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As far as I know, the DNA sequence becomes shorter every time a cell divides.
A shorter sequence results in information loss and aging characteristics.

As a countermeasure, shouldn't one stay young and healthy at least longer, if the DNA in a cell is changed with a "better" DNA (from years ago... let's say your DNA was extracted with the age of 20 and you reinsert it every 10 years or so)?

If this is true, then (as for today's technology)
- is it already possible to perfectly reproduce the DNA? (I only found articles about XNA)
- is it possible to easily replace the DNA in all human cells? (I found articles about bacteria inserting pieces of DNA into human cells)

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The shortened DNA sequences are repaired by the telomerase enzyme. So, there is no actual loss of information. –  Sam Jan 21 at 5:33
    
I think this idea is being implemented for stem-cell therapy but not to control ageing. So, stem cells such as HSCs can be cryo-stored and used for therapy later. Umbilical cord cells can also be employed. With iPSC technique it is possible to convert a cell to any cell type. It might be theoretically possible to replace every old cell but we haven't completely understood the importance of ageing. So there is a possibility that it can have disastrous consequences (not social implications but biological ones such as cancers etc) –  WYSIWYG Jan 22 at 4:54

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Lots of questions here. I recommend taking college level courses in Biochemistry, Molecular Biology, and Medical Physiology, and Geriatrics.

There are other issues than just DNA. Systemic failure is system-wide, and probably most of the issues are still yet undiscovered. Good news I guess if you want to be a researcher in this exciting field! As for DNA-ends, read up on Telomerase. As for DNA copying learn about PCR and sequencing methods. Generally speaking, reverse transcription can be dangerous. Please see many of diseases we are having a hard-time solving, which often use these pathways. Seriously avoid that one. Not everything in DNA is used by the organism at any given time, and some information is elsewhere. Moreover, what if we use DNA as DATA storage besides blueprint information? Hint: while the jury may be still out on Neurons, environmental factors may be passed as epigenetic traits by not just primary shape such as DNA methylation but by obstructing access via conformation and histone wrapping. Not even mentioning mitochondrial dna or prenatal development factors.

Instead: A good diet and exercise.

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well, as I already am an engineer, taking college courses and focusing on that subject may be a little late. Because of a friend, I came across this strange idea, but it seems not to work. Thank you a lot for your explanation. –  user1435792 Jan 21 at 17:33

As far as I know, the DNA sequence becomes shorter every time a cell divides. A shorter sequence results in information loss and aging characteristics.

Neither telomeres nor aging are nearly that simple.

is it already possible to perfectly reproduce the DNA?

No. Any method that would be used to replicate DNA requires using enzymes, and they always have a non zero failure rate. You could use whole genome amplification to make a pile of DNA that was on average the right sequence, but each individual chromosome would have errors. And forget copying over epigenetic modifications.

There are ways to get cells to incorporate new DNA, but every cell in an organ of a living organism? And a whole mammalian genome's worth? And to magically tell the cell that the new genome should replace the old one?

None of that is feasible.

(I found articles about bacteria inserting pieces of DNA into human cells)

Okay, let's get some scale here. Most bacteria have a genome of 5-10 million bases. Humans have a genome of 3 billion bases. Even if a bacteria can transduce a small plasmid into a human cell, that doesn't scale up to bacteria replacing the whole genome of every cell in an organ.

And because there's always a relevant xkcd comic

http://xkcd.com/793/

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