Here on Earth, Carbon is the base of all life from plants to fungi to animals. The most common alternative that exists is Silicon, but surely out of the over 100 elements other than Carbon and Silicon, there are other options for life bases. Out of the elements that exist, which ones can possibly work as a base for life? What are their advantages and disadvantages when compared to Carbon?
Wikipedia has a full article just for that: https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry - Most of this answer is based on this article.
Basically, alternative biochemistry would be:
Almost all of Earthly amino acids have a L form and sugars have a D form. But, life in some other planet could adopt L-L, D-L or D-D.
Silicon: Silicon is the most frequent candidate as a replacement for carbon, but molecules created with long sequences of silicon (those are called silanes) tends to be much less chemically stable than their carbon counterparts. Further, silicon is much more common than carbon in Earth, and even with that, life here is carbon-based. Anyway, it remains as a possible candidate.
Silicone: Alternate sequences of silicon and oxygen (aka silicones) are much more stable than long sequences of just silicon, so silicones tends to work better than silanes. Polysilanols are the silicone componds analogue to carbon-based sugars, and they are soluble in liquid nitrogen.
Boron: Boron sequences, called boranes are highly explosive in Earth's atmosphere, but might be viable in some other planet. However, boron is relatively too rare to be seen as a viable alternative.
Sulfur or Phosphorus: Those also are able to create long chains in some situations, however they tends to be even less stable than silanes.
Metals: Some metals mixed with oxygen are very capable of creating significantly complex molecules. However, many metals are relatively rare, so you only get a few options left, namely titanium, aluminium, magnesium and iron, which are even more abundant than carbon.
Water is essential because it is a liquid capable of acting as a solvent to a large set of substances and also has a pretty large liquid temperature range. Few substances have those characteristics, but there are some, namely:
Ammonia: In low temperatures (i.e. planets which orbit reasonably far from their host stars), ammonia becames a liquid capable of act as a solvent to a large pletora of substances and has a reasonable large range of liquid temperature. The liquid range is not as large as water is, but in those cold planets where it remains liquid, maybe the temperature does not variate as much as it does in Earth. In high pressures environments, however, ammonia has a liquid range even larger than water. Also, ammonia dissolves many metals even better than water does.
Hydrogen fluoride: Has a suitable liquid temperature range, similar to water, but in colder temperatures. It is also able to act as a solvent for many substances. However, it is considered too rare to be a viable candidate.
Hydrogen sulfide: A good solvent, but not as good as water or ammonia. However, mixed with a small proportion of hydrogen fluoride, it becames great. Unfortunately, it features a narrow range of liquid temperature, but it might be workable at high pressures where its liquid range is better.
Methane and other simple Hydrocarbons: Titan has lakes made from hydrocarbons, mostly ethane and methane. By the way, accordingly to the wikipedia article, some people as Darrell Strobel from John Hopkins University suggests that Titan actually features hydrocarbon-based life, which combines hydrocarbons to hydrogen, reducing ethane and acetylene to methane (something analogous to what many organisms here in Earth do in order to breath). Water is a stronger solvent than methane, however this is not necessarily a con to methane, since this might mean that it is more able than water to selectively preserve molecules useful for biologic reactions. Also, computer models shows that cell membranes based on carbon, hydrogen and nitrogen are capable of working in liquid methane (Earth's cell membranes uses carbon, hydrogen, oxygen and phosphorus). See more here. Also, accordingly to Chris McKey, methane-based life could be more common in the universe than water-based life.
Silicon dioxide: In Earth this becames glass or sand, a solid. However, at higher temperatures (higher than what we see in Venus), it becames a liquid. It still needs high pressure to keep a possible workable liquid range that is not too hot for organic reactions.
And there are other possible substitutes for water given a suitable temperature and pressure range: Hydrogen chloride; hydrogen cyanide; sulfuric acid; formamide; methanol; liquid nitrogen; liquid hydrogen; supercritical hydrogen; supercritical carbon dioxide; liquid sodium chloride (aka salt); and water mixed with hydrogen peroxide (H2O2).
