If you thought we had explored all the important parts of our genome, think again. Scientists are puzzling over a collection of mystery DNA segments that seem to be essential to the survival of virtually all vertebrates. But their function is completely unknown.

The segments, dubbed 'ultraconserved elements', lie in the large parts of the genome that do not code for any protein. Their presence adds to growing evidence that the importance of these areas, often dismissed as junk DNA, could be much more fundamental than anyone suspected.

David Haussler of the University of California, Santa Cruz, and his team scanned the genome sequences of man, mouse and rat1. They found more than 480 ultraconserved regions that are completely identical across the three species. That is a surprising similarity: gene sequences in mouse and man for example are on average only 85% similar. "It absolutely knocked me off my chair," says Haussler.

The regions largely match up with chicken, dog and fish sequences too, but are absent from sea squirt and fruitflies. The fact that the sections have changed so little in the 400 million years of evolution since fish and humans shared a common ancestor implies that they are essential to the descendants of these organisms. But researchers are scratching their heads over what the sequences actually do.

The most likely scenario is that they control the activity of indispensable genes. Nearly a quarter of the sequences overlap with genes and may be converted into RNA, the intermediate molecule that codes for protein. The sequences may help slice and splice RNA into different forms, Haussler suggests.

Another set may control embryo growth, which follows a remarkably similar course in animals ranging from fish to humans. One previously identified ultraconserved element, for example, is known to direct a gene involved in the growth of the brain and limbs.

To solve the conundrum, experts predict a flurry of studies into the enigmatic DNA chunks. "People will be intrigued by this [finding]," says Kelly Frazer who studies genomics at Perlegen Sciences in Mountain View, California. "It is the kind of stuff that blows people away."

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'Junk' throws up precious secret

A collection of mystery DNA segments, which seem to be critical for the survival of many animals, are causing great interest among scientists.
Researchers inspecting the genetic code of rats, mice and humans were surprised to find they shared many identical chunks of apparently "junk" DNA.

This implies the code is so vital that even 75 million years of evolution in these mammals could not tinker with it.

But what the DNA does, and how, is a puzzle, the journal Science reports.

Excess baggage?

Before scientists began laboriously mapping several animal life-codes, they had a rather narrow opinion about which parts of the genome were important.

According to the traditional viewpoint, the really crucial things were genes, which code for proteins - the "building blocks of life". A few other sections that regulate gene function were also considered useful.

The rest was thought to be excess baggage - or "junk" DNA.

But the new findings suggest this interpretation was somewhat wanting.

David Haussler of the University of California, Santa Cruz, US, and his team compared the genome sequences of man, mouse and rat. They found - to their astonishment - that several great stretches of DNA were identical across the three species.

To guard against this happening by coincidence, they looked for sequences that were at least 200 base-pairs (the molecules that make up DNA) in length. Statistically, a sequence of this length would almost never appear in all three by chance.

Not only did one sequence of this length appear in all three - 480 did.

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This is just so fasinating to me.