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The Tube With No Plug
Helen wanted one short letter stored in a form that could sit in a drawer with the power off. Disks and tapes still ask for a machine that understands their format. The information in DNA sits in the order of four bases: adenine, thymine, guanine, and cytosine. If that order held her letter, the tube could wait.
The first code she wrote was the obvious one. Two bits became one base: 00 for A, 01 for T, 10 for G, 11 for C. She threw it out before anyone synthesized it. A long run of the same base, a homopolymer, is a known source of sequencing errors. A strand that swings too far toward G and C, or too far away from them, is a poor candidate for synthesis. The code she kept broke up long repeats, held the