Just how long can DNA survive after the death of a cell or without cellular activity to maintain itself? I addressed this question a few months ago (Young Earth Creationism and Ancient DNA) and suggested that we would learn much more in the coming year. Already we have seen multiple complete genomes from fossil bones of humans and a variety of other organisms and this week we have evidence that viruses can survive in frozen tundra for 10s of thousands of years.
Scientists exposed single-celled amoebas to melted ice water from 30,000 year old frozen soil cores from Siberia. Soon after exposure the amoebas were attacked by a virus and killed. Electron microscopy and DNA sequencing revealed the killer to be a radically different virus from all previously identified. Presumably this virus had lay dormant in the permafrost soil until it was released in the lab.
Not only ancient but viable
What is remarkable about this virus, with respect to studies of ancient DNA, is that its genome of over 600,000 base pairs was still viable. As I said before DNA begins to degrade once there is no cellular activity to maintain the integrity of a genome. How quickly DNA degrades is of great interest and great debate. What we do know is that under very cold conditions the process of DNA degradation is greatly reduced. In the case of the 500,000+ year old horse sample from under glacial ice in Alaska the genome was completely reconstructed but it was still broken into many small pieces (A Horse is a Horse According to Answers in Genesis). For a virus to still be able to infect a host the essential genes for infection and replication must not have been damaged so badly that they didn’t work anymore. This is quite a remarkable feat of longevity but viruses are strange in that their genomes can be far more pliable or adaptable than many other “living” things and thus they may be able to survive much longer than typical cells under these conditions. There have been viruses that are older than this one that have been sequenced but their viability has never been confirmed.
Larger than a bacteria and a similar-sized genome
Also noteworthy is the size of this virus. It is a new record holder for size. It is larger than many bacteria and is visible with a light microscope. Its genome is not a record breaker but it’s no slouch either. Its genome was completely sequenced and comes in at 610,033 base pairs which encodes for 467 proteins. Contrast that with our common flu virus at 14,000 nucleotides in length encoding just 8 genes. In fact this virus has more genetic information than some parasitic bacteria. The genome itself is full of unknown genes and even the genes that are recognized (32% of the 467) have sequences that are only distantly related to known genes. This genetic uniqueness of this virus is quite remarkable given that billions of DNA sequences that have been generated for organisms all over the earth with which this genome can be compared. Most of its genes are as distinct from other viruses as they are from bacteria, plants or animals making their origins very unclear.
Like the Pandoravirus which I intend to write about soon these megaviruses are causing scientists to rethink the definition of life since viruses typically haven’t been viewed as being “alive.” These viruses have properties that certainly skew the distinction between living and non-living with this virus being much more than just a parasitic DNA strand. Rather it has a much more complex “life” cycle than most other viruses.
Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology. Legendre et al. 2014. Proceedings National Academy of Sciences. http://www.pnas.org/content/early/2014/02/26/1320670111