that report. Such reports are supposed to be confidential, but Watson and Crick happened to know someone on the Medical Research Council who had a copy of the report and was willing to show it to them. When Crick saw the evidence in the report, he recognized the type of crystal symmetry Franklin described, and realized something that she hadn't. If DNA crystals could be flipped upside down and backwards, and still look the same, the strands of the backbone must be identical, and they must run in opposite directions.

piece of the puzzle happens to come Crick's way

By this time, Franklin had also concluded that DNA was a two-chain helix, composed of two intertwined sugar-phosphate backbones. Figuring out the shape of the backbones, though, still left the bases an open question. She knew from details in her X-ray images that the phosphates were on the outside of the helix, which meant that the bases must point toward the center. But how did they fit together? Each base is a slightly different size, but the smooth twists of the sugar-phosphate chain never varied. How could the bases fit inside the chains without touching and repelling one another? She was sure there was a clue in DNA's unique base ratios — one of the puzzle pieces discovered before Franklin had even begun to study DNA — but she still wasn't sure exactly what that clue meant. By February 23rd, her notes show that she realized that if A were physically interchangeable with G, and C with T, then the amount of A would have to equal T, and likewise for C and G. She was getting close — but she had yet to put the pieces together into a complete hypothesis. Meanwhile, back in Cambridge, Watson and Crick were working on the same problem …


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