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Genome may be full of junk after all: cross-species analysis suggests most DNA has no function.

Most of the human genome may actually be junk.

In recent years scientists have stopped dismissing as nonfunctional the part of the genome that doesn't produce proteins. But a new study comparing the human genetic blueprint with 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 with those of other mammals, Sidow shows that about 85 percent of the conserved DNA (a bit more than 6 percent of the total genome) is in spacers between genes or between protein-producing bits within genes. This positioning suggests that these DNA regions may play a role in regulating how proteins are made, Sidow said November 3.

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 puffer fish has a genome of only about 390 million DNA letters, but is still a sophisticated organism.

But some of Sidow's colleagues think his analysis maybe missing some crucial elements. Recent studies of RNA molecules that don't code for proteins show that those molecules have definite functions, even though they 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.

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Title Annotation:Genes & Cells
Author:Saey, Tina Hesman
Publication:Science News
Geographic Code:1USA
Date:Dec 4, 2010
Words:357
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