Phosphorus is an essential atom in biochemistry, participating in the structure of DNA and RNA and transporting energy as part of ATP. It might be replaceable by arsenic. While poisonous to almost all life in Earth, there is at least one known species of bacteria that is able to replace at least part of its phosphorous by arsenic. However, arsenic is somewhat rare and features poorly chemically in comparation to phosphorus.
Replacing oxygen and CO2
Some bacteria actually "breath" sulfur instead of oxygen. In fact sulfur-reducing bacteria were widespread in the primitive Earth before we had an oxygen-rich atmosphere.
Also, if the life-in-Titan hyphotesis is correct, Titan's bacteria should breath ethane and acetylene instead of oxygen and expell ethane instead of carbon dioxide. Or perhaps absorb hydrogen, ethane and acetylene, producing methane.
Life that uses hydrogen sulfide as a solvent could breath sulfur monoxide producing carbon monoxide and/or carbon dioxide.
Replacing nucleobases, amino acids, DNA and RNA
DNA is composed by four nucleobases - adenine, cytosine, guanine and thymine. RNA is similar, but replaces thymine with uracil. However there are other possible nucleobases that were sythetized in labs or that might had occured abiotically in early Earth's and were not used by Earth's life, which might not be the case in other planets. Notably is an experiment performed in 2014 in the Scripps Research Institute (see here) where two artifical nucleobase pairs, namely d5SICS and dNaM, were added to genoma of Eschera Coli and were able to be replicated in their DNA with high fidelity without reducing the capability of the bacteria to reproduce nor reducing its ability to self-repair DNA's damages nor losing those artificial bases with time.
Further, the genetic code uses triplets of nucleobases to synthetize amino acids, from which 21 are naturally synthetized, but many of nonstandard amino acids are know. An alternate biochemistry could use perhaps quads or pairs of nucleobases to form amino acids, perhaps using nonstandard nucleobases and/or producing nonstandard amino acids.
There are many alternative forms to DNA and RNA. Those alternative forms for RNA and DNA are collectivelly called XNA. Some of them are:
PNA, which is a candidate for a DNA substitute, and it was suggested that very early Earth's life was PNA-based instead of DNA-based since PNA may polymerize spontaneously in water at high temperatures and pressures. PNA is not known to occur naturally in Earth, but its backbone molecule is produced naturally by some cyanobacteria.
TNA and GNA. Those were produced in labs and known to be as good to store genetic information as DNA. TNA behaves similarly to RNA and GNA could replace either RNA or DNA. However, those are not known to occur in nature, they were synthetized in labs, but it is hypothesized that they may have occurred in the primitive Earth and could be DNA's and RNA's precursors. Also, it is notable that GNA is simpler, more stable and more heat-resistant than either DNA or RNA. TNA is also structurally simpler than RNA.
LNA: A possible substitute for RNA in some xenobiologic environment.
xDNA: An artificial DNA substitute, which features four extra nucleobases in adition to the common four nucleobases. Those four alternate nucleobases are modifications of the standard nucleobases, where an extra benzene ring was added to each one. The resulting molecule (xDNA) is more stable than regular DNA in high temperatures and is not compatible for integration into otherwise regular DNA.
I have no idea of what kind of molecule could be a nucleobase, amino acid, DNA or RNA substitute that is not carbon-based and I doubt that present-day science have enough knowledge for proposing any candidate of that.
Possibilities for alternate biochemistry
Based on all of that, I think that some possible alternate biochemistries are:
(Titan's environment): Low temperature. 45% larger atmospheric pressure than Earth. Methane and ethane are the solvents. Carbon-based life which breathes methane and ethane and expels acetylene. Other possibility is breathing hydrogen, ethane and acetylene, producing methane. Those two possibilities could be complementary and living in the same environment, just as plants and animals does. Don't know if some replacement for DNA and RNA would be necessary or not nor what it could be. Accordingly to this such world would benefit from orbiting a red-dwarf star, but this is not obligatory.
