In this paper Crick, Brenner, and their collaborators described a very elegant series of genetic experiments by which they proved that the genetic code for protein was a triplet code. They used an acridine dye, proflavin, to induce mutations in a specific, well-studied gene of a virus, a so-called bacteriophage, that attacked the bacterium Escherichia coli. The effect of proflavin was to either eliminate a single nucleotide from the gene or to add a nucleotide, each of which had the effect of rendering the virus incapable of synthesizing a particular protein. After a sequence of three such additions or deletions, the nucleotide sequence of the gene once again come into frame, and synthesis of the protein resumed. This proved that the nucleotide sequence which carried the genetic information was to be read three (or, less likely, six) nucleotides at a time proceeding from a fixed starting point.. Simultaneous with but independent of Crick and Brenner's genetic experiments, Marshall Nirenberg, a biochemist at the National Institutes of Health, developed direct biochemical evidence of the triple code. Together, they laid the foundation for the deciphering of the genetic code over the course of the next half-decade.. Crick and Brenner's experiment with acridine mutants was another fruitful application of genetic studies of the rII region of bacteriophage T4, an experimental system developed by Seymour Benzer in the early 1950s that allowed fine-structure mapping of a gene at the nucleotide level, two decades before techniques for direct sequencing of DNA were first developed.
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