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DNA lives long after death, and holds the key to many stories

Monday, October 25, 1999

By Michael Woods, Post-Gazette National Bureau

Scientists have recognized a kernel of biochemical reality in one of Halloween's favorite spine-tingling tales -- the notion of the living dead.

Deoxyribonucleic acid, or DNA, the molecule that embodies the code of life, is a zombie. It survives after death and remains functional. This "living death" has opened up a whole new field of the life sciences, called ancient DNA, or "molecular archaeology."

It has been the centerpiece in scientific research on ancient life forms and criminal cases involving re-examination of old evidence, sometimes obtained from exhumed bodies.

"It could be DNA from a 1990 crime victim or a 40,000-year old mammoth," explained Evgeny I. Rogaev, a Russian Academy of Sciences expert on ancient DNA.

Rogaev's analysis of DNA from nine skeletons found near Yekaterinburg, Russia, confirmed that that the bones were remains of Czar Nicholas I, his wife Alexandra and their children. It helped resolve a long controversy over fate of Russia's last royal family.

How does zombie DNA do it? How does it stay when the rest of the body goes? How can it tell stories about the dead?

DNA makes up the chromosomes inside cells and carries genetic instructions for making, reproducing and operating the whole animal or plant.

DNA is made from four chemicals, called nucleotides: adenine, guanine, thymine, and cytosine. Those four chemicals are equivalent to letters that spell out DNA's genetic instructions, just like 26 letters in the English alphabet spell out written instructions. It takes about 3 billion letters to spell out the instructions for a human.

DNA's instructions remain long after the body dies. In fact, as Dr. Kenneth V. Iserson pointed out in his 1994 book, "Death to Dust: What Happens to Dead Bodies," there is no magic moment at which all life disappears. Legally, death occurs when the brain no longer works. But biologically, death is a slower process.

"Even though a person may be dead, because his heart stops working, some muscle, skin and bone cells may live on for many days," explained Iserson, director of bioethics at the University of Arizona Health Sciences Center. Eventually, lack of oxygen and the buildup of waste products kill off those little outposts of cells.

DNA also starts to degrade from the moment of death, as the bonds holding the molecule together break apart under assault from water, oxygen and microbes.

Some of the best sources of ancient DNA are samples sealed off from the environment, or in natural deep freezes. Movie scenes from "Jurassic Park" of dinosaur DNA extracted from mosquitoes preserved in amber had roots in scientific fact. Amber has a drying effect, and seals DNA away from attack by oxygen and microbes.

Molecular archaeologists have claimed success in extracting samples of DNA from termites and bees encased in amber 30 million years ago, and a weevil that lived 125 million years ago. Other scientists dispute those findings, claiming that modern DNA contaminated the experiments.

Heat is another foe. Freezing can slow the rate of damage to DNA by 10-20 fold. Among the best sources of ancient DNA have been ancient mammoths and humans frozen in ice in permafrost in Siberia and other regions of the world.

Dry places are another. Last year a scientific group led by Hendrik N. Poinar of the University of Munich, announced the development of a technique to isolate DNA from 20,000-year-old dung preserved in dry caves in Arizona and Nevada. The dung came from the ground sloth and about 20 other species of extinct large animals.

Animals shed DNA-containing cells in their feces. Studying the material could pinpoint genetic reasons to explain why the animals became extinct.

"This adds new dimensions to the study of ancient animals," said Robert Wayne, an evolutionary biologist at the University of California at Los Angeles.

How long does DNA remain after death? Depending on conditions such as moisture and temperature, it may remain for millions of years. Scientists have claimed extraction of DNA from a variety of ancient sources, including the remains of a quagga, a zebra-like animal that became extinct 145 years ago; ancient Egyptian mummies; an American mastodon that lived 46,000 years ago; Ice Age hunters frozen about 10,000 years ago, and 40-million-year old bacteria isolated from the gut of bee.

No matter how well preserved an ancient organism may be, scientists think that most DNA deteriorates after 50,000 to 100,000 years.

A laboratory technique called the polymerase chain reaction, or PCR, however, has made it possible to recover even the most minute traces of the genetic code. Years ago, scientists had to find hair, hides, bones, and other big samples to extract enough DNA for analysis.

PCR changed that. It uses an enzyme to amplify DNA -- almost like a photo-copy machine transforms one document into a hundred or a thousand identical copies. If even a few molecules of DNA remain in an ancient sample, PCR can reproduce plenty of material for analysis.

What does ancient DNA reveal when recovered from an exhumed body?

Genetically speaking, humans are 99.9 percent identical. But with 3 billion letters in the genetic code, even that 0.1 percent of variation allows for individuals to have a unique genetic "fingerprint." This can be used in a variety of ways, such as linking a bloodstain at a crime scene to a specific suspect; determining paternity or establishing the identify of people long dead.

Scientists at the Armed Forces Institute of Pathology in Washington used it to identify more than 150 missing servicemen from World War II, Korea and Vietnam.

In 1998, for instance, they identified the Vietnam-era soldier in the Tomb of the Unknowns in Arlington Cemetery. Scientists used PCR to amplify DNA from bone fragments and compared it to DNA samples of relatives of servicemen killed within a 25-mile radius of where the body was found.

They concluded the unknown was 1st Lt. Michael Blassie and his remains were reburied near his hometown in Missouri.

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