Second DNA Code: Human DNA Sends 2 Messages At Once, Presenting New Implications For Evolutionary Biology
It has long been known that the genetic codes contained within DNA perform two functions: creating the building blocks of cell function and designating which genes to turn on and which to turn off. Those two simple tasks determine basically everything about us as humans. Now, scientists have discovered that 15 percent of codons, the three-letter instructions that make up the genome, give out both instructions at the same time.
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In a paper published Friday in the journal Science, researchers from the University of Washington explained their findings and named these special codons "duons." While these dual-purpose codons were the subject of the university's excitement in its press release announcing the research, the fact that those functions already existed is not new, as several geneticists and science reporters have observed.
What is new is that these so-called duons may play a greater role in evolution that previously thought. "The fact that the genetic code can simultaneously write two kinds of information means that many DNA changes that appear to alter protein sequences may actually cause disease by disrupting gene control programs or even both mechanisms simultaneously," said lead author John Stamatoyannopoulos in a statement. In other words, those 15 percent of codons could be weilding outsize influence.
To help make sense of what all this means, here's a primer on genetics. In each of your cells, there's a nucleus. Inside those are chromosomes, and inside those are genes. You can think of each gene as basically a telegram with a series of codes on it (your DNA) containing the letters A, C, G and U in groups of three. These codes (the codons) give directions to proteins about which amino acids to produce, and the amino acids, in turn, make up the proteins that carry out various cell functions.
Meanwhile, there are these other proteins called transcription factors, which latch onto the DNA and tell them which genes should be turned on and off. They serve a sort of regulatory function, ensuring that liver cells don't get turned on in skin cells, for example. They make sure the body gets built the right way. Other times, when they're corrupted, they mean the difference between cancer cells and healthy cells, all by managing which codes get dispatched and when.
What Stamatoyannopoulos found was that 15 percent of codons give two sets of instructions simultaneously: specifying amino acids and telling transcription factors where to go. As the Science editors explain, "The distribution of the TF binding sites evolutionarily constrains how codons within these regions can change, independent of encoded protein function. Thus, TF binding may represent a widespread and strong evolutionary force in coding regions."
As Forbes pointed out in a piece called "Don't Be Duped By 'Duon' DNA Hype," the story about a "second code" got somewhat hyperbolized and misguided. Stamatoyannopoulos, quoted in the news release, certainly didn't help by saying, with exaggeration, "Now we know that this basic assumption about reading the human genome missed half of the picture." Leonid Kruglyak, a genetecist at UCLA, responded to the news in a tweet: "horribly ahistorical. And control regions within coding [sequences] is not new."
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