READING DNA
Many techniques for decoding genomes capitalize on the complementary base-pairing rule of DNA. The genomic alphabet contains only four letters, elemental units called bases—adenine (A), cytosine (C), guanine (G) and thymine (T). They pair with each other (A with T; C with G) to form the rungs of the classic DNA ladder. The message encoded in the sequence of bases along a strand of DNA is effectively written twice, because knowing the identity of a base on one strand reveals its complement on the other strand. Living cells use this rule to copy and repair their own DNA molecules (below), and it can be exploited to copy (1–2) and label DNA of interest, as in the sequencing technique developed by Frederick Sanger in the 1970s (3–4) that is still the basis of most sequencing performed today.