The Earth was formed largely out of the heavier elements, including carbon and oxygen. Somehow, some of these atoms came to be arranged in the form of molecules of DNA. This has the famous double-helix form, discovered in the 1950s by Francis Crick and James Watson in a hut on the New Museum site in Cambridge. Linking the two chains in the helix are pairs of nitrogenous bases. There are four types of nitrogenous bases—adenine, cytosine, guanine and thymine. An adenine on one chain is always matched with a thymine on the other chain, and a guanine with a cytosine. Thus the sequence of
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