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Does DNA really have a “half life”? Physicist Rob Sheldon is skeptical


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.

The Long Ascent, Volume 2

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?

Genesis: The Long Ascent and The Long Ascent, Volume II

This horse deserves a beating even after it's dead. The point is that DNA can certainly be destroyed a lot sooner (within minutes after putting it on a charcoal grill, for example), but even under the most ideal conditions, it still degrades at a minimum rate determined by background radiation--DNA's half life. There no way, no how for there to be any such thing as 700,000 year old DNA. The specimen must either be contaminated or the specimen is a hell of a lot younger. There are no other realistic options. -Q Querius
@Polistra, you mean languages devolve over time, we started from complex language systems in the Mesoppotamia dispersion to very simplistic languages that we have today which keep degrading by comparing them even to the Middle Ages, alongside everything else that degrades. All of this supports perfectly the Genesis narrative, most likely the pre-flood language was similar to traditional Chinese since the gospel message and a lot of other key passages are preserved there Kirikagure
RS@4: "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." Maybe we are talking past each other. The 512 year half life of DNA is the supposed ideal under best conditions. There are lots of reasons why it could be less as you point out. No one that I know of is using DNA to date fossils. It's just setting an upper boundary on when you can expect to still find it in a specimen. Much like C-14 isn't used in things assumed to be much over 50,000 years. They don't expect to have enough present to accurately use the method. So, question. What do you do with DNA identified in Hadrosaur cartilage when it is presumably at least 66 million years in age? Why is there even cartilage in a specimen that old? Does this strike you as reasonable that there would be any? I know, you're a physicist not a biologist but from the biology side there should be no way...yet there it is. These findings should be a red flag....something is not right. A complex bio-molecule like DNA should never be found in something at least 66million years of age, period, full stop. So....perhaps (just a glimmer in the dark, just maybe) the hadrosaur isn't really that old. After all no one uses nuclear dating on fossils (it's expensive). The date is based on the strata that it's found in. And how do they date the strata? Why that's based on the fossils found in it! I kid you not! I suppose somewhere back there in what my kids consider ancient history someone radiometrically dated that strata, but if so something is radically wrong. Latemarch
The reason to be "overly fussy" is the certainty of the logic or conclusions. If I use radioisotope half-life to date an artifact, I should expect accuracy to at least 10% or better. Calibrated C-14 should be <1% accuracy. But if I use "DNA half-life" to predict the presence or absence of DNA in a dino bone, I might be wrong by 10,000% or more! As someone once pointed out about cold fusion, that is the difference between a dollar and the national debt. (Yeah, I know, the national debt is a moving target.) Robert Sheldon
And what will you do with this? Evidence of proteins, chromosomes and chemical markers of DNA in exceptionally preserved dinosaur cartilage Published in National Science Review Jan. 2020
"This study provides the first clear chemical and molecular demonstration of calcified cartilage preservation in Mesozoic skeletal material, and suggests that in addition to cartilage-specific collagen II, DNA, or at least the chemical markers of DNA (for example, chemically altered base pairs that can still react to PI and DAPI), may preserve for millions of years."
As Polistra says "overly fussy". Half life of drugs are frequently listed as well as increased or decreased half life related to disease states or age. It allows the physician to properly dose an individual patient. Half life....it's just a calculation. Latemarch
One thing one can say legitimately is that DNA does have a maximum half life based on background radiation. Other factors such as high temperature, humidity, pH, chemical reactions, and location (such as being buried in a natural uranium deposit) will hasten the breakup of DNA.
In 2013, a 700,000-year-old horse fossil frozen in permafrost became the oldest DNA ever sequenced.
If this is true, then please explain how DNA survived 700,000 years of background radiation. Was it encased in a box made of lead? -Q Querius
Overly fussy. We already use the term in many different ways. At least they're getting the half * half * half aspect right. Most of the time it just means something goes away or fades. Language DOES evolve over time. Can't stop it. polistra

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