Ah, possibilities. To any science fiction author or fan, this is a no brainer. We are, after all, in the business of considering new possibilities. We will ignore the fact that there are already life forms based on RNA, since that is really a technicality, and RNA works on the same basic chemical principles as does DNA, it just is more susceptible to mutation and holds half the information per unit length. If we want true possibilities for something new, then we want entirely new coding systems for life to work on.

One classic sci-fi approach would be to consider artificial life-forms - those based on computer technology. In this case, all the necessary data for creating and maintaining a life form would be stored in some computerized format, possibly digital media, but also possibly other formats that are not our standard. That's a cop out though, as most people don't want to talk about machines when they consider "life".

A chemical-based life form should have some form of encoding that provides it with a constant set of functional instructions. In our system DNA fills that role. The only real requirements for encryption are that the method be stable, accessible, and reproduceable.

Stability is a must, or else the organism will not be able to remain the same over time, and probably cease to function as a result. Small mutations (as with DNA) might be permissible, but large ones would be lethal. As a result, whatever chemicals are involved in the encoding should either have strong stable bonds, or else be protected from interacting with other chemical species. Single bonds or aromatic structures would be most favorable in providing structural stability. (Yes, both are found in DNA - guess why.) A protective casing could act to protect a less stable molecule from corruption, though this seems less effective. In human cells, the nuclear membrane can be regarded in this way, as a second line of defense.

Accessibility is important. The encoded information is useless if the organism can't "read" from it. This means that some mechanism must exist by which the chemical information can be reached. Either the coded material must be on the outer surface of the molecule then, or else a structural transformation must be possible to allow access to the encoded surfaces. (In DNA the structural transformation is necessary.)

Reproducibility is a must for the continuation of life. The lifeform would have to be able to not only copy the

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