I was reading Alan Steele’s Coyote, which describes the attempts of a group of people from Earth to colonize an alien moon, called Coyote.
Upon arriving after a 230 year journey, they conduct a test on the plant life. As Steele writes:
Now, I’m aware that all life on Earth has DNA that twists in one direction. There is no life form on Earth (that we are aware of) that has opposite-twisting DNA.
However, are the two inherently incompatible? Would it be impossible to grow right-DNA crops in left DNA soil? Would eating an animal (there were native animals on Coyote) or plant that is left-DNA harm a right-DNA organism. After all, we safely eat things that don’t have DNA at all (salt, water, etc.) and it doesn’t cause death.
In short, is Steele’s passage credible, or just nonsense?
I believe the problem is that if you were to eat an opposite-DNA plant or animal, you would be unable to digest the opposite-twisted amino acids, and thus would get almost no nutrition from the food, and die.
Since all life on earth depends on eating each other, it is vital that there be a large supply of same-twisted creatures about to munch on.
I don’t know about DNA, but there are some left-handed versus right-handed molecules that would have no nutrient values if they were found in the wrong variety… some simple sugar like glucose pops out to me.
Basically, our digestive and metabolic systems are capable of breaking down starch or double sugars into simple sugars, but would not be able to break down a ‘backwards’ sugar. (They’ve been researched extensively as artificial sweeteners because they trigger the taste buds in the same fashion as regular sugars, but have no to low effective caloric content.)
You’re right that we eat a lot of things that have no backwards form like salt and water… and as far as I know, it’s not like any of these backwards organic molecules would really be toxic or dangerous. But they wouldn’t feed us.
The big question, though, is if Earth plants would be able to grow strong in a ‘backwards’ ecology. Most plants are capable of synthesizing all their organic requirements out of water, CO2, sunlight, and suitable trace elements such as nitrogen, potassium, sulfur etc in the soil right??
Hi, Friedo. Also, I suspect that the reference to DNA is meant to be ‘cool’ and adds nothing of substantive scientific value. The ‘handedness’ of DNA is structural, not chemical, and I suspect that you could break down a DNA helix into its component parts and reconstruct one that twists the other way with little effort. Since DNA is already broken down in digestion, (and has little nutritive value, though it might be a useful source of the raw materials required to synthesize new DNA in human cells,) this seems like a crimson fish.
What he seems to be talking about, despite the the reference to a “left-handed genetic structure,” is the type of amino acids present in proteins (“dextro-configured amino acids”).
Amino acids exist in two forms, known as levo (left) and dextro (right). For poorly understood reasons, almost all proteins in life on Earth contain only L-amino acids. See the section on isomerism in this Wiki article.
It is possible this pattern developed in early life on Earth purely by chance, in which case there would be a 50:50 chance that extraterrestrial life would be based on D-amino acids. (It is also possible that there is some subtle reason that life uses these forms, in which case all carbon-based life may do so.)
A life form based on L-amino acids would not be able to feed on life forms based on D-amino acids, and vice versa, since it would not be able to incorporate the other form into its own proteins.
However, the passage quoted doesn’t seem to indicate that the author knows what he is talking about; it seems to be mostly a throw-away idea based on some cursory facts he half remembered from college intro biology.
The DNA chirality shouldn’t matter at all: Nucleic acids are essentially nutritionally irrelevant. The amino acid chirality would matter, if you intended to eat them (right-handed amino acids and proteins made from them would be indigestible to us, and might taste funny), but I know of no reason why the chirality of amino acids must be the same as that of DNA (although this might just be my ignorance: It’s presumably not just coincidence that all 19 of the chiral amino acids (there’s one achiral one) have the same chirality).
But even granting that, there’s no reason that Earthly plants couldn’t grow in alien soil: Plants don’t generally need much more than carbon dioxide, oxygen, a few simple nitrogen compounds, and water, all of which are achiral, and to the extent they do need anything chiral, it’s in trace amounts which could be synthesized or produced by other Earthly life.
In fact, it would likely be an advantage if the planet being colonized has chirality opposite Earth’s, since that would mean that indigenous diseases couldn’t infect us, the local predators couldn’t digest us, and the native pests couldn’t digest our crops (which might not stop them from trying, of course, but it might discourage them).
Aside from all that, there’s still the mistake that the author claims that Earthly biochemistry is right-handed, when it’s left-handed.
Depending on conditions, earthly DNA exists in both left and right handed helices: DNA Helix Geometries
Now maybe the author meant to imply the use of L-deoxyribose, rather than the usual D-deoxyribose for the sugar-phosphate backbone ( Mirror Image DNA ), but if so, he sure didn’t say that.
