for National Geographic News
Scientists have sequenced the DNA of two cave bears that roamed the Austrian Alps some 40,000 years ago. It marks the first time researchers have been able to completely sequence the DNA of a species that has long been extinct.
The research opens the door to sequencing the DNA genome of other extinct species, including the Neandertals (often spelled "Neanderthals").
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"We have shown that it is possible to sequence the genome of a long-extinct organism, something previously considered to be in the realm of science fiction," said James Noonan, a geneticist and postdoctoral fellow in the genomics division at the Lawrence Berkeley National Laboratory in California.
Noonan is the lead author of the study, which appeared in the June 2 online edition of the journal Science.
Until now scientists have been unable to extract pure DNA from the cell nuclei of ancient animals. Not only does DNA start to degrade at death, but the microbes that eat away at dead animals contaminate genomic DNA, which is found in cell nucei. Other contaminants may include human DNA left through contact, such as when a scientist handles the bones.
Rather than use genomic DNA, most studies of ancient DNA have used mitochondrial DNA. A sort of cellular power plant, mitochondria have their own type of DNA and are believed to have evolved separately from genomic DNA. (See our quick overview of genetics.)
Only genomic DNA, however, can help scientists understand the functional differences between extinct and living species.
Because unbroken strands of ancient DNA are so hard to come by, previous ancient-DNA studies have used a biochemical amplification method to create a string of DNA. In effect, they take an unbroken fragment of DNA and copy it over and over to create a complete strand. But this only works for mitochondrial DNA, not genomic DNA.
Needle in a Haystack
This time the scientists took a different approach. First they extracted genomic DNA from two 40,000-year-old cave bear bones from Austria.
Extinct for more than 10,000 years, cave bears (Ursus spelaeus) were related to the ancestors of modern brown bears and polar bears. Cave paintings show that ancient humans encountered cave bears.
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