In Search of the Human Genetic Code

Scientific terminology is intentionally precise: One would not confuse a peptide with a peptidase, or DNA with RNA.Unfortunately, the public has been slow to embrace the word "genome" because of a continuing confusion with the term "genetic code" that is perpetuated by editors, writers, and even a few prominent scientists.The dictionary defines "code" as "a system of signals used to represent letters or numbers in transmitting messages." Substitute "amino acids" for "letters or numbers" and you have an approximation of "genetic code." Francis Crick led the team that deciphered the code in 1961,1 and soon after, Monroe Strickberger's classic genetics text defined it well: "A table of all the code words or codons that specify amino acids." The code's elegance lies in its universality. The same codons specify the same amino acids, in a human or a hyacinth.Yet when the first draft of the human genome sequence was announced in June 2000, headlines hailed the deciphering of the "human genetic code." Time magazine proclaimed "Cracking the Code!" beneath DNA gurus J. Craig Venter and Francis Collins, coaxed by the magnitude of the occasion to actually share a magazine cover. Alas, when I tried to correct the editor at Scientific American, after a leading story made the "human genetic code" gaffe in the first sentence, she declared, "Francis Collins uses the term, so it's correct." Venter uses it, too.What they all meant, of course, is that a genome had been sequenced. Maybe I'm too picky, but "deciphering the genetic code" and "sequencing the wombat genome" are just not the same thing. The code is the correspondence, not the sequence. The oversimplification might confuse someone wondering how foods can be genetically modified if the rules differ for each species.The error persists. The Wellcome Trust Web site links to an article from ABC NewsOnline Australia warning that the effects of modern living "may be damaging the human genetic code." News releases from the Institute for Genomic Research announce the deciphering of new genetic codes. And a news headline in a mid-September issue of Science magazine reads, "Report Upholds Public Access to Genetic Codes," implying that there's more than one. We learn in the article that scientists "have released the genetic codes of more than 1,100 viruses and 150 bacteria."2Fortunately, a report in another mid-September journal, Nature, has finally put usage of "genetic code" on the right track, not to mention revealing an astonishing new piece to the puzzle first laid out in the 1960s. Joseph Krzycki, Michael Chan, and colleagues at Ohio State University transferred from an archaeon to Escherichia coli the ability to incorporate the amino acid pyrrolysine, which is not one of the canonical 20, into protein.3E. coli normally cannot do this. Conventional wisdom had held that pyrrolysine was a mere modification of lysine, just as selenocysteine can arise from tweaking cysteine. The investigators showed otherwise by giving E. coli an aminoacyl tRNA synthetase specific for pyrrolysine, a supply of the rare amino acid, and mRNA from a gene in the archaean that naturally incorporates pyrrolysine. The bacteria faithfully produced the foreign protein.Like all good science, the work raises new questions. Since the synthetase targets the "stop" codon UAG, why doesn't it insert pyrrolysines at other sites in E. coli's genome, not just in the gene in question? The specificity may come from neighboring sequence that contorts the mRNA into hairpin loops.4The new work also, once again, dramatically demonstrates the universality of the code. The general public can be let in on these sorts of studies if only the media would be more precise in bandying about "genetic code." Attention editors and writers: The fact that spellcheck spits back "gnome" for "genome," creating a single-base deletion, is no excuse for not using this term.More profoundly, E. coli's incorporation of this unusual amino acid shows that there is never a final word in science; new information continually modifies what we thought we knew. And that's something that the media, with its focus on "scientific proof," has yet to learn – but that's another topic.Ricki Lewis rlewis@the-scientist.com

In Search of the Human Genetic Code

The public has been slow to embrace the word “genome” because of a continuing confusion with the term “genetic code.”

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