AR-News: Humans, animals share more DNA than previously thought

[jim robertson]

Humans, animals share more DNA than previously thought
Santa Cruz study shows common 'junk' fragments

Keay Davidson, Chronicle Science Writer
  Saturday, May 8, 2004

The next time you play with a dog or shriek at a mouse or slice into a 
chicken, keep this in mind: They're your distant cousins, genetically 
speaking.

New evidence for the unity of earthly life was revealed Thursday by 
scientists at UC Santa Cruz and elsewhere, who reported discovering a 
breathtaking number of perfectly matched DNA fragments in these creatures 
and humans.

What's equally amazing to the scientists is how many of the matching 
fragments are part of material once scorned as "junk DNA," genetic detritus 
that has accumulated over the eons and that, it was long assumed, serves no 
useful purpose.

Now, the evidence of the long-run endurance of so much "junk" suggests that, 
on the contrary, at least a fraction of the "junk" does serve some kind of 
evolutionary purpose, although scientists don't know what it is.

A team led by UC Santa Cruz postdoctoral researcher Gill Bejerano used 
high-speed computers to compare the gene sequences of humans and animals, 
they report in Thursday's Science Express, a Web site managed by the journal 
Science.

They found that the chemical sequence of certain segments of DNA in specific 
vertebrates precisely matches some DNA segments in humans. In all, they 
identified almost 500 segments that were "completely unchanged" despite tens 
or hundreds of millions of years of evolution in animals as seemingly 
unrelated as mice and men.

Metaphorically speaking, it's like finding planks from Noah's Ark in the 
bulkhead of the Jeremiah O'Brien.

It's hard to understand how the oldest of these DNA fragments -- some more 
than 400 million years old -- could have endured unchanged over such an 
incredible length of time, the scientists said. By comparison, the dinosaurs 
went extinct relatively recently, a mere 65 million years ago.

These DNA fragments "are now evolutionarily frozen. We don't know of a 
biomolecular mechanism that would explain them," said Professor David 
Haussler of UC Santa Cruz, a computational biologist who runs the lab where 
his postdoctoral colleague, Bejerano, did most of the work. The research 
team also included scientists from the University of Queensland in Brisbane, 
Australia. Their computer-crunching uncovered 481 matched DNA segments, each 
of which is at least 200 "base pairs" long. The longest is 800 base pairs. 
(Strands of DNA are composed of "base" molecules that pair up, forming a 
continual genetic code that controls growth and activity in virtually all 
life forms.)

The long-run survival of the genetic segments is especially puzzling because 
for DNA, time is a ruthless gauntlet. Over the eons, genes routinely suffer 
mutations caused by accidents, such as when a high-speed space particle or 
cosmic ray smashes through the DNA coil, or by errors that occur when DNA 
copies itself.

How could the 481 matching fragments have endured intact for so long and in 
such widely separated life forms? After all, the human evolutionary line and 
chicken lines went their separate ways 300 million years ago, while the 
human and fugu-fish lines said sayonara more than 400 million years ago.

Neo-Darwinian theory holds that genes coding for beneficial traits are more 
likely to survive because they enhance the organism's chance of survival; 
those genes that don't enhance survival tend to die out because the organism 
carrying them is less likely to reproduce.

By this reasoning, Haussler explains, the matching DNA fragments might have 
been preserved in pristine shape because they serve some useful evolutionary 
purpose -- its nature still unknown.

E-mail Keay Davidson at kdavidson at sfchronicle.com.


http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2004/05/08/MNGA16H1GD12.DTL




"Just remember it's the birds that's supposed to suffer, not the hunter." 
—George W. Bush, advising quail hunter and New Mexico Sen. Pete Domenici, 
Roswell, N.M., Jan. 22, 2004