I actually think that alien life won't be very alien at all.
Given that the laws of physics are constant throughout the universe/s in question, I think they're probably very similar. Why? Well, DNA and RNA are self replicators that, to the best of our knowledge, arose independently. Given that the emergence of self-replicators is improbable to begin with, it's even more improbable that two different types of self-replicators arose that are so similar to each . This probably means that there is some about the way natural laws are shaped that means that certain types of self-replicators are more probable than others; and those that use heterocyclic aromatic organic compounds as code fall under this category.
What about other aspects of life? All large forms of life are modular; that is, composed of many smaller but similar types. It's likely that alien life will follow this basic structure as well, because it's much easier to evolve that way. Evolving a larger body without modularity requires fundamental changes in structural composition which may be difficult to achieve with incremental changes.
The temptation with exploratory thinking is to deliberately think as much outside the box as you can. Doing so is good, but it's not always correct.
Given that the laws of physics are constant throughout the universe/s in question
That actually turns out to be a pretty big given, and there's some evidence that the "laws" as we know them are contingent and could be subject to variation across different regions of the universe. Of course, those regions are in principle unobservable by us, but if it can vary on a large scale it'd be rash to rule out variation on a smaller scale.
Not really. What you are describing might happen on a bad episode of Star Trek TNG (Q changes the gravitational constant, for example), but in reality stuff that happens on a planet orbiting a distant star are not going to be all that different than what happens here on Earth. Chemical bonds work the same way in Sunnyvale as they do in the Oort Cloud or in a galaxy billions of light years away. We know this by studying the spectra of the light coming from these places.
At the chemical level, I do not anticipate that extraterrestrial life would be very different than that found here. Water is a terrific solvent and is liquid at a wide range of temperature. Unlike methane and ammonia, its molecules are dipoles (one end is more positive and the other more negative), which makes it easy for it to form hydrogen bonds and a strong surface tension at the surface.
Carbon and Oxygen are both fairly abundant. They are formed during the carbon-cycle of aging stars (3 He -> C, 4 He -> O). I have read SF stories that tell of creatures that breathe Florine or are Silicon-based, but these are pretty far-fetched. Florine is a lot rarer than Oxygen, and though it is more electro-negative than O, because of this it is likely to stay locked up in rocks than remain in the atmosphere. Silicon has the same number of valence electrons as Carbon does, but the bonds that it forms are at a higher energy state (n=3, vs. n=2 for C). This means that it is hard for Si to form double or triple covalent bonds; this is something C can do very easily. For a more detailed discussion on this subject, see: http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemis...
Note that DNA is NOT a self-replicator. It requires ribozymes (RNA + Proteins) to actually replicate. To my knowledge there has not yet been a enzyme made of DNA (thermodynamically it's favoured structure is not catalytic).
to my knowledge this is what happens when DNA is replicated:
they take the tangled dna string thats inside the core of a cell, it is transported outside of that core and then some enzyme copies the DNA but it replaces the T protein with a U (could be another one). This is then called RNA.
Why do they do this? well the T protein is very vulnerable. By replacing it, they can now safely transport the rna to the place where they're duplicated.
What you are trying to describe is transcription, the process of reading a DNA template to produce a RNA transcript. This is done in the nucleus for eukaryotes, and in the nucleoid for prokaryotes (which don't have a nucleus). RNA Polymerase is the protein that synthesizes mRNA (from the DNA template) and the nitrogenous base Uracil (U) is used in place of Thymine (neither are proteins).
In fact Uracil is less stable that Thymine and the hypothesis is that DNA uses Thymine due to its lower rate of point mutation. (Note I mean less stable in the sense that it can be mutated more easily due to functional groups).
Expanding on this is the concept of panspermia, life spreading across worlds on its own. For all we know, the building blocks of life could be older than the Earth itself. So, aside from playing by the same rules, different forms of life on different planets could actually have the same ancestors.
Sulfur is capably [sic] of forming long-chain molecules like carbon. Some terrestrial bacteria have already been discovered to survive on sulfur rather than oxygen, by reducing sulfur to hydrogen sulfide.
Terrestrial bacteria use H2S an an energy source, not as the basis of life. Extraterrestrial life will almost certainly be carbon-based---or rather, its ancestors will be. ET, if we find it, will almost surely be a machine intelligence.
Machines have a lot of disadvantages that aren't readily apparent. Machines have to build collection systems or mine energy sources to thrive. We live on stuff that grows out of dirt-- that's pretty handy.
Of course, machines could become more and more advanced if they didn't go extinct in those first pivotal years, but I think it'd be to the end of looking organic. And if you're going to life out of something, make sure it's low on the periodic table. We're carbon-based because that's what works.
Who's to say that aliens will even be matter-based?
It doesn't seem to imporbable to me that life forms, for example, on the surface of stars, where energy is abundant. Self contained, self replicating energy patterns are just as much "life" as bacteria, and I don't see why they can't evolve intelligence either.
Somehow, Hawking observes, "some of these atoms came to be arranged in the form of molecules of DNA. One possibility is that the formation of something like DNA, which could reproduce itself, is extremely unlikely.
Or it could be quite likely. There are theories that amino acids can form without life. Just get something resembling our planet's crust and they'll form and then they can form peptide chains and and so on. This means that any planet similar to earth could end up not just with carbon based life but with DNA based life.
Carbon don't enter into it. Just because something has a lot of carbon in it doesn't mean it acts like we do. There are plenty of other things to do with carbon than the ways organisms on Earth use it. DNA? Forget it, that was evolved. Alien biology will be completely alien.
"Completely alien" is a misnomer—it's like saying alien computers won't be Turing machines, just because they're "so alien." DNA is simply a way of sticking polymers together to form a storage device and self-stable electrochemical API for accessing it. Organisms that want to reproduce, especially sexually, need storage devices. Likewise, oxygen is the simplest and most robust carrier of electrochemical capacitance. Aliens will have these things—not because they evolved in the same way we did, but because those are the most fit mutations in the early chemical soup. Sure, given an incredibly constrained soup containing lots of sulphur and no oxygen, sulphur-dependent life might evolve, but by the time it has evolved enough that it meets up with us after some good ol' intergalactic travel, it will probably realize that oxygen is a lot easier to manage and rewrite its genetics to use it instead. Or, to take that concept further, it'll be a post-Singularity machine intelligence, as mhartl said.
Well, you're probably right about the oxygen thing. I just meant that the chemical mechanisms behind their functioning will be completely different than ours. I mean, some Earth organisms have completely different RNA from ours. I do not expect aliens to have guanine, adenine, etc. as their storage device, but that's not to say they won't have one.
Given that the laws of physics are constant throughout the universe/s in question, I think they're probably very similar. Why? Well, DNA and RNA are self replicators that, to the best of our knowledge, arose independently. Given that the emergence of self-replicators is improbable to begin with, it's even more improbable that two different types of self-replicators arose that are so similar to each . This probably means that there is some about the way natural laws are shaped that means that certain types of self-replicators are more probable than others; and those that use heterocyclic aromatic organic compounds as code fall under this category.
What about other aspects of life? All large forms of life are modular; that is, composed of many smaller but similar types. It's likely that alien life will follow this basic structure as well, because it's much easier to evolve that way. Evolving a larger body without modularity requires fundamental changes in structural composition which may be difficult to achieve with incremental changes.
The temptation with exploratory thinking is to deliberately think as much outside the box as you can. Doing so is good, but it's not always correct.