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1977
Walter Gilbert (1932-) and Frederick Sanger (1918-) devise techniques for sequencing DNA
Molecular biologists by the 1970s had deciphered the genetic code and could spell out the sequence of amino acids in proteins. But inability to easily read off the precise nucleotide sequences of DNA forestalled further advances in molecular genetics and all prospects of genetic engineering. Walter Gilbert (with graduate student Allan M. Maxam) and Frederick Sanger, in 1977, working separately in the United States and England, developed new techniques for rapid DNA sequencing. Sanger and Gilbert each took advantage of recently discovered enzymes and both methods benefited from improvements in gel electrophoresis, a method used for imaging the order of nucleotides. The Gilbert-Maxam method involved multiplying, dividing, and carefully fragmenting DNA. A stretch of DNA would be multiplied a millionfold in bacteria. Each strand was radioactively labeled at one end. Nested into four groups, chemical reagents were applied to selectively cleave the DNA strand along its bases—adenine (A), guanine (G), cytosine (C) and thymine (T). Carefully dosed, the reagents would break the DNA into a large number of smaller fragments of varying length. In gel electrophoresis, as a function of DNA's negative charge, the strands would separate according to length—revealing, via the terminal points of breakage, the position of each base. The Sanger method revealed the precise nucleotide sequence of DNA by using "chain-terminating" or "poison" molecules that revealed the positions of the bases. Single-stranded DNA was employed. Complementary copies were synthesized with the help of DNA polymerase. The resulting sample of DNA was divided into four parts. To each part was added one of the four DNA bases, together with a small percentage of the slightly altered chemical analogues. These "dideoxy" versions of the bases, when incorporated into the growing chain, terminate it. This process generated various lengths of the DNA chain that, as in the Gilbert-Maxam method, revealed the sequence of bases through gel electrophoresis. The methods devised by Sanger and Gilbert made it possible to read the nucleotide sequence for entire genes, which run from 1,000 to 30,000 bases long. For discovering these techniques Gilbert and Sanger received the Albert Lasker Medical Research Award in 1979, and shared the Nobel Prize in Chemistry in 1980.
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