Left-right asymmetry

[mmcirvin]

I like cool biology articles because I really understand this stuff so poorly; I'm almost guaranteed to learn something new. Here, PZ Myers tells us how cilia that rotate clockwise end up nudging early embryonic development in such a way as to generate the usually consistent left-right asymmetry that exists in our bodies. As commenter Rosie Redfield points out, the direction in which the cilia rotate is in turn determined by the handedness of our proteins, so it all comes down to a molecular asymmetry going back to the beginning of life.

I know that physicists have often wondered whether the handedness of protein helices in living organisms is something that was just randomly picked out and amplified by early organisms, or if there's some fundamental physical basis for it, maybe involving parity-violating weak interactions. I'd say "just random" is probably the way to bet. But I'm not sure how it could be known for sure, absent finding life similar to ours elsewhere in the universe that has the other handedness in its proteins (and even that might not be entirely conclusive). Then again, there's this strange result from the Murchison meteorite.

Comments

[pompe.livejournal.com]

I'd go for random too. But don't forget the ribose in the RNA/DNA, it is also chiral. So you have another handedness than just the proteins which might differ.

However, lets speculate about the idea that the polarized light in the early solar system affected chirality. If we were indeed born near that big blue Progenitor we probably also had the corpse of that star, a neutron star, nearby. And the polarized UV light from that young neutron star might have* affected the chiral makeup of the amino acids in the solar nebula.

So maybe the Progenitor gave us handedness, well, on a molecular level, and our left-right assymetry is a legacy older than even our planet. Just like the wristwatch you (probably) wear is a legacy of the World War, the hand you bear it on might be the legacy of the closest supernova our solar system ever has experienced.

*Probably not on a large scale. But it is a nice idea to play with.

[mmcirvin.livejournal.com]

Reading the Wikipedia article I was trying to figure out how that circularly polarized light would come about, but something with a hellacious magnetic field would probably do nicely.

At the molecular level chirality isn't a big puzzle

[(Anonymous)]

One thing it took me a long time to realize is that, at a molecular level, chirality is not an issue. The "L" and "D" forms of a sugar (or an amino acid) are entirely different molecules with entirely different shapes. At a macroscopic level (e.g. as crystals) they have the same properties, but at the molecular level this is irrelevant - what matters is how they interact with other individual molecules.

So I've come to think that there's nothing special about the question of how the first successful metabolism settled on D-sugars rather than L-sugars, or L-amino acids rather than D-amino acids. It's not fundamentally different than questions of how metabolism settled on any particular amino acid or sugar for any particular function.

Rosie Redfield (posting anonymously because I haven't figured out the alternative)

Re: At the molecular level chirality isn't a big puzzle

[mmcirvin.livejournal.com]

It could still be argued that there's a question of why that whole collection of other molecules came about instead of their mirror images. Do a parity transformation, not just on the sugars or amino acids, but on the whole ecosystem and you'd get another one that is equally viable.

But either would be more viable than a racemic mixture of the two, so it just comes down to a sort of autocatalytic coin flip that may not have anything particularly interesting behind its decision.

Re: At the molecular level chirality isn't a big puzzle

[(Anonymous)]

But either would be more viable than a racemic mixture of the two, so it just comes down to a sort of autocatalytic coin flip that may not have anything particularly interesting behind its decision.

I'm surprised that as a recovering particle physicist you were able to resist uttering the words "spontaneous symmetry breaking" here, since of course that's exactly what it is.

This is one of two examples I like to use when explaining the concept of spontaneous symmetry breaking. The other one is a round dinner table, where the water glasses are placed halfway between the place settings. At first, nobody's sure whether to use the water glass to her left or to her right. As soon as one person picks, breaking the symmetry, her neighbors are forced to pick the same way. Even if there's no overall preference for one solution over the other, everyone (at least in one region of the table) ends up going the same way.

At actual dinner parties, this is often not really spontaneous symmetry breaking, since some people have studied their Miss Manners and know which way it's supposed to go. If there are no Miss Manners types, though, and if the table is large enough, the symmetry may get broken in different ways in different parts of the table. If you want to impress your friends, you can point out to them in that case that a topological defect has formed on the table, and that the mechanism is similar to the way cosmic strings form (or would form if they existed).

Assuming, of course, that you have the sort of friends who are impressed by such things.

-Ted Bunn

Re: At the molecular level chirality isn't a big puzzle

[mmcirvin.livejournal.com]

I'm surprised that as a recovering particle physicist you were able to resist uttering the words "spontaneous symmetry breaking" here, since of course that's exactly what it is.

Exactly, though it's not the kind of simple Lagrangian system in which I used to study spontaneous symmetry breaking.

Re: At the molecular level chirality isn't a big puzzle

[westacular.livejournal.com]

Yes, exactly. Some small imbalance at a critical point was enough to spread across the entire system.

Still: When and how did this happen? Where? And was the initial imbalance purely random, or did some external factors (such as that polarized light hypothesis) cause it? (I realize I'm just repeating your post in simpler terms.) Interesting that the Murchison meteorite suggests an extrasolar origin.

This all seems quite similar in nature to the matter/antimatter question -- although in that case, it's harder to conceive of external factors that could be responsible.

[iayork.livejournal.com]

I like cool biology articles because I really understand this stuff so poorly

Heh. This morning I had a consultation with a statistician, about a problem I'm working on related to how particularly antigenic epitopes are generated from the background of all cellular proteins. The consultant turned out to be a PhD candidate in the math department; a very nice lady, but completely, and I mean cmopletely, blank on biology. I mean, I started to give a brief explanation of the problem -- three words in, I said "amino acids" and a look of utter panic crossed her face. So I dropped down about five levels of