# Left handed DNA incompatible with Right? **URL:** \ **Category:** Factual Questions\ **Created:** [March 20, 2007, 3:37pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724 "2007-03-20T15:37:59Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![asterion](https://avatars.discourse-cdn.com/v4/letter/a/4da419/32.png) [@asterion](https://boards.straightdope.com/u/asterion)\ **Post date:** [March 21, 2007, 12:57am UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/21 "2007-03-21T00:57:04Z")
[QUOTE=Strain of Thought] 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. [/QUOTE] Again, it works inside the story, though I tend to agree with you otherwise. I think it’s been said, but just to be sure, I’ll say it. Optical activity is not an indicator of absolute stereochemistry in and of itself. This is especially true in any natural product, with many many stereocenters and lots of possible diastereomers (even though none of them will exist in nature.) ---
**Author:** ![K364](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k364/32/5_2.png) [@K364](https://boards.straightdope.com/u/K364)\ **Post date:** [March 21, 2007, 2:01am UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/22 "2007-03-21T02:01:13Z")
[QUOTE=Strain of Thought] 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… [/QUOTE] Arthur C. Clarke wrote a short story called “Technical Error” synopsis [here](http://math.cofc.edu/kasman/MATHFICT/mfview.php?callnumber=mf169) if you’re interested. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 21, 2007, 2:35am UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/23 "2007-03-21T02:35:44Z")
[QUOTE=Pleonast] 1. The first life on earth has a single chirality. 2. Chirality is heritable, making the original chirality dominant. 3. A mutant with opposite chirality suffers a large disadvantage. 4. Universal chirality is maintained. [/QUOTE] You misunderstand my point. This explains why individual amino acids would all have the same chirality, but not why all the _different_ amino acids have to have the same chirality. AFAIK, you can make peptide bonds between amino acids of different chirality, although the side chains will be on different “sides” of the chain. I am not clear why you couldn’t build a protein using D-alanine, L-leucine, L-glycine, and D-cysteine, for example. ---
**Author:** ![Strain\_of\_Thought](https://avatars.discourse-cdn.com/v4/letter/s/3be4f8/32.png) [@Strain\_of\_Thought](https://boards.straightdope.com/u/Strain_of_Thought)\ **Post date:** [March 21, 2007, 7:54am UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/24 "2007-03-21T07:54:45Z")
Colibri, using artificial means of synthesis you _can_ make proteins using amino acids of different chirality. It’s just that earth DNA _won’t_. All the different amino acids are labeled with the same chirality expressly because they are the set that earth DNA codes for. The set that DNA codes for is not a result of the labelled chirality, the labelled chirality is a result of which set the DNA codes for. ---
**Author:** ![Roog](https://avatars.discourse-cdn.com/v4/letter/r/a87d85/32.png) [@Roog](https://boards.straightdope.com/u/Roog)\ **Post date:** [March 21, 2007, 9:09am UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/25 "2007-03-21T09:09:40Z")
From [here.](http://www.chemistry.ucsc.edu/~fink/200lecture/1-97.htm) > [@](#): > > It is believed that there are only 20 common aa’s because of evolutionary selection. These 20 are sufficient to handle a wide variety of tasks in protein structure and function. The combinations of shape, size and polarity are the minimum necessary “to do the job”. The selection of the L-configuration may be chance, or L-aa’s may be inherently more stable (at least in proteins). The mirror image of HIV protease has been synthesized by Steve Kent, i. e. made from all D-amino acids: it’s structure and properties seem to be identical except for the chirality aspects. Polypeptides of mixed D and L aa’s are of lower stability than those with all L or all D. The presence of the b-amino acid would lead to much more flexibility in the polypeptide, and probably prevent folding! ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 21, 2007, 1:46pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/26 "2007-03-21T13:46:50Z")
[QUOTE=Strain of Thought] Colibri, using artificial means of synthesis you _can_ make proteins using amino acids of different chirality. It’s just that earth DNA _won’t_. All the different amino acids are labeled with the same chirality expressly because they are the set that earth DNA codes for. The set that DNA codes for is not a result of the labelled chirality, the labelled chirality is a result of which set the DNA codes for. [/QUOTE] Sure. But this does not explain _why_ DNA codes for only one kind of chirality. Actually, in looking into this a little further, I think I have found the reason. All the rest of the amino acids are synthesized from [glutamate.](http://en.wikipedia.org/wiki/Amino_acid_synthesis) Therefore, if the primordial glutamate happened by chance to be L, this explains why all the rest now follow suit. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [March 21, 2007, 2:21pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/27 "2007-03-21T14:21:23Z")
