On the thread about alien virus’, people keep mentioned alien DNA. Is DNA something that ALL lifeforms have/would have, or would an alien have another clever way of remembering how our cells were put together? Or on the other hadn, is it a gimme if the lifeform were carbon-based?
Possibly. We only have the one example of course. It’s also possible there are other life chemistries out there, but one based on carbon seems to be the most likely, and if you have carbon-based life, then DNA seems like the simplest solution to passing on genetic information. We won’t know until we have a larger sample size, however. And that doesn’t seem too likely any time soon.
Please ask after we have more aliens to play with.
I ass-u-me that many or most biochemistries that develop in the Earth -life range
(which is pretty broad, considering the various kinds of extremophiles)
will use DNA, or possibly RNA alone.
But the proteins and enzymes described by the base bairs will probably be very different;
or a much larger macromolecule could perhaps develop with data storage capacity;
a huge protein or metalloprotein perhaps, arranged like a memory chip of some sort;
this might allow a flexibility not seen in DNA, perhaps in some cases allowing Lamarckian-type evolution;
also there is the possibility of completely unsuspected chemistries, perhaps outside Earth’s temperature range; these might use a solid state data bearing compound perhaps, or be organised in a completely novel way that doesn’t require data replication…
(I’m just waffling here- what alternatives to DNA would you suggest?)
SF worldbuilding at
As other people said, the question is kinda speculative.
But it seems to me that extraterrestrials would have some kind of DNA-like chemical that has the role that DNA has for us. IMHO this is a natural consequence of evolution by natural selection.
At the same time, I think that such extraterrestrial-DNA is probably NOT DNA but some other chemical. This is based on the observation that most engineering problems have more than one adequate solution.
Just MHO of course.
Almost certainly not. While the production of some from of analogous transcription system seems probable, the chances of it being DNA is extremely remote.
DNA/RNA is only one possible polymer that could be used for genetic data storage. It itself presumably only came about as a result of random reactions within a very specific environment. There is no reason to assume that any alien lifeform would be working on a system of ribose sugars rather than a system of arabinose sugars. There’s no particular reason to assume they would use sugars at all.
The big problem is that we have no idea how life began. The RNA world hypothesis relies on the ability of RNA to catalyse its own production, but we have no idea how the original RNA may have been produced or how many other self-catalysing molecules might be possible. If RNA is the only sch molecule possible and if life only ever evolved in Earth type environments then other lifeforms may well have RNA. If life has evolved in non-Earthlike environments or if there are other self-catalysing molecules or if the RNA world hypothesis is not correct then the chances of other lifeforms having DNA is pretty remote.
Peptide Nucleic Acids (PNA).
Blake nailed it here. There’s no reason to believe that alien life has evolved organic molecules resembling ours. However, I would speculate that whatever such molecules they possess evolve via natural selection.
What does this mean? How do you know what the “simplest solution” is?
Unless DNA has traveled through space, and we are really extraterrestrial offspring!
Well, I don’t, which is why I said “seems to be”. It is the tendency of natural systems to select the simplest structure that will accomplish a given task, and I see no reason to assume differently with DNA. Of course, we have only the one system from which mto make observations, but given the billion-and-a-half years that chemical evolution had to work with and the fact that DNA was the winner, it seems likely that it is, in fact, the simplest carbon-based solution to carrying information between generations of organisms.
I disagree entirely. It is the tendency of natural systems to select the simplest structure that will accomplish a given task from the materials available.
And that is the important qualifier. The mammalian eye for example is not “the simplest structure that will accomplish a given task”. It’s a complete pig’s breakfast and made unnecessarily complex by having a big blind spot in the back that we’ve had to patch up using the hardware and software systems if the brain. It’s unnecessarily complex, but it’s the best we could do with what we had to work with. The same is true of human reproduction. Years of complete dependance on our parents because we can’t have a big brained infant and a big bodied infant and still walk upright. It’s required a system of hideously complex and impractical pair bonding rituals coupled with excessive maternal and paternal drivers and social evolution just so we can survive.
DNA is the same. It’s not simple. The whole triplet thing is unnecessarily complex. It codes for far more amino acids than we actually have access to, even allowing for stop and start codons. The trouble is that having twin codons doesn’t allow for enough. Of course the truly ‘simplest’ system probably wouldn’t rely on triplest at all. It would have a one for one correlation between the ‘codon’ and its corresponding AA. That would allow for much shorter DNA strands that would be less prone to damage. It would also probably help remove the need for the complex transcription systems we have between DNA and protein that we have now.
Like the mammalian eye or human reproduction, DNA exists after half a billion years of evolution because it is all we can produce with the materials to hand, not because it’s the simplest solution. We can’t engineer a better system form the ground up so we tinker with the one we’ve got.
