The amber and ochre codons mentioned in the letter (the triplets UAG and UAA, respectively) were called nonsense codons because it became clear from experiments with certain phage mutants that they do not specify any amino acid during protein synthesis. UGA was discovered to be the third nonsense codon, after Nirenberg had ruled out that it coded for either tryptophan or cysteine. All three nonsense codons, Crick and others soon determined, instead signaled the termination of the polypeptide chain.. Robert Holley's experiments with soluble RNA, or sRNA, soon to be renamed transfer RNA (tRNA), were designed to sequence the seventy-seven nucleotides of the tRNA that transferred the amino acid alanine during protein synthesis in yeast. These experiments, carried out over seven years and not yet completed at the time of Crick's letter, showed how the shape of tRNA made the molecule capable of capturing alanine and carrying it to the positions in the polypeptide chain where it was called for.
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