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While I’ve never taken a course in biochemistry for credit, over the years, I have learned the basics of biochemistry through independent reading as well as hanging around actual biochemists. when it comes to nucleic acids, the basics are fairly straightforward. The DNA (deoxyribonucleic acid) that resides within the nucleus of each cell of an organism is the archive of all the information required to create and run the individual it belongs to. However, just looking at an organism’s DNA doesn’t tell you which genes are the ones that are necessary for the survival of that organism at any one time. As an analogy, if you were dumped into a vast library without a card catalogue (showing my age here), it’s not likely that you’d be able to find a particular book you’re looking for. By various means, when a cell determines a need for a particular protein, a message is sent to the nucleus, where, in a process called transcription, the section of the DNA that codes for that particular protein is used to form a strand of RNA (ribonucleic acid) with a sequence of bases that are complementary to the ones on that section of the DNA. (A more detailed description can be obtained by watching this video.)
The resulting strand of RNA is called messenger RNA, or mRNA (yes, the stuff in the COVID vaccine is artificial messenger RNA) because, in order to start producing the desired protein, the mRNA has to leave the nucleus and find a ribosome, which is an enzyme complex that uses the information coded into the mRNA to construct the protein. Once there is sufficient protein, the cell used other enzymes to chop up the mRNA into its subunits which can be used again to make other strands of mRNA, or possibly other purposes, as needed.
As DNA is the informational archive for an organism, it has evolved to be chemically inert. RNA, on the other hand, is used only on an as-needed basis, and so it degrades quite rapidly under normal circumstances.
I have read interesting reports and written diaries about recovering both DNA and proteins from fossils of great age, including extinct species, but I would never have expected to see a study on the recovery and analysis of RNA from an extinct species. This is why I was surprised to see the results of a new study that does exactly that on the naturally freeze-dried remains of three Siberian wooly mammoths, whose ages range from 39,000 to 52,000 years old. Preservation of the RNA was no doubt helped by the fact that these specimens froze immediately after they died, but to me, it’s still a surprise that after that long, there’s anything to find.
Why is this important? Again, while DNA is the full library of information required by an organism, the RNA reflects the portions of that DNA that were being used to make proteins needed by the organism at its time of death (in the case of these specimens). The analogy that’s used here is that the DNA are the blueprints for a building, while the mRNA represents are the instructions provided to workers working on particular rooms in the building. Thus the RNA can give very specific information on what was going on biochemically in the organism.
He and his colleagues used enzymes to convert any existing RNA molecules within the sample into short strands of DNA, then sequenced them and reverse-engineered what the corresponding RNA sequence must have been. After screening out contaminating material from environmental sources or modern handling, they succeeded in identifying ancient RNA from three of the 10 mammoths, each dated to between 39,000 and 52,000 years old. The Siberian deep freeze likely halted the molecules’ degradation very shortly after the animals died.
Although most of that RNA was chopped up and fragmentary, one mammoth in particular, nicknamed Yuka, preserved extraordinary detail. For one, the team spotted a few RNA sequences from genes only found on the Y chromosome. That came as a surprise: Scientists had thought Yuka was female.
Other RNA sequences recovered from Yuka contain instructions for building and maintaining muscle tissue. In one sense, Dalén says, that observation is “a bit boring,” as those same genes are active in many other mammals. “But there might be in the future avenues where we find, for example, RNA from hair follicles,” he adds, “and learn which genes made woolly mammoths woolly.”
In any case, this research has opened a new door for biomolecular archaeology.
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