Crick and Watson's first attempt to solve the structure of DNA in the fall of 1951
was brief and unsuccessful. Thinking like Pauling, they quickly came up with a model
of three DNA strands wound around each other in a helix, phosphates at the core. It
seemed to fit the density data, the x-ray data was compatible with anything from two
to four strands per molecule, and it solved a theoretical problem. If DNA was the
genetic material then it had to say something specific to the body; it had to have
a language that could be translated somehow into the making of proteins. It was already
known that the sugars and phosphates were simple repeating units, unvarying along
the DNA strands. The bases were the variables. The bases varied, but the x-ray pattern
indicated a repeating crystalline structure; ergo, the core - the part of the structure
giving rise to the repeating patterns - must contain the repeating subunits, the sugars
or phosphates, with the bases sticking out where they would not get in the way.
The only major problem was explaining how one could pack phosphates into the middle
when at normal pH they would be generally expected to carry a negative charge. All
those negative charges at the core would repel each other, blowing the structure apart.
The triple helix they had devised was so pretty, though, and fit so much of the data
that Crick and Watson figured there had to be a place for positive ions at the core
to cancel out the negative charges. They grabbed a copy of Pauling's The Nature of the Chemical Bond, searched for inorganic ions that would fit their needs, and found that magnesium
or calcium might fit. There was no good evidence for the presence of these positive
ions, but there was no good evidence against it, either. They were trying to think
like Pauling, after all, and Pauling would certainly have assumed - as he had with
the alpha helix - that the structure came first and the minor details fell into place
later.
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