It’s exciting how much DNA we are finding from how far back:
In 2013, a 700,000-year-old horse fossil frozen in permafrost became the oldest DNA ever sequenced. Before that, the oldest sequenced genome was from the remains of an 80,000-year-old Denisovan.
Then, earlier this year, scientists announced they’d sequenced DNA from a 1.2-million-year-old mammoth tooth – which currently holds the record for the oldest recovered and sequenced DNA.Jacinta Bowler, “The Trouble With Dinosaur Bones” at ScienceAlert
We are told not to expect too many more DNA windows into time:
DNA has a half-life of 521 years, meaning that after 521 years, half of the bonds in its molecular backbone break. After 1,042 years, half of that remainder would be gone, too.Jacinta Bowler, “The Trouble With Dinosaur Bones” at ScienceAlert
After a million years, they say, forget it. But our physics color commentator Rob Sheldon paused at the term “half life” and commented,
As a physicist, I would like to point out that biologists are misusing the word “half-life”. DNA does NOT have a half-life of 521 years. Radioisotopes have a half-life, because the nucleus is unstable to natural decay through the weak force (for isotopes of interest).
The weak force is unaffected by temperature, pressure, time, or chemicals, so it is accurate to say that “after X time, 1/2 the material will have decayed, making X=half-life.” Now of course, if the nucleus itself is bombarded with electrons or neutrons, the decay can be accelerated, but at this point we are entering the realm of atom smashers, and one shouldn’t call this “natural decay” any longer.
This is definitely NOT true of DNA. DNA can be destroyed by heat. Otherwise why do we boil baby bottles or put hospital sheets in an autoclave? And heat is a statistical process, (hot molecules are not all moving at the same speed), so what takes 1 minute at 100C might take 20 minutes at 80C or 1000 minutes at 60C.
Accordingly DNA has a half-life depending on temperature. It also depends on pH, on free-radicals, on UV light and presence of water. In other words, it depends on everything in the environment. This completely destroys what physicists mean by “half-life”, and makes hash of the word.
Mammoth DNA from the frozen tundra of Siberia has been resurrected after 1.2 Million years. Denisovan DNA over 60,000 years ago was recovered from a cool cave in Eurasia. In the tropics, no DNA is recoverable after a mere 1000 years or so.
So please, don’t use the word “half-life” with regard to biological molecules unless you are very carefully specifying the environmental conditions as well.
But the very term “half-life” makes it all sound so much more scientific. Who could resist?