At the same time, other groups were trying to solve the DNA structure puzzle by building three-dimensional models. The group that succeeded was that of Francis Crick (born 1916) and James Watson (born 1928), in Cambridge, England. Watson and Crick built their models using data from X-ray crystallography, a technique for measuring interatomic distances through analysis of the scatter patterns made by X rays bouncing off a pure crystal. Rosalind Franklin (1920-1958), working in London, had made the best X-ray pictures of DNA, and Watson and Crick had been given access to these (without Franklin's permission) at a critical time in their model-building endeavors and saw features in it that Franklin had not yet discovered. Shortly after, Watson and Crick deduced the correct structure, and published their work in April 1953.
DNA is a double helix, in which two sugar-phosphate strands wind around each other, forming a structure that looks like a broad spiral staircase. The sugar molecule has a head and a tail, and each sugar-phosphate strand therefore has a direction, like a chain of arrows. The two helical strands point in opposite directions.
The bases project toward the inside like stair treads. In contrast to the monotony predicted by the tetranucleotide hypothesis, the bases along one strand can be in any sequence whatsoever. Critical to the stability of DNA is the hydrogen bonding between the bases across the interior. These weak chemical attractions form only when the base atoms are positioned just so— in particular, Watson and Crick discovered, only when adenine projects across to meet a thymine, and guanine a cytosine. ChargafPs ratios reflect this essential base pairing.
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