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I'm not a chemist—I'm an artist and I occasionally write. But I'm starting to get more and more interested in xeno/speculative biology. When it comes to these things I'm pretty much just looking at stuff dumbfounded and trying to understand it. So every now and then I would like some advice from people who are much more galaxy brained than me.

My latest strange fixation is nucleic acids. I'm researching stuff for a personal sheet with alternative biochemistry things, and I sort of noticed that most backbone sugars I see being used are monosaccharides like threose, hexitol, altritol, and the naturally-occurring ribose and deoxyribose. So I've been kind of making the safe assumption that any monosaccharide can fill the role. I have the feeling that I'm very wrong. But I really wanted to know before assuming.

Thank you so much in advance <3

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Probably not, but other things can be.

The fact that DNA and RNA use a pentose backbone doesn't mean that any DNA analogue needs to have a sugary backbone, and not any random monosaccharide will necessarily work. The important thing about ribose is the structure, meaning the actual literal 3-dimensional shape and connectivity of the molecular fragment that makes up the backbone. In fact, it needn't be a sugar at all! Anything "close enough" to the shape of ribose and deoxyribose can theoretically be used as backbone alternatives in XNA. Things like peptides and even cyclohexene have been suggested.

This website has a good intro to a few suggested ribose backbone analogues, some of which are just altered ribose, some of which are completely different. For a more digestible and broad treatment, check out the Wikipedia pages on nucleic acid analogues and xeno nucleic acids.

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It must be a Pentose or a Hexose*

*may have exceptions

From what I've seen the sugar must be a pentose, having 5 carbons, or a hexose, having 6 carbons. There are notable exceptions to this general rule of thumb, such as threose which is a tetrose having 4 carbons. It is also of vital importance for there to be a phosphate "pathway". Xylose may be a good alternative sugar, and I like that it has "X" in it like XNA. Xylonucleic acid (formally XyloNA) is also a thing, along with Deoxyxylonucleic acid (not sure of the abbreviation here). There are at least six currently investigated backbone sugars that are used in XNA research as shown in this paper from 2020. XyloNA research has been conducted and this paper from 2015 shows a comparison to RNA.

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    $\begingroup$ Thank you lots, but threose is a tetrose and hexitol is a hexose though? And they're pretty much the first things you find when searching about XNA. I'm kinda confused? $\endgroup$
    – They
    May 20, 2021 at 17:11
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    $\begingroup$ It doesn't necessarily need to be a pentose. See the wikipedia page on xeno nucleic acids $\endgroup$
    – R. Barrett
    May 20, 2021 at 18:03

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