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I am neither a student nor really advanced in biology, I am just writing a tabletop role-playing game scenario that I want as realistic as possible. In the story, the players find an old laboratory where genetic mutations were studied, but as the electricity is off since 40 years, most of the embryos are "rotten". I still need to have some embryos still "fresh" so someone could get DNA from it and continue the experiment. Problem is, I thought about formol (the experiment is supposed to have taken place in the 1980's, so the health problem is not one right now), but I've read that formol alters and destroys the DNA. I've seen older ways with alcohol, but the process doesn't seem as efficient.

What liquid (or technic) could have been used in the 80's to preserve embryos and their DNA so we could still exploit it 40 years later, and with not electricity?

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    $\begingroup$ 70% alcohol preserves DNA well. $\endgroup$ – user438383 May 12 at 13:10
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    $\begingroup$ Hi Lyzvaleska, and welcome to Biology.SE! You mention you've read that formol destroys DNA, and that alcohol preservation seems less efficient. Can you link to sources for these claims? $\endgroup$ – acvill May 12 at 14:13
  • $\begingroup$ dry/lyophilized DNAs are also rather stable. idtdna.com/pages/support/faqs/… $\endgroup$ – Maximilian Press May 13 at 4:21
  • $\begingroup$ @acvill For the formol destroying DNA, I've seen it in a french source (but can't seem to put my hand on it right now anymore) that it could alter the DNA. For the alcohol, I saw claims about being less efficient, but apparently it was more of a visual problem and the evaporation. But it looks like that alcohol fits my needs perfectly :) $\endgroup$ – Lyzvaleska May 13 at 19:23
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There are a few plausible ways:

As someone mentioned in the comments; Alcohol works well for this. We have samples from a hundred years ago (e.g. Thylacine), which have been stored at room temperature in a museum for that period of time and have had DNA easily extracted from them.

Unfortunately for many species, alcohol preservation fell out of favour due to it shrinking (by dehydration) specimens, so that they no longer look "realistic" in terms of morphology (something that is very important for classical identification methods). In addition, ethanol and other alcohols tend to evaporate fairly fast if not sealed properly. For these reasons aldehyde based solutions came into favour about 100 years ago. The downside to this is that formaldehyde and other aldehydes cross-link the genetic material, making it more or less impossible to extract and use.

Another possible method is dehydration - mummification. Small samples like embryos or tissues can be dehydrated quite fast. In the absence of water, these samples should last more or less forever - we can extract DNA from mummies in many places around the world, but these ones might interest you in particular.

There are also specialist chemical solutions such as RNAlater that are routinely used in labs for preservation of precious genetic material.

Given that we knew these sorts of things in the 1980's and that the lab was staffed with competent scientists, and that they knew that the lab was going to shut down but would re-start at some time in the future, then it is entirely plausible that they would have used alcohol or something like RNAlater.

In the absence of this knowledge, but still competent scientists, it is much more likely that they would have used some sort of cold (frozen) storage method; whether this was cryopreservation (liquid nitrogen), such as is used for storage of cells or a specialist -80 $^\circ$C freezer or a -20 $^\circ$C freezer like you might find at your house. Some samples may have been still stored in chemical preservation in the interests of space-saving in freezers though.

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