symmetry in genetics

[gusl]

coding for phenotypes
How do our genomes code for phenotypes that are informationally redundant, such as the fact that we have two hands, eyes, etc? Consider polydactyly: is there a mutation that can put extra fingers on one side but not the other? The occurrence of bilateral polydactyly suggests that some information gets reused between the two hands, but my understanding is that most cases are one-sided.

chirality
Why don't we observe the mirror image of DNA and proteins? Are they just as plausible? Is there an asymmetry in physics or in origins of the Earth that favoured this particular chirality? Is this evidence that life on Earth started only once?

These musings are so SFI-ish, that I'm tempted to ask CSSS alumni.

Comments

[easwaran.livejournal.com]

I was going to suggest chirality as something that gets inherited without being encoded genetically. If you've got ribosomes and proteins and everything with one chirality, then they'll make more of the same, and when the cells split, the new cell has the same chirality. Since new organisms start out as cells created by their parent, they also inherit that chirality.

At least, that's my assumption - I don't know if it's accurate.

As for the informational redundancy, I think that's a hot topic in biology these days, and some words I know related to it are "evo-devo" and "HOX genes". Dawkins talks about this with respect to segmented organisms (with arthropods like centipedes as the clearest cases, but even vertebrates as further examples - note the repetition of the spine and ribs). Since all segmented organisms are subphyla of Bilateria, maybe there's a connection there too.

[gustavolacerda.livejournal.com]

<< I was going to suggest chirality as something that gets inherited without being encoded genetically. If you've got ribosomes and proteins and everything with one chirality, then they'll make more of the same, and when the cells split, the new cell has the same chirality. Since new organisms start out as cells created by their parent, they also inherit that chirality. >>

yes, that is clear. The question is: if life started more than once (as some respectable people believe), why do we observe that all life on Earth seems to have the same chirality?

[gustavolacerda.livejournal.com]

thanks.

Btw, I just realized that "information redundancy" has at least two distinct meanings: the one I gave (phenotypes are compressible), and the redundancy that enables error-correction in DNA.

[easwaran.livejournal.com]

I guess I hadn't realized that many respectable people believed that life started more than once. Do they think that the different starts have interbred, or that they're totally separate phylogenies? In any case, if it started more than once, there's just as much of a mystery about why it all uses DNA and RNA (and maybe even why it uses proteins) as there is about the chirality. Chirality there's at least a 50/50 chance that any two starts would be the same.

[(Anonymous)]

I wonder if life has converged on one chirality, e.g. if you think about any lifeform that consumes molecules such as sugars and proteins - if your DNA can only encode for digesting one chirality then survival of the fittest will prefer individuals that can digest the predominant chirality. And then if higher animals start eating you the effect propagates up through the food chain.

If so there's a random 'first past the post' reason why we see the chirality we do. For low level organisms like bacteria we know that their DNA isn't from a strict lineage - some bacteria actually adopt DNA from their surroundings (as well as the molecules they use to grow) so once again there's an advantage to working with molecules and DNA with the predominant chirality. Whether this would completely eliminate all other life is potentially something that can addressed mathematically.

[the-locster.livejournal.com]

BTW that was me - forgot to login.

[gustavolacerda.livejournal.com]

I find it plausible that the DNA-RNA-protein architecture is the only solution for life that is reachable by chance in an Earth-like environment within the relevant timespan. (This could probably be strengthened)

If you met an intelligent alien, wouldn't you expect their number theory to be just like ours? (i.e. use concepts like primality, etc.)

[gustavolacerda.livejournal.com]

I was wondering if there was some kind of matter-antimatter -type dynamic (but I don't know whether or how DNA and mirror-DNA would react).


<< there's an advantage to working with molecules and DNA with the predominant chirality >>

Indeed, I've heard that molecules of the opposite chirality can be poisonous to eat. That is, I've heard of molecule pairs with this property. (However it looks like humans can handle both dextrose and levulose)

[gustavolacerda.livejournal.com]

for the last point, see http://web.media.mit.edu/~minsky/papers/AlienIntelligence.html (which incidentally seems to make an argument for Occam's razor, if you squint)

[the-locster.livejournal.com]

I think typically the mirror image molecules (L-form?) tend to be fairly harmless and just pass through the gut and/or get filtered out by the kidneys.

[easwaran.livejournal.com]

Number theory yes, but I would have guessed that there would be other ways of encoding information, especially given that actual life does use a few other techniques (epigenetics, the chemicals in the metabolism, and other developmental pathways). But of course, it uses these others to a much smaller degree.

[easwaran.livejournal.com]

There's also ones that we interact with differently, like the smell of wintergreen and the smell of carraway, which I believe are mirror images of one another.

[gustavolacerda.livejournal.com]

Someone told me that you can distinguish Enantiomers (i.e. molecules that are mirror images of each other) by pH! I have a hard time believing it...

[easwaran.livejournal.com]

Seems implausible to me, unless they're talking about some sort of effect in biological solution.

[gustavolacerda.livejournal.com]

right, you'd need to have asymmetry to begin with...