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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 share of G and C away from the extremes , and wrapped the letter in a fountain code . That code makes many strands , and any large enough handful of them can rebuild the page . Losing some strands is expected . The rebuild does not need every strand . What came back from the order was phosphoramidite synthesis , the chemical method , a dry speck in a tube . Reading it was not like opening a file . She amplified the strands by PCR , sent them through a sequencer , and let the decoder repair what synthesis and sequencing had smudged . The letter returned . It returned slowly . Nothing in the afternoon behaved like random - access memory . A chosen pair of primers can fish for one region , and that fish is still a wet - lab step , not an address you open between keystrokes . Behind the tube lay a paper from 1994 . Leonard Adleman had used DNA to solve a small Hamiltonian path , a route that visits each chosen point once . That was a calculation , carried by a crowd of molecules , and it did not become a general computer in place of the one on her desk . She left the paper closed . Storage and that experiment share a molecule . They do not share a job . She wrote the label in pencil : one letter , recoverable from a large enough subset of strands , not a memory chip . Then she set the tube in the drawer and turned the sequencer off .
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