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Scientists to Resurrect Ancient Gene to Replay Evolution

By Michael Schirber, Astrobiology Magazine

posted: 02 May 2009 11:09 am ET
Woolly Mammoth
Woolly mammoths were driven to extinction by climate change and human impacts. 
Credit: Mauricio Anton
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Woolly Mammoth
    Woolly mammoths were driven to extinction by climate change and human 
impacts. Credit: Mauricio Anton
    Cells are the fundamental working units of every living system. All the 
instructions needed to direct their activities are contained within the 
chemical DNA (deoxyribonucleic acid). Credit: U.S. Department of Energy Genome 
Programs
    Apart from reproductive gametes, each human cell contains 23 pairs of 
chromosomes, each a packet of DNA. Every strand of DNA is a natural polymer of 
repeating nucleotide units, each of which comprises a phosphate group, a sugar 
(deoxyribose), and a base (either adenine, thymine, cytosine, or guanine). 
Every strand thus embodies a code of four characters (A's, T's, C's, and G's), 
the recipe for the machinery of human life. In DNA double helix, the strands 
are linked by hydrogen bonds between adenine and thymine (A,T) and between 
cytosine and guanine (C, G). Each such linkage is said to constitute a base 
pair; some three billion base pairs constitute the human genome. Credit: U.S. 
Department of Energy Genome Programs

The movie "Jurassic Park" was a lesson in how resurrecting extinct organisms 
can go awry. A new project plans to take a safer route: resurrect a single gene 
from an extinct species of bacteria. This tiny snippet of DNA will be implanted 
in modern-day bacteria, with the goal of seeing whether evolution can be 
replayed in the lab.

In previous work, paleogeneticist Eric Gaucher from the Georgia Institute of 
Technology and his colleagues reconstructed earlier forms of a common gene by 
computing the way different lineages diverged to create the bacterial family 
tree.

"It is a bit like what historical linguists do when they infer the spelling or 
pronunciation of an ancient word from its modern derivatives," Gaucher says. 
"Except, we are working with the DNA alphabet."

As part of NASA's Astrobiology: Exobiology and Evolutionary Biology program, 
Gaucher and Betul Kacar, also from Georgia Tech, now plan to plug one of their 
reconstructed genes in a modern organism's DNA.

If this genetic anachronism evolves forward along one of the branches that the 
researchers have computed, then this will provide some verification of this 
molecular genealogy technique, as well as give support to the notion that 
evolution is repeatable and not simply a matter of chance.

Ghosts in the code

It is hopeless to think that dinosaur DNA could be recovered from mosquito 
blood trapped in amber (or from anywhere else for that matter), as the 
molecular code isn't likely to survive 65 million years.

The chances are far better for more recent extinctions. A nearly complete DNA 
sequence of the woolly mammoth (which died out about 11,000 years ago) was 
published last November, giving some people ideas about bringing these giants 
back to life.

However, finding frozen hair and tissue samples is not the only way to isolate 
extinct DNA. Gaucher and his colleagues have shown that it is possible to 
estimate the genes in organisms that lived several billions of years ago by 
doing a genetic survey of their family tree.

It is a bit like guessing what color your great-great-great grandmother's eyes 
were by cataloguing the eye colors of all her living descendants and playing 
back the rules of inheritance. In the case of gene reconstruction, Gaucher's 
team estimates the DNA code of an extinct life form by comparing the codes of 
its living descendants and using theories of genetic mutations.

Sick with age

Gaucher and Kacar now plan to insert one of these ancient genes into a modern 
E. coli bacteria.

"These bacteria are going to be sick," Gaucher explains. That's because this 
gene codes for an essential protein, but the outdated version being inserted 
into the organisms works best at a temperature of 55 degrees Celsius, far above 
the 37 degrees Celsius that E. coli prefers.

Like a molecular Rip Van Winkle, the ancient EF gene will feel strong 
evolutionary pressure to adapt to its new cooler surroundings.

"It's difficult to see evolution, short of building a time machine," Gaucher 
says, but their technique may be the next best thing.

The scientists will be verifying whether the mutations in the inserted gene 
follow the same path as was taken by the line of ancestor bacteria as they 
evolved over millions of years.

"I do believe that it is now possible, with tools that have recently been 
developed, to 'replay the molecular tape of life,' even if it is one (or a few) 
molecules at a time," says Belinda Chang of the University of Toronto, who is 
not involved in this work.

    * News and Information about Extinctions
    * Extinct Tasmanian Tiger's DNA Revived in Mice
    * Gallery: Mammals Facing Extinction

This article was provided to LiveScience by Astrobiology Magazine.


© Imaginova Corp. All rights reserved.


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