Not really, unless it all has a common ancestor. Ultimately, biology and chemistry all traces back to physics (though the physics might be so incredibly complicated that we have no hope of understanding it as such), and the only assymetry in physics is an exceedingly small and subtle effect at the subatomic level. Neutral kaons are basically irrelevant to anything relating to life, and even there, the difference between left and right is only at about one part in a thousand.
To get a biological effect, you’d have to first have a kaon produced by a high-energy cosmic ray, which would have to hit an organism (a long shot already, since their lifespans are short enough that the vast majority would never reach the surface). Then, that kaon would have a slightly larger chance to decay one way versus another. Then, one set of decay products would have a somewhat larger chance of interacting with biological molecules, depending on their chirality, and then, that interaction might cause a mutation, which might be beneficial or detrimental, and if you get many such mutations, the cumulative effect might be to give D or R aminos a slight competetive edge. Compared with all of the other potential causes for mutation, all the rest of which are symmetric, this would be an effect so far burried in the noise as to be completely irrelevant.
Of course, an interesting question (to me) is if we’ll find carbon-based lifeforms on other planets who evolved on a different basis and have no DNA at all as we understand it.
One thing I have never understood myself is why all the amino acids should have the same chirality. Note that, as the Wiki article mentions, the assignment of chirality is a convention, and does not actually reflect the direction light is rotated by each amino acid:
Now I might be able to understand why life might use only the D or only the L form of each amino acid; but why all of them should have the same chirality is obscure to me.
It wouldn’t surprise me. Steele’s world-building, quite frankly, sucks. It’s just a setting for a series of political short stories and novellas from Asimov’s.
Maybe, but carbon can combine in so many ways that it makes a versatile basis for life. A good essay, for the layman, on the chemical requirements for life is Planets Have An Air About Them by Isaac Asimov.
Isaac Asimov himself thoroughly explored this phenomenon in his book The Left Hand of The Electron. Philip K. Dick’s A Scanner Darkly also treats it as important.
Spatially, a ‘right-handed’ object can never be rotated through three-dimensional space to become congruent with an otherwise identicaL ‘left-handed’ object- but there will be no other way of distinguishing the two without comparing them to another ‘handed’ object. This is why people are dislexic, and why putting your right shoe on your left foot hurts.
All DNA on earth does, in fact, twist in the same direction- but whether you decide to call that direction left or right is arbitrary. For what might be called ‘mechanical’ reasons, this DNA imparts it’s ‘handedness’ to every protein it codes for- so biochemically, all life on earth has only left hands and left feet, and can only combine properly with left gloves and left shoes. Not all organic molecules are complex enough to have handedness, but most of them do.
Some of the confusion with optical effects arises because observing how a molecule twists light is a good way to test which way the molecule twists- but the way it twists light is not directly indicative of the way it relates to DNA. You could compare this to the way, if you made a handprint with your left hand and cut it out with a pair of scissors, then affixed it ink-outwards to a glove, it would match with the right glove, not the left. The transfer of pattern often reverses symetry, but the point is that all the ‘left’ handed organic molecules originate in the same chain of chemical reactions.
A biosphere which used ‘right’ handed organic molecules would have to use them entirely, because all biochemical reactions would be related to eachother there in the same way that they are here. If you were enter that biosphere, all your enzymatic processes would be futzed, because you’d only have left hands and feet and the environment would only present right gloves and shoes. You couldn’t digest anything, from pollen in the air to fatty acids. Eating anything would give you a nasty, nasty stomach ache, and prolonged exposure to the atmosphere would clog your pores and respiratory system with unprocessable organic matter.
The really implausable thing about the story you excerpted is that a spacefaring civilization that was unsure about a planet’s organic makeup would gamble like that with a colonization attempt.
Okay, I made a stupid arrogant assumption in that previous post. All Earthly DNA does not twist in the same direction, it merely has the same ‘handedness’. See Squink’s previous post on this thread.
I don’t know much about this subject, but I remember years ago reading about the attempt to develop left-handed sugar that would taste the same but would not be digestible. So you may be on to something.
No, amino acids of abiotic origin (like those found in comet tar, say) are split 50-50. It’s the Earthly ones (almost all of biotic origin) which (strongly) favor one chirality. So what makes the interplanetary ones distinctive is not that they’re predominantly right (they aren’t), but that there are any rights at all.
Just trying to figure out how this fits into my hijack, if at all… is that supporting the theoretical possibility of carbon-based life with other structural bases than earth life… (other ‘genetic’ codes than DNA-RNA based, say, or possibly even non-proteinaceous life to take a much bigger conceptual leap.) Because carbon is so darned versatile?
PS: The spelling of ‘proteinaceous’ was brought to you by dictionary.com
