How long do DNA samples last 1

Researchers determine the half-life of DNA

Our genome has a half-life: after the death of a living being, its DNA disintegrates at a constant, predictable rate. This is what an international team of researchers discovered when examining fossil bird bones. In these, half of the mitochondrial DNA was destroyed by degradation processes after 521 years. This corresponds to a constant decay rate of 5.5 DNA breaks per million molecules per year, the researchers report in the journal "Proceedings of the Royal Society B". “Our results show for the first time that DNA in fossil bones decays at an almost unchanged rate over time,” write Morten Allentoft from Murdoch University in Perth, Australia, and his colleagues.


Finding intact DNA from the Cretaceous period is unlikely

This is important information for paleontology, say the researchers. Because on the basis of this decay rate, one can now predict whether there is still intact genetic material in a fossil find. “Our half-life calculations show, for example, that it is extremely unlikely to be able to isolate intact DNA fragments from 80 to 85 million year old bones,” say the researchers. In 1994, however, a research team claimed exactly that in a publication. In view of the new findings, however, everything suggests that the supposedly fossil DNA was a contamination of the samples, the researchers state. However, they consider it quite possible that primeval genetic material could last for several hundred thousand to even a million years.

As the researchers emphasize, the half-life of DNA changes depending on the temperature and environmental factors: at minus five degrees, for example, the half-life for small pieces of DNA in bones is 158,000 years, at higher temperatures it is shorter. What is decisive, however, is the knowledge that the biological degradation of DNA in a fossil buried in the ground for thousands of years basically follows a calculable curve.

Bones of an extinct bird as test objects

For their study, the researchers examined 158 leg bones of the moa, a bird originally native to New Zealand but now extinct. The bones were between 602 and 7,839 years old. All fossils were found only five kilometers apart on New Zealand's South Island and were found in the same type of rock. As a result, all bones were exposed to the same chemical-physical conditions and were preserved at around 13 degrees, the researchers report. They isolated the genetic material from the bones and compared how well a 242 base pair fragment of mitochondrial DNA (mtDNA) was preserved in the various samples.

"When we related the DNA content to the age of the bones, we found an exponentially decreasing curve," write Allentoft and his colleagues. From this, an mtDNA decay rate of 5.5 decays per million molecules per year can be calculated for the fossil sites. The DNA from the cell nucleus, on the other hand, disintegrated two to two and a half times faster in an additional test. According to the researchers, the longer shelf life of mitochondrial DNA could be related to the fact that this genetic material exists as a small ring and is therefore more stable than the loose strands of nuclear DNA. (doi: 10.1098 / rspb.2012.1745)

(Proceedings of the Royal Society B, 10.10.2012 - NPO)

October 10, 2012