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XNA is a nucleic acid analogue that could hypothetically be used by an organism in place of DNA. and is "invisible" to natural biological systems. In medical applications, it has the advantage of being resistant to breakdown by nucleases. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2909387/ https://gizmodo.com/xna-is-synthetic-dna-thats-stronger-than-the-real-thing-5903221

So would an organism that uses XNA not be affected by any bacteria, viruses, etc., that would affect a normal, DNA using organism?

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  • $\begingroup$ I think accuracy is a goal worth pursuing in itself. $\endgroup$ Mar 22, 2023 at 22:16
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    $\begingroup$ Could you perhaps give a clearer indication of what you're trying to achieve? $\endgroup$ Mar 22, 2023 at 22:20
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    $\begingroup$ I don't see any worldbuilding here at all, just a vague "what if". $\endgroup$
    – Nij
    Mar 23, 2023 at 7:46
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    $\begingroup$ @Nij I don't see the problem there, why do you? It definitely has world building elements: how would creatutes that not get sick, ever, evolve and what would happen if they meet creatures that do? A predictable "whites meets First Nations in america" scenario? Also, i hope you realize that World Building is too .... a giant "What If" exercise, eh? Tugboat or Titanic, it's all STILL .... boats. $\endgroup$ Mar 23, 2023 at 9:39
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    $\begingroup$ That's not what the question asks, and no, that's not how the site works. I suggest getting better acquainted with site scope, Worldbuilding SE is very definitely not just a "giant What if exercise". $\endgroup$
    – Nij
    Mar 23, 2023 at 10:17

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So would an organism that uses XNA not be affected by any bacteria, viruses, etc., that would affect a normal, DNA using organism?

Viruses yes; bacteria no. The "immunity" to these different things depends on how the pathogen utilizes the host organism.


Bacteria have their own cellular machinery to grow and reproduce and the host they infect is simply a source of readily-available nutrients. If they are DNA-based bacteria infecting an XNA-based host the fats and sugars and other nutrients are all still there to be taken regardless of the XNA/DNA mismatch so the bacteria can still eat and thrive.

The same is true for protozoan and or fungal infections.

(Some details will matter though. For example, humans don't digest cellulose so we can't just live off of wood/grass/etc. and similarly couldn't live off a XNA plant that stores its energy as cellulose. If that XNA plant stores it's energy as everyday fructose and glucose, however, it's fair game. The same concept applies to whatever specifically it is that the bacteria/fungi/protozoans are digesting.)


Viruses are different because they hijack the host's cellular machinery in order to replicate; if the host's cellular machinery only reads/writes XNA then a DNA-based viral payload is not going to be able to be read or copied even if managed to sneak in.


Prions would form another category. Unlike viruses they do not hijack the DNA/XNA machinery to replicate, but they do still require that the organism in question have a matching protein structure that can cause a misfolding cascade. It doesn't really matter whether the protein in question was synthesized from DNA or XNA instructions, but the likelihood of convergent evolution creating two proteins analogous enough to one another that they can prion-misfold eachother is vanishingly unlikely.

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  • $\begingroup$ It should perhaps also be noted that nearly all prion diseases we know about come from the same protein getting misfolded. It is speculated that MSA may be caused by a different protein misfolding; otherwise, all known prion diseases are from PRNP. Given the enormous variety of diseases we know about, this suggests that the vast majority of proteins are incapable of misfolding in a prion-like way, and so an organism without common ancestry would probably be immune. $\endgroup$
    – Kevin
    Mar 24, 2023 at 7:36
  • $\begingroup$ But, as @Mary correctly points out, RNA viruses (which are actually the majority of human viruses, IIRC) would continue to operate unless you also replace RNA with something else. $\endgroup$ Mar 24, 2023 at 10:53
  • $\begingroup$ @JackAidley I think that "replace RNA with something else" is almost surely assumed. The "cellular machinery" that viruses "hijack" isn't separate DNA-machinery and RNA-machinery... it's machinery that unzips DNA to create a half-DNA carbon-copy analogue (called RNA) that ribosomes then use as protein blueprints. Most XNA have an entirely different sugar backbone to DNA, so it seems crazy to think that an XNA carbon-copy analogue would gain a DNA/RNA style backbone rather than naturally have an XNA analogous backbone. $\endgroup$
    – DotCounter
    Mar 24, 2023 at 14:57
  • $\begingroup$ @LetEpsilonBeLessThanZero I was assuming a synthetic organism substituting XNA for DNA, in which case the use of RNA is very likely indeed and many RNA analogues would bind to the same tRNAs and so RNA would still function. However, I realise doubly checking the question that it does not state that - if we're talking alien life then, yeah, totally different. Even bacteria are in trouble there. $\endgroup$ Mar 24, 2023 at 16:47
  • $\begingroup$ @JackAidley Yes, and to be fair to you, OPs linked articles absolutely do involve some scientists creating XNA-to-DNA/RNA transcoders... but I presume that was because it was a matter of convenience versus inventing all of the necessary XNA-codon machinery from scratch (and not a natural biological necessity). $\endgroup$
    – DotCounter
    Mar 24, 2023 at 17:26
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It might complicate the existence of retroviruses, because they insert themselves into the host's DNA. RNA viruses would not be affected unless the RNA also changed.

