How long does DNA last?

Factual Questions
1

How long does DNA last? Here’s why I wanna know:

I saw a documentary (narrated by Alec Baldwin, dunno the name) that mentioned that Neanderthal man died out around 25,000 years ago. They also lived in some harsh, cold environments; considering that fact, could there be some well-preserved Neanderthal DNA somewhere on ice, as yet unfound? Also, I read somewhere that Neanderthals could and did interbreed with Modern man, although the practice was not believed to be common. If so, could Neanderthal sperm be harvested to fertilize a woman? Would it be ethical to reintroduce an extinct human species?

Art

2

I am not an expert on this by any means, but my understanding is that it IS possible for some DNA to still be around if thoroughly frozen, but it is highly unlikely that we would have an entire genome, and even less likely to have a specific Neanderthalis sperm cell with a genome. IIRC, the most likely DNA to survive is the mitochondrial DNA, which is not part of the regular genome but rather a specific subset contained only in cell mitochondria (the energy factories). This is maternal (passed down from the mother only) which again limits the information we can gain from it.

It is believed that Homo sapiens could breed with Neanderthals, however I don’t know whether offspring would be fertile or not. Not knowing the full genome of the Neanderthal makes it hard to predict - if they have 24 pairs of chromosomes as do many primates, then infertile offspring are likely. I am not familiar enough with the body of evidence showing how often or even IF matings did occur.

As for ethical - I don’t know. Too many complicated issues for me to think about!

3

Well, on the one hand, purified DNA is very stable and can be left at room temperature for years and frozen indefinitely without much damage at all. On the other hand, cells have a number of enyzmes that break down DNA which become active as the cells die, so DNA that was is cells (like human DNA) generally gets broken down pretty quickly. Also, virtually every microorganism known to man will eat DNA for nutrients.

Occasionaly, you have a situtation where someone was frozen/dried/preserved fast enough that neither microorganisms nor their own enzymes had a chance to completely break down the DNA before they froze and/or dried out. There is still almost certainly some breakdown from enzymes, and freezing itself breaks long
DNA strands like chromosomes. DNA analysis is possible, since this uses short strands and many cells worth of DNA, but any given cell (such as a sperm cell) will have LOTS of breaks and missing bits and therefore would not have enough of the genome to survive, much less fertilize an egg.

More importantly, a sperm needs a lot more than just DNA to fertilize an egg - the DNA needs to be surrounded by chromatin proteins and the entire apparatus of the sperm cell (proteins, lipids, RNA, and what-all) need to be fairly functional. That’s not going to happen with frozen bodies.

You probably know that it is possible to freeze functional sperm (duh, sperm banks), but this requires special solutions and carefully controlled cooling rates. The solutions involved make holes in cell membranes, and are toxic to non-frozen living tissues, so there’s no chance of the conditions inside a body being accidentally right, and the cooling speed required is much faster than anything inside a human body would freeze, even if you dipped the whole corpse in liquid nitrogen.

There’s a vanishingly small possiblity that some prehistoric sperm got frozen in just the right way and managed to survive somehow, but given that these hypothetical sperm probably froze OUTSIDE of someone’s body, it would be awfully hard to find them. And, realistically, there’s just not very many places on earth where they wouldn’t have been eaten by microorganisms long ago. Furthermore, humans rarely end up in the few places so dangerous that microbes won’t go there.

So, I wouldn’t worry about it too much.

mischievous

4

I could have sworn I read in a thread here (or maybe it was on TV) about a study where some researchers did find some intact Neanderthal DNA from skulls and compared it to humans only to come to the conclusion that we didn’t interbreed. I remembered it because I was disappointed; I thought maybe we had interbred after seeing another science show suggesting the possibility (and noticing that my dad definately appeared to have some “neanderthal traits”… think big heavy apeish features and stronger than average back in his prime). I guess they only compared the parts of the genome they could find…

5

Yes, some mitochondrial DNA sequences have been extracted from Neandertal bone, to be precise a right humerus (upper arm bone). These show that Neandertals most likely did not interbreed with our ancestors:

DNA Shows Neandertals Were Not Our Ancestors

This is not absolutely conclusive, since the analysis did not include nuclear DNA. However, it does make it very unlikely that Neandertals and the ancestors of present humans interbred.

6

Here’s another study with the same conclusion.

7

The oldest confirmed DNA sample, from plants, was recovered from permafrost and was 400,000 years old. However, as the article says the samples are highly fragmentary. You couldn’t clone a plant from them.

There have been claims of DNA from 80-million-year-old dinosaurs, and from 425-million-year-old bacteria, but these are strongly disputed by some researchers.