[QUOTE=Strain of Thought] Colibri, using artificial means of synthesis you _can_ make proteins using amino acids of different chirality. It’s just that earth DNA _won’t_. All the different amino acids are labeled with the same chirality expressly because they are the set that earth DNA codes for. [/QUOTE] Earth DNA does **not** determine the chirality of amino acids. It merely provides the information which the cell’s [translation](http://en.wikipedia.org/wiki/Translation_(genetics)) apparatus uses to construct peptide chains. On earth those chains happen to be made of 20 different L-amino acids, but there’s no reason the same DNA sequence couldn’t serve as the directions for chains comprised of 40 different types of D-amino acids. Incidentally, [D-amino acids _do_ appear](http://www.google.com/search?hl=en&safe=off&c2coff=1&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=Uqf&q=%22occurrence+of+d-amino+acids%22&btnG=Search) in a variety of earthly peptides and peptidoglycans. These isomers are usually proceed by [Posttranslational modification](http://www.google.com/search?hl=en&c2coff=1&safe=off&client=firefox-a&rls=org.mozilla:en-US:official&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=%22d-amino+acids%22+%22posttranslational+modification%22&spell=1) of pre-existing L-peptides. [QUOTE=Colibri] I am not clear why you couldn’t build a protein using D-alanine, L-leucine, L-glycine, and D-cysteine, for example. [/QUOTE] The preference for L over D is probably an artifact of the evolution of [Aminoacyl-tRNA synthetases](http://www.biochem.ucl.ac.uk/bsm/xtal/teach/trna/trna.html), the enzymes which hook amino acids onto transfer RNAs. In the beginning there were probably only 1 or 2 different versions of this protein, with a chance preference for L isomers of some amino acid. Since the chemistry depends on the position of the amino group within the enzyme’s binding pocket, rather than the position the side chains, mutations to the amino group binding region are more likely to be deleterious than mutations to the side chain binding region. Switching the substrate from D to L _requires_ such movement of the amino group binding region. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [March 21, 2007, 2:27pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/28 "2007-03-21T14:27:22Z")
[QUOTE=Colibri] [glutamate.](http://en.wikipedia.org/wiki/Amino_acid_synthesis) [/QUOTE] That’s a _terrible_ article. The actual story of [amino acid synthesis](http://www.rpi.edu/dept/bcbp/molbiochem/BiochSci/sbello/new_page_2.htm) is much more complex. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 21, 2007, 4:35pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/29 "2007-03-21T16:35:18Z")
[QUOTE=Squink] That’s a _terrible_ article. The actual story of [amino acid synthesis](http://www.rpi.edu/dept/bcbp/molbiochem/BiochSci/sbello/new_page_2.htm) is much more complex. [/QUOTE] OK. But to what extent are the 20 amino acids synthesized from the a limited number of precursor molecules? (It’s a bit difficult to wade through the link to sort this out, but it appears that there are only a few, even if glutamate is not the only one.) If many of the amino acids are derived from the same precursors, this would be a partial explanation of their common chirality, together with your own point concerning the tRNA synthetases. As per **Roog’s** cite, perhaps stability issues of mixed D and L proteins could be involved as well. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [March 21, 2007, 5:04pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/30 "2007-03-21T17:04:57Z")
[QUOTE=Colibri] it appears that there are only a few, even if glutamate is not the only one [/QUOTE] There’s like 5 or 6 different main pathways, and a bunch of little ones that aren’t major contributors as things happened to evolve, but could have been. You’re right that the production of chiral centers via a limited number of paths will contribute to an overall bias towards D or L. But just as with the aminoacyl-TRNA synthetase story, an improbable mutation back at the dawn of life _could_ have given us a mixed DL world, regardless of what the DNA (see OP) looks like. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 21, 2007, 5:17pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/31 "2007-03-21T17:17:06Z")
