In theory how many types of life forms could their be?

[ChrisBooth12]

Humans and all plants and animals on earth are carbon based. We have water as a solvent. What other types of life could their be knowing what we know about chemistry? Silicon I think I have heard thrown around a long with nitrogen, any others. Could life exist without some sort of DNA or some other type of system similar to our DNA? Are any of these engergy type lifeforms we see on sci-fi possible

[bordelond]

This Wikipedia article will be of interest, and can be a launching point for discussion: Hypothetical types of biochemistry

Another solvent that is sometimes speculatively paired up with silicon is ammonia (NH[sub]3[/sub]). As for other elements that may serve as molecular building blocks and solvents, phosphorus, chlorine, sulphur, methanol (CH[sub]4[/sub]O) and hydrogen flouride (HF) get thrown around. Two other ideas from the Wikipedia link:

Isaac Asimov, the biochemist and science fiction writer, suggested that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane or liquid hydrogen.

A proposal has been made that life on Mars may actually exist and be using a mixture of water and hydrogen peroxide as its solvent. A 61.2 percent (by weight) mix of water and hydrogen peroxide has a freezing point of -56.5 degrees Celsius, and also tends to super-cool rather than crystallize. It is also hygroscopic (attracts water molecules from the surrounding environment), an advantage in a water-scarce environment.

[Chronos]

The big problem with using elements other than carbon as the basis is that carbon can do everything. Silicon can form long chains like carbon, and nitrogen and phosphorous can produce small structures nearly as complex as those produced by carbon, but nothing can do both as well as carbon does.

That, and the fact that carbon is more common than silicon, nitrogen, or phosphorus.