Genome may be mostly junk after all

Cross-species comparison suggests most human DNA has no known function

WASHINGTON — Most of the human genome may actually be junk.

Scientists had recently stopped dissing the part of the genome that doesn’t produce proteins. But a new study comparing the human genetic blueprints to those of other mammals concludes that very little of the human genome is really necessary.

About 7 percent of the human genome is similar to the DNA of other mammals, said Arend Sidow of Stanford University. Because it is similar, or “conserved,” geneticists assume this DNA is the most integral. In all, Sidow concludes, these important parts of the genome comprise only 225 million of the 3 billion chemical letters of DNA found in the complete human genetic instruction book.

But only a small portion of the conserved DNA is translated to produce proteins. Comparing the human genome to those of other mammals, Sidow shows that about 85 percent of the conserved DNA (and a bit more than 6 percent of the total genome) is found in spacers between genes or between protein-producing bits within genes. This positioning suggests these DNA regions may play a role in regulating how proteins are made, Sidow said November 3 at the annual meeting of the American Society of Human Genetics.

Sidow’s studies rely on the principle that if certain pieces of DNA are retained throughout evolution, they must be important. Things that aren’t conserved by evolution are less likely to be required for basic functions. “I think the rule is that important stuff stays,” he said.

And all the extra DNA isn’t necessary to add complexity, Sidow argues. After all, the pufferfish has a genome of only 380 million DNA letters, but is still a sophisticated organism.

But some of Sidow’s colleagues think his analysis may be missing some crucial things. Recent studies of RNA molecules that don’t code for proteins show that those molecules have definite functions, but aren’t conserved in the DNA codes of other mammals, said Job Dekker of the University of Massachusetts Medical School in Worcester. “Lots of things that are important are not conserved,” he said. And current computer programs may not be very good at picking out small DNA regions shared among many species, he added.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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