> [@](#): > > 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? That depends on how different you consider to be “different”. You could certainly swap out a few base pairs in a nucleic acid-like molecule: We even see this to some extent on Earth. One of the bases in RNA is different from the corresponding base in DNA. Given that one possible substitution exists, I see no reason to doubt that there could be others. One might have a long ladder-shaped molecule which had an entirely different set of building blocks than Earthly DNA, or a different number of them. But there are going to be some commonalities. Anything describable as “life” would have to have some way to store the information which tells it how it’s supposed to fit together, which pretty much means an aperiodic polymer of some sort. And this information-molecule is going to have to be easily replicable. I’m sure there are other ways of doing this, but I don’t know of any simpler methods than the ladder-shape used by DNA. For instance, if you made your genetic molecule out of two matching sheets stuck together, rather than two strands, it’s going to be hard to get at the information stored in the middle. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [March 21, 2007, 5:39pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/32 "2007-03-21T17:39:53Z")
[QUOTE=Chronos] One might have a long ladder-shaped molecule which had an entirely different set of building blocks than Earthly DNA, [/QUOTE] For example, you could build your genetic coding molecule around [peptide nucleic acids](http://en.wikipedia.org/wiki/PNA). ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [March 21, 2007, 5:55pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/33 "2007-03-21T17:55:04Z")
[QUOTE=Strain of Thought] Colibri, using artificial means of synthesis you _can_ make proteins using amino acids of different chirality. It’s just that earth DNA _won’t_. All the different amino acids are labeled with the same chirality expressly because they are the set that earth DNA codes for. [/QUOTE] Why? There are different amino acid translation schemes on Earth: why can’t there be an amino acid translation scheme that results in some of them having opposite chirality? It might have to involve a process unlike tRNA due to chirality constraints, but the evolutionary advantage would be enormous for the first adopters considering that others would not be able to eat them nutritiously. ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [March 21, 2007, 6:10pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/34 "2007-03-21T18:10:26Z")
One other thing to consider is that the ribosome, which translates mRNA into protein, excepts amino acids of the same chirality. The entire translation machinery that holds the mRNA, guides in the tRNA, and proofreads is huge and complicated, but the subenzyme which actually creates the peptide bonds doesn’t care about nucleotide sequence and all that. It does however care about amino acids with the correct chirality. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 21, 2007, 8:51pm UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/35 "2007-03-21T20:51:38Z")
> [@](#): > > It might have to involve a process unlike tRNA due to chirality constraints, but the evolutionary advantage would be enormous for the first adopters considering that others would not be able to eat them nutritiously. Evolution won’t change everything at once at start from scratch with new proteins for every purpose. So what you’d see would be an organism which incorporated a novel amino acid (not necessarily a dexter one; there are many “amino acids” which aren’t used much or at all in terrestrial life) in a few places in a few proteins. The other proteins in the organism would still be entirely digestible, and even the modified ones would still be mostly digestible. Eventually, you might see an organism evolving to use more and more novel AAs, but that would take a long time, and give the predators time to adapt to eating them, so it wouldn’t be all that huge an advantage. ---
**Author:** ![Bob55](https://avatars.discourse-cdn.com/v4/letter/b/e9c0ed/32.png) [@Bob55](https://boards.straightdope.com/u/Bob55)\ **Post date:** [March 22, 2007, 4:01am UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/36 "2007-03-22T04:01:16Z")
So they’re advanced enough to colonize another planet, but not advanced enough to do a little organic chemistry and convert some of that planet’s amino acids to isomers we can use? Why didn’t they just genetically engineer bacteria or plants to do this for them before they left earth? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 22, 2007, 4:16am UTC](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724/37 "2007-03-22T04:16:12Z")
[QUOTE=Bob55] So they’re advanced enough to colonize another planet, but not advanced enough to do a little organic chemistry and convert some of that planet’s amino acids to isomers we can use? [/quote] I think it’s more than just a “little organic chemistry;” you would essentially have to break the amino acid apart to rearrange it. Probably a rather expensive process. > [@](#): > > Why didn’t they just genetically engineer bacteria or plants to do this for them before they left earth? That’s not really possible for reasons already explained: Earth life is not set up to handle the opposite chirality at all. [Previous page](https://boards.straightdope.com/t/left-handed-dna-incompatible-with-right/396724.md?page=1)