The genetic code had to be a "language" — using the DNA alphabet of A, T, C, and G — that produced enough DNA "words" to specify each of the 20 known amino acids. Simple math showed that only 16 words are possible from a two-letter combination, but a three-letter code produces 64 words. Operating on the principle that the simplest solution is often correct, researchers assumed a three-letter code called a codon. Research teams at University of British Columbia and the National Institutes of Health laboriously synthesized different RNA molecules, each a long strand composed of a single repeated codon. Then, each type of synthetic RNA was added to a cell-free translation system containing ribosomes, transfer RNAs, and amino acids. As predicted, each type of synthetic RNA produced a polypeptide chain composed of repeated units of a single amino acid. Several codons are "stop" signals and many amino acids are specified by several different codons, accounting for all 64 three-letter combinations.


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      NSDL_SetSpec_BEN,NSDL,dna alphabet rna molecules university of british columbia national institutes of health free translation letter combinations amino acids translation system letter combination codons polypeptide chain genetic code three letters signals strand principle math accounting institutes of health,oai:nsdl.org:2200/20110722012728959T



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