Similar to Earth, but featuring GNA and TNA as a replacement for DNA and RNA. Possibly with different amino acids.
Low temperature. High pressure. Ammonia solvent. Carbon-based life. Don't know if some replacement for DNA and RNA would be necessary nor what it could be.
Low temperature. Earth-like pressure. Liquid nitrogen solvent. Silicone-based life. Something very different than DNA and RNA would be needed for genetics, but I have no idea what it could be.
Low temperature. High pressure. Hydrogen sulfide solvent, mixed with bit of hydrogen fluoride. Carbon-based life breathing sulfur monoxide producing carbon monoxide and/or carbon dioxide. Don't know if some replacement for DNA and RNA would be necessary nor what it could be.
Low temperature. Low pressure. A mixture of water and water peroxyde is used as a solvent. Carbon-based life. No idea about what would be the metabolism or what would substitute DNA and RNA, but it would not need to be so much different from what happens in Earth when compared to the other possibilities.
High temperature. High pressure. Silicon dioxide and silicates are used as solvents. Silicione or aluminum-based life. No idea about what would be the metabolism or what would substitute DNA and RNA.
High temperature. High pressure. Sulfuric acid solvent. Don't know if carbon-based or silicone-based, nor what would be the metabolism, nor what replaces DNA and RNA.
High temperature. High pressure. Supercritical carbon dioxide solvent with small quantities of supercritical water. Carbon-based life. Don't know the metabolism nor what would replace DNA and RNA.
There could be a lot of more possible combinations, but I think that listing those are already good enough. Alternate biochemistry knowledge is still a baby, so there is a lot of speculation and uncertainties and very few certain knowledge about what could be or not be the possible biochemistry for alien forms.
A while back I wrote a story, Learning Curve, about silicon based life living in molten sulfur aquifers on the Jovian moon of Io. Io seems to have the physical characteristics that would allow complex molecules based on silicon and sulfur to be stable since it is totally desiccated. Nevertheless there is certainly an energy gradient on Io both thermal and electrical. Life can be defined as a self replicating system that uses an energy gradient to locally reverse entropic stasis or collapse. Of course such life would have very alien ways of making use of that energy. One of the interesting things about sulfur is that, unlike water, it has an absurdly low vapor pressure at or significantly above melting temperature. This makes it an ideal fluid for an organism living in a near vacuum. Protective skins would not have to be as pressure resistant as our space suits though conductive loss of heat would still be a problem. Most objections to sulfur as a life fluid are based the characteristics of pure sulfur, something sulfur on Io would definitely not be. Impurities in molten sulfur would drastically change viscosity, solubility and other characteristics increasing the range of possible chemical interactions. A lot of people assume silicon life to be crystalline but that's as silly as assuming carbon based life would look like a double helix. On a macroscopic level silicon based life might not look any more crystalline that earth life. One danger is that silicon sulfur life might catch fire or even explode when exposed to terrestrial conditions.
Since Voyager we have learned a great deal more about Io and the possibility of underground sulfur seas has been diminished but not entirely eliminated. I'm hoping to one day see a 'critter' rolling across the ochre sands of Pele Patera on a space probe viewer.
If you're looking for specific molecular polymers:
- dimethylsiloxane (Si, O, CH3)
- phenylsilicone (Si, O, C6H5)
- oiphenyllead oxide (Pb, O, C6H5)
- diphenylltin (Sn, C6H5)
- butylpolystannoxane (Sn, O, OH, C4H9)
- silazane (Si, N, H, CH3)
- phosphonitrilic chloride (P, N, Cl)
- dimethyl polyborophane (B, P, H, CH3)
- silyl orthoborate (Si, O, B, CH3)
- dimethylated polygermane (C, H, Ge, CH3)
The parentheses indicate elements or radicals that are in the molecule. I got this from 1. As far as I can tell, you could replace the hydrocarbons with biomolecules of your choice.