Bacteria would not be affected at all. They consist of their own cells and multiply on their own, without reference to the host. Likewise with fungi and other lifeforms that can be infectious.

If XNA is to be any obstacle to them, it would be in consequences that otherwise make the organism resistant to infection. Such as merely making the biochemistry too different. Many diseases can not survive in animals other than a limited subset that serve as hosts.

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    $\begingroup$ If the intention is to make the organism immune to viruses, it would be trivial to change the "coding table" of RNA->AA in the artificial organism thus rendering the RNA of the virus into nonsense. $\endgroup$ Mar 23, 2023 at 8:03
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    $\begingroup$ @JackAidley trivial in the "It's just software, how hard can it be?" sense? $\endgroup$
    – fectin
    Mar 23, 2023 at 23:34
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    $\begingroup$ @fectin: You know, I was preparing a comment explaining why it was actually trivial - and changing the coding table is - but while writing it, it occurred to me that there are string of potential knock-on problems to be solved while doing so that render it anything other than trivial. $\endgroup$ Mar 24, 2023 at 8:09
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"Affected" is a broad term. If the creature operates under completely separate information carrying molecules, but everything else is familiar, then on a couple counts maybe. If not then the two lifeforms may still chemically interact. A wet cozy place is a wet cozy place, whether it's in the cracks between some salty rocks or in the orifices or respiratory systems of some incompatible alien from a different tree of life. Gaia's bacteria can't tell the difference.

XNA creatures could be "infected" by DNA creatures who experience XNA organisms as no different than the surrounding environment.

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Replacing the alphabet for how the organism internally work might make it safe from infections due to DNA/RNA based organism, making the two mutually incompatible.

However you cannot exclude that the metabolic products of one species will not affect the other.

While virus make their host sick by hijacking their genetic expression machine with the introduction of their genome in it, bacteria do it by the byproducts of their metabolisms, on which the genetic material has little influence.

Imagine a protein, or more generically, a molecule produced by the DNA organism which is active also on something inside the XNA organism. Despite not carrying any DNA nucleotide out of which was synthesized, it might still lead to effects on the receiving organism which can be, for all intents and purposes, seen as "being affected by it".

Cyanide is not directly produced out of DNA, yet can affect a human who happens to ingest it.

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If you're completely replacing the CTGA alphabet with something different, then existing pathogens wouldn't be able to interact with it. It would generate a completely different set of proteins which would be indigestible to existing life. Possibly even poisonous.

This is one of the basic lessons of pathology. Biological viruses are like computer viruses in that they're targeted at specific bits of genetic code. Without the targeted code, they're harmless, inert. Bacteria graze off of the products of our metabolism. Multi-cell organisms wouldn't fare any better. We could eat the carbohydrates produced by them, and we could probably break down their molecules, but we couldn't use them.

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    $\begingroup$ There is no reason proteins would have to be fundamentally different. Aside from proteins that directly interact with the genetic molecule, like histones and synthases and some components of ribosomes, a different genetic code could be designed to still encode the same set of amino acids. $\endgroup$ Mar 23, 2023 at 3:44

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