classification of structure, it was a revelation whose implications opened up a vast area of exploration. Why so momentous? Because that structure facilitates DNA's role and function to such an extent that the whole process of decoding and eventually altering the basic genetic information was suddenly glimpsed by drawing the curtain back on what has come to be known as the alphabet of life.
The structure of DNA was at once realized to be dramatically suggestive of how the molecule actually functions to store and deliver coded information. By weak chemical bonding between complementary bases—adenine with thymine and cytosine with guanine, and each pair vice versa—the hereditary store of information in all life forms takes shape as a coded sequence of simple signals. The signals are arranged in the double-helix structure discovered by Watson and Crick. Picture two strands of rope side by side, each with a string of chemical bases along its length (Figure 5.1). When a base on the first rope is adenine (A), the base opposite it on the other rope