Is it possible to outlive a DNA sample?

Possible stupid question alert

If I understand correctly, every time your DNA replicates, it’s very very very slightly different than the previous replication. So compounded over decades, is it possible to outlive a DNA sample?

If my DNA was harvested the moment I was born and i lived forever. How long would it take for the age 0 sample to no longer identify as my own?

100 years?
500 years?
A million years?

DNA does not change. For example they can solve a crime from 30 years ago if they have that crime scene DNA and compare it to DNA from a person today because the DNA of the criminal is the same now as 30 years ago.

there can be slight mutations in DNA but that is not enough to change the DNA in any meaningful way.

I think you are thinking about it backwards. The ends of DNA have a section of junk sequences called telomeres which lose a base pair every replication. It’s sort of like if every time you photocopied a book, one word was erased from each end. Some people think this is one of the causes of aging. Not all species are unable to restore their telomeres, and even in people, the germline somehow manages to stay intact. So, the only way you would stay immortal is if your DNA remained the same. The original DNA sample will eventually break down, but it won’t change it’s sequences.

The germline mutation rate (between generations, what you pass on via sperm/egg) gets a lot more attention, because its the substrate for evolution. Over the very long term, the difference between species represents the accumulation of germline mutation.

But what you’re looking for is called the somatic mutation rate. It’s difficult to estimate because the rate it’s extremely low, so it’s technically challenging. You have to sequence a lot of DNA, and you have to correct for comparable or higher error rates in the sequencing process.

But there does appear to be a recent paper giving some estimates for mice and humans here:
https://www.nature.com/articles/ncomms15183

The estimate is of the order of 10^-8 per cell division. So… if we take your question at face value, replication fidelity is so high that it would take millions of cell divisions for your genome to become unrecognizable.

But it wouldn’t really work like that, of course. With just a few mutations, cells would die or become dysfunctional. That’s how cancer happens. Among the things that would become dysfunctional would be the enzymes that copy and proofread DNA, so even if the cell lineage managed to survive the mutation rate itself would skyrocket at some stage.

If OP has somehow managed to become immortal, then presumably their DNA replication has also become 100% accurate, else errors would accumulate until the cells become cancerous or non-viable. Further, they would presumably also have a mechanism to accurately correct other errors that occur due to, e.g., chemical or radiation damage.

So, to answer the OP, not fighting the hypothetical, since they are immortal their DNA would remain exactly the same in vivo forever and ever.

Now the question is, what steps are being taken to preserve the initial DNA sample?

Not really a question it is possible to answer. To live forever you need major changes to cell replication in your body, and what changes we assume affects the answer. We also need to make assumptions about what kind of test we’re using, what the threshold for “not the same organism” should be and so on.

As for humans and current DNA-testing for genealogy, identical twins get a result of “self or identical twin” through their life-time, no problem.

Yes, an important point is that mutations happen slowly. So slowly that effectively, there is no noticeable difference in the DNA samples taken from a person over their lifetime. IANABiologist, but I think the process of DNA testing goes like this:
There are certain known points on each of our chromosomes, common to almost everyone. To create a DNA “fingerprint”, in very basic words they treat the sample with chemicals that “snip” the DNA into segments at known points. the snipped DNA is put on one edge of gel on a film and a small voltage is applied across the gel. This voltage causes the DNA samples to migrate - the bigger they are, the more junk segments in that active segment, the slower they migrate. So for certain known segments of DNA, they get a weight ( equals length) number. Given a few dozen random numbers, the odds that any two people match within any accuracy is astronomical. (Unless they both got that DNA from a common ancestor)

This is the DNA test used to identify relatives, criminals, etc. Even then, the change is slow enough that for example, to solve the question of whether Thomas Jefferson had children with his slave, Sally Hemmings - the DNA test between descendants on both sides showed a connection. However, the test could not distinguish between Jefferson and his uncle or brother as the father. (IIRC they looked for direct male descendants and compared the Y-Chromosome, since it would be the one passed directly down.) Similarly, I recall reading about how some body of a man found ceremonially killed and buried in a bog in England several thousand years ago was preserved enough that they matched his DNA to people still living in the area.

Also note that a lot of DNA changes are to “junk” DNA, segments of DNA between the active parts that appear to serve no function - so changes to them do not hurt the body functions. Similarly, changes to chunks of the DNA which actually make the cells work and produce results - usually are not good for the cell or the body - think “cancer”. But generally there’ a feedback mechanism where significantly changed DNA will cause that cell to die rather than split. Plus, and changes that occur randomly in some odd part of the body don’t migrate to other parts of the body, so each location’s DNA likely diverges a bit from the original blueprint.

So to answer the OP’s question… so long that it’s unlikely to be relevant.

This is not the same as analyzing every segment of your DNA small bit by small bit looking for similarities. Presumably, doing this one can find some changes.

That is the kind of test used in forensic DNA testing. 23andMe, Ancestry and other’s doing testing for genealogy and (mostly bogus) medical reasons use a different kind of process and test hundreds of thousands of sites on your chromosomes for what specific nucleotide you have there, each of which is a so called SNP (pronounced snip), a single nucleotide polymorphism, i.e. a place where not all humans have the same single letter in the sequence.