Ah, but your ‘simplest’ system would require a much more complex base-pairing system (or a completely different system of replication and transcription–btw, you meant translation, not transcription… or, well, both )… and would be MUCH more susceptible to mutations (right now certain mutations do nothing, and others switch to a somewhat similar amino acid; yours would definitely switch to a different amino acid).
All that said… thanks for bringing up the triplet codons, because I’ve been meaning to say this: even if aliens would use DNA (the jury’s still out on that), they’d almost definitely have a different triplet (or doublet or quadruplet or whatever) code than us. Of course, if they didn’t, that would make us take a closer look at how it works, because it would mean that the code wasn’t random at all…
Nah. A really simple system would allow mirror halves. A-A, G-G. T-T etc rather than the A-G T-C system we have. That’s not complex. Alternatively we can just have one active strand and a cap strand that only exists to protect the active strand and prevent acidental transcription (or tarnslation ). Much simpler.
As for functional mutations being more common, I have to say “so what?”. As you point out, ATM when we get a point mutation it often codes for nothing and the whole gene is rendered useless. Under my system a point mutation always susbtitutes a new AA. Quite often this will produce a less effective but functional protein, and rarely a more effective protein.
It averages out about the same. Under the current system we get nonsense codons that jam the whole process, similar codons that produce slightly different proteins and occasionally completely different codons. Under the simplified system we never get nonsense codons that cause abreakdown but we do get completely different substitutions more often and slightly different substitutons less frequently. On the whole the only difference I can see is that the simplified system would allow for novel mutations more often because we never get nonsense codons.
You’ve gotta love baseless speculation about things that don’t exist.
What are you defining as a “nonsense codon”. I actually said quite the opposite–since the genetic code is degenerative, mutations often result in absolutely no change to the protein (different codon, but same AA); other times they result in only small changes (different codon, but similar AA); other times somewhat large changes (different codon, unrelated AA, and in an important part of the protein)… but only in those rare cases that the codon mutates to a stop codon does the protein get prematurely terminated…
A nonsense codon is any sequence for which no appropriate AA exists. Essentially a termination codon.
It’s a bit out of my field, but do you have any figures on just what the ratio is for a point mutation producing an alternative triplet for the same AA rather than a different AA or nonsense? I would have thought it would be pretty rare.
Ah, that’s what I thought you meant. There’s no such thing. Every codon does SOMETHING, and only 3 of the 64 combinations are stop codons.
Let me do some quick math and see if I can give you an answer to that second part. We were going to look into it in depth as a sort of side project in grad school, but nobody ever got around to it while I was there (and I was really the person who would have been most likely to look into it). We were doing molecular evolution stuff, and we wanted to figure out “real” mutation rates (ie, what the protein-level mutation rate would be based on a known DNA-level mutation rate).
Ok, I’m just going to do one somewhat randomly-selected codon as an example (sorry for this total hijack )
Original codon: GAU
Amino acid: aspartic acid (relatively small, polar sidechain)
Possible point mutants: UAU, AAU, CAU, GUU, GGU, GCU, GAC, GAA, GAG.
Amino acids coded for by mutants:
1 mutant = identical
2 mutants (GAA, GAG) code for a very similar amino acid
The switch to asparagine will do very little if the size is the most important factor, but could be a drastic change if the charge is most important.
The switch to histine would probably be a big problem, as would tyrosine.
Valine is similar in size but not in charge, alanine is smaller, and glycine is still smaller.
Ok, turns out in most cases bad things happen there… but you NEVER get a stop codon starting there.
Just out of curiosity, I’ll run another one. This time I’ll use UCA, which codes for serine (which is also coded for by 3 other codons).
In this case, 3 are exactly the same, 2 are stops. Threonine is close to serine, leucine is large, alanine is small. Proline would likely cause a BIG problem in the protein, since it changes the shape of the backbone.
This would really take a big statistical study to get to where I thought it was… sorry, my statement about a point mutation not always doing much might have been a bit premature…
Huh? What do you mean there’s no such thing. I’ve seen the term ‘nonsense codon’ is used in post-grad texts and it’s used in all undergrad texts I’ve ever seen.
Hmm, the Genetics Glossary that I usually use for starting points for definitions agrees with you… as another term for the 3 stop codons. I had never seen that term used, but I guess it makes sense, since a nonsense mutation is a mutation to a stop codon (even though my usually useful Genetics Glossary doesn’t have a complete definition there).
In any case, most point mutations are NOT nonsense mutations (since only 3 out of 64 codons are nonsense codons, and many codons can’t get to a stop codon through a single mutation).
I never said otherwise.