Like a bottle of milk at the supermarket, date-stamped "Sell by . . . ,"DNA has a time-limited shelf life.
The double-helix molecule that defines all life on earth decays by theprocess of depurination: Free radicals in water perform thisdegradation by breaking certain nucleic acids loose from their sugar-phosphate backbone, and thus chops up the DNA strands.
But unlike perishable groceries, the depurination process takes years,centuries, millennia _ depending on ambient conditions _ rather thandays or weeks. But can DNA hang in there for millions of years?
Leaving Jurassic Park to one side, at least two Americanpaleontologists have recently discovered dinosaur bones, one in Utah,the other in Montana, in which they identified putative DNA. (SeeBioWorld Today, Nov. 23, 1994, p. 1.)
And a' la Jurassic Park, other scientists have identified DNA from ahoney bee and a weevil embedded in amber millions of years ago.(See BioWorld Today, June 10, 1993, p. 1.)
When a paleontologist's hammer strikes gold, in the shape of a fossilbone, he or she can date it in the field by identifying the geologicalstratum in which it was buried. This low-tech age-by-association canthen be fine-tuned by high-tech potassium-argon dating, whichcompares the rates at which radioactive argon and potassium decay.
Now an international team of microbiologists and molecularbiologists has contrived and tested a similar system for construingDNA depurination in relation to the known time-definedtransformation of essential amino acid molecules from optically left-facing (L) to right-facing (R) mirror images.
This method relies on the high-tech amplification of DNA andamino-acid molecules by PCR.
Their report, in today's Science, bears the title: "Amino acidracemization and the preservation of ancient DNA."
Racemization is the switch-over from 100 percent L to 50 percent R,producing a stable mixture of an amino acid's two chemicallyidentical forms. The protein content of all but a very few livingorganisms on earth consist of L amino acid isomers. Racemization ofan individual's proteins begins when it dies.
In fact, a classic whodunit ("The Documents in the Case," byDorothy Sayers) nailed its murderer by revealing that his victim'sstomach contained the R isomer of toad stool toxin, rather than thenutritious L mushroom protein.
Amino Acids Know Left From Right
Marine chemist Jeffrey Bada co-authored the Science paper withscientists at the University of Munich, Germany, led by moleculargeneticist Svante Paabo.
Bada, at the Scripps Institution of Oceanography, in La Jolla, Calif.,had worked out the rates at which the 20 amino acids essential to liferacemized, and their correlation with DNA degradation. Hedetermined that of all amino acids, the fastest to racemize is asparticacid. Paabo and his colleagues developed and applied the method forusing this lock-step correlation to authenticate specific samples ofancient DNA.
First they tackled 26 "archeological" specimens, ranging in age froma five-year-old living horse to a Siberian mammoth that died 50,000years ago. Then they analyzed DNA from 16 "paleontological"samples, including the Tyrannosaurus rex skeleton in Montana, andthe more publicized slivers of dino bone from the roof of a Utah coalmine. Both go back 65 million years.
The amber-embalmed honey bee and weevil date from 35 millionyears ago.
Microbiologist Scott Woodward, of Brigham Young University, inProvo, Utah, provided his bone material to the Bada/Paabo team toverify their new method for authenticating ancient DNA. The dinosamples scored a ratio of 0.21 on the aspartic-acid test, which rates0.8 as the upper limit of authenticity.
This, and comparable test results on other samples, led Bada toconclude, "What we have shown is that you cannot get meaningfulDNA out of dinosaur fossils."
But Woodward, in an interview with BioWorld Today, expressed his"reservations." He objected that those negative results were based on"indirect measurements" and so represented only "a good firstapproximation" of a valid DNA-authenticating test.
He is working toward such a direct measurement system, but allowsthat when and if it will be realized is "a $10 million question."
Old DNA Tackles Useful Chores
Meanwhile, Woodward is actively conducting analysis of animalDNA in the parchment of Israel's Dead Sea Scrolls, which exist as ajigsaw puzzle of 10,000 to 20,000 fragments. By matching up theDNA sequences in these, he and his associates at the HebrewUniversity hope to "verify the joints and improve the manuscripts" ofthe scrolls.
Separately, he is analyzing the DNA of ancient pathogens in some ofthe 500 mummies excavated at the Fayum archeological site inEgypt. "Looking at these microorganisms," Woodward observed,"will help us see how those ancient bugs evolved into today'spathogenic organisms."
Paabo told BioWorld Today that he hopes the system described inScience will be of great methodological help for the authentication ofancient DNA. "It allows you to very rapidly screen thousands ofamino acid samples to find the very small minority that actuallycontain DNA."
He and his colleagues in Munich "are focusing on retrieving ancientDNA, studying the type of damage that exists in it, and seeing if wecan actually repair the DNA from samples, which today areimpossible to work with."
Bada is partially funded by the National Aeronautics and SpaceAssociation's exobiology program, with which he is collaborating inresearch to find present or past life on Mars.
Paabo explained: "If there had been life on Mars, you would expectskewed ratios of the two forms of amino acid, D and L. So one thingthat Jeff is planning is to analyze racemization on the surface ofMars." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.