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Why are DNA samples stored in cryogenic conditions as biological (or bio-chemical) samples and not as binary data streams of nucleotide sequences in a hard disk or tape drive?

A few reasons I can hazard a guess at are -

  1. Cost to sequence
  2. Cost of data storage (?)
  3. Reliability of sequencing?

Is there any motivation - financial or academic to achieve this?

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    $\begingroup$ I think it would be really useful if you gave more information about specifically what DNA samples you are talking about and why you think they’re stored cryogenically rather than as sequence data. $\endgroup$
    – user438383
    Jun 9 at 19:42
  • $\begingroup$ @user438383 - I was thinking of the 23&me kind of services and forensic evidence. A DNA of a fugitive can be made available worldwide and matched with active investigations across borders if the DNA were in digital form. $\endgroup$
    – Lord Loh.
    Jun 9 at 21:49
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    $\begingroup$ @LordLoh. 23&me does not fully sequence their customer's DNA. Rather they sample some important spots that are known to be informative, and check those sequences. DNA identification for law enforcement generally doesn't using sequencing at all. They rely on a technology called DNA Fingerprinting. Law enforcement has to hang on to samples so that the defense can examine them with their own experts. $\endgroup$ Jun 9 at 22:57

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One reason DNA samples are kept is because we might want to do further analysis using them. If we relied on digital records we'd have to synthesize the physical DNA from the digital record, adding an additional step and possibly introducing errors.

The larger reason is that DNA is not a digital code, it's a large, complex molecule. It happens that many important properties of DNA can be modeled as a simple digital code using the sequence of the four bases, but this is by no means the whole story. For example, in cells with nuclei, the DNA is wound around complex protein structures called histones. The histones in turn can have subtle chemical modifications that have profound influences on whether the DNA wrapped around them is active or not. Furthermore the DNA itself is subject to chemical modifications that aren't usually noted in the standard genetic code. The most important of these is DNA methylation, which again is critical in determining the activity of a region of DNA.

Many people spend their entire careers studying DNA via data streams of nucleotide sequences on computers, but when you have new insights, you often want to test them in a wet lab using physical DNA.

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    $\begingroup$ It's also most common to store DNA not as a molecule but as part of an organism that contains all the machinery to make more DNA (and everything else) when you need it; spores, cells, seeds, eggs. $\endgroup$
    – Bryan Krause
    Jun 9 at 18:58
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    $\begingroup$ It’s also often not trivial to just synthesize DNA whenever you need it, and this is especially true for larger sequences. Even for something small like an expression vector, one would typically have only the insert synthesized and then subclone and transform it into some organism. All this takes time and money. $\endgroup$
    – canadianer
    Jun 9 at 20:58
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The answer by Charles Grant is absolutely correct, especially with respect to our limitations with sequencing DNA of biological origin.

I'd like to add that DNA is also a much more stable storage medium than digital storage. DNA can effectively retain encoded data for at least 2000 years and can still produce living organisms after >30,000 years. Digital storage becomes corrupted within several decades.

DNA also has an extremely high information storage density, and there have been several efforts at encoding digital data as DNA.

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