I am in the process of creating my universe, and have based it on semi-hard science. The universe that I have created is quite extensive, and I thought that it would be unreasonable for the only kind of genetic material to exist to be DNA and or RNA, and yet I don't just want to come up with some outlandish, unrealistic alternative genetic material.
So my question is: Are there any molecules that could realistically serve as genetic material in place of DNA and RNA?
Xenonucleic acids (XNAs) (see also Wikipedia) may be what you're looking for.
XNAs are nucleic acids related to DNA, some of which can store information for organisms in the same way that DNA does for life as we know it. These six are
Of these, the latter four are perhaps the best-studied.
DNA is useful because it is a complex molecule which can hold lots of information, is made of relatively common chemical elements and has a structure which allows replication. Alternatives to DNA will have to have similar properties, or life "sort of as we know it" isn't going to be possible.
The first possible alternative is to revive an idea of how DNA was created on the first place. One hypothesis is that organic molecules were "templated" by gathering on the crystal structures of certain types of rocks. If you imagine rock shelves on the ocean floor near thermal vents, then you can imagine the organic molecules in the water sticking to the crystal boundaries of the exposed rock faces. Since the crystal structure is regular or at least semi regular, only certain molecules can fit in the spaces, and only certain patterns can emerge.
In a very alien life form, this patterning could be extended to having the creature uptake certain minerals in the diet to ensure that "templates" could exist inside the cell analogues. This would be limited to very simple life forms with short "DNA" strands, and would essentially nullify evolution since the DNA never changes (based as it is on a crystal pattern template). Every creature is functionally a clone.
Scaled up, larger creatures would be stuck to exposed rock faces like mats, or perhaps "peel off" to live independently for a while after reaching a certain amount of growth. In form this sort of life would resemble possible reconstructions of Ediacaran biota, but once again, would be constrained by the amount of "template surface" available. A mudslide could conceivably lead to the extinction of the entire biome.
You can see why precursor DNA formed in this manner wold be very limiting, and once the pro to DNA molecules were unstuck from the template and started combining in the water, far more flexible life become possible.
Other potential alternatives to DNA would use different chemicals as the base elements of the molecule. Earthly DNA consists of adenine (A), thymine (T), guanine (G) and cytosine (C), but perhaps other nucleotides are possible.
The final idea would be for the alien DNA analogue to use more than 4 bases. A DNA with six or eight bases would allow for far more complex genes to be created or expressed. The downside is the more complex genome would probably be more prone to errors during replication with more potential areas where mismatches can occur. Creatures based on this sort of DNA analogue might evolve far more quickly since mutations in the genome happen more frequently. They would also be much more prone to get the sorts of hereditary diseases that can affect Earthly creatures as the DNA molecules become disordered or improperly matched up during reproduction, causing harmful mutations in the offspring.
I suspect DNA represents a sort of lower boundary where issues like error correction and stability are strong enough to prevent widespread mutations and diseases from overtaking the organism(s), but has enough flexibility to allow for evolution to happen.
If you think of sand with other minerals you could have electronic/rock life. Magnetised iron to carry information. Piezo crystals for manipulating and sensing the environment. It energy source could be solar, radioactive isotopes, or a thermocouple between earth and air.
It would be formed by one or more of the trillions and trillions of lightening strikes (or asteroid strikes) in the universe fusing silica into semiconductors.
You could use RNA. It's very similar to DNA, but it is different.
If you're looking for specific molecular polymers:
The parentheses indicate elements or radicals that are in the molecule. I got this from this page. As far as I can tell, you could replace the hydrocarbons with biomolecules of your choice.
You don't want to be TOO scientific about it, because that's just boring.
All life that we know of is built primarily out of the three most abundant elements in the universe (excluding helium, which doesn't react with much). And It's not merely that these elements are by far more common than any others.
Carbon has the unique qualities of forming compounds with a long list of other elements and readily forms polymers. From Wikipedia: ". . . it resists all but the strongest oxidizers. It does not react with sulfuric acid, hydrochloric acid, chlorine or any alkalis." And it does all this at earth-normal temperatures and pressures.
Water (made out of the other two most common elements) also has some unusual properties. Its boiling point is super high, it's got crazy cohesive strength, and as a solid -- not only does its solid form take up more space than its liquid form, which is wierd, but it's actually less dense than its liquid form, so it floats.
Any chemistry based on other substances would be less robust by several orders of magnitude, assuming any plausible combination of elements exist that can form even a fraction of the many compounds we know of with hydrogen, oxygen, and carbon (I know of none).
So Alien life likely has basically the same chemistry as ours, and since it has to accomplish all the same things that terrestrial life did (photosynthesis, reproduction, etc) it's going to have found a lot of the same solutions. Especially when, in many instances, there's either no real alternative or only alternatives that are way less efficient, it's really not a stretch to assume that it would be a lot more similar to terrestrial life than we're conditioned to expect.
That's no fun. Humans with green skin or funy ears? It's been done.
More fun is something like Noodles suggestion with the piezoelectric rock monsters -- enough science to sort of sound plausible and some interesting creatures crawling around.
In theory all you need is a sufficient energy gradient and some mechanism for harvesting the energy and you potentially have some form of life.
Reproduction isn't even nexessarily a requirement: Noodles' rock monsters might be created on some hell planet when the right type of magma solidifies under a wild magnetic field that lays down the piezoelectric material in the right pattern, and they live asexually until they get destroyed somehow.
It's more important to be internally consistent within the setting and sort-of plausible than to let science kill your soul. It's probably not a cooincidence that most sciences, especially in the "hard" sciences, have little interest in science fiction.
Ultimately the science in the background doesn't do anything useful to keep players or readers or whomever engaged in the setting -- whatever you create has to do something interesting and interact in interest ways with the other denizens of the world. It only has to make enough sense to keep them occupied until they get caught up in events; if someone spots a flaw at that stage, they're apt to be too invested in things to pay any attention.
Basically you need something with these properties:
Given the age of the solar system, and the fact it's made up of stuff from earlier solar systems, if there was another physical mechanism with these same properties and the tendency to actually occur naturally, it's likely it would have happened by now, and we would see lots of it everywhere.
The only explanation for not seeing it is that the conditions on Earth, and on other bodies we've visited, aren't suitable for this stuff to exist.
