In the five years after 1961, when conclusive genetic and biochemical evidence for a three-letter genetic code was first presented, the laboratories of Marshall Nirenberg, Severo Ochoa, and Har Gobind Khorana, as well as Crick and Brenner's laboratory in the newly-founded Laboratory for Molecular Biology in Cambridge, raced one another in a friendly competition to decipher the entire genetic code. This involved assigning all sixty-four triplets of nucleotides to the twenty amino acids which they specified, an effort that was nearly complete by the time this paper was published. Indeed, this article was one of the last contributions Crick made to the deciphering of the code, a problem that had preoccupied him for a dozen years.. In the paper, Crick, Brenner, and their co-workers reported another application of their system of using frame shift mutants, mutants produced by the insertion or deletion of a single nucleotide in the gene of a bacterial virus, the result of which was to move the sequence of bases, or letters in the genetic alphabet, to the right or the left by one letter. When read three letters at a time from the fixed starting point of the gene, the sequence was "out of frame" from the location of the mutation on forward. This meant that most of the codons beyond the location of the mutation contained a new combination of bases, and thus coded for different amino acids. (Another two such mutations would bring the sequence back into frame.). Here the authors presented genetic evidence that the triplet UGA was a nonsense codon, that it did not specify a particular amino acid. Instead, they hypothesized that UGA served as a space to separate adjacent genes.
This item may be under copyright protection; contact the copyright owner for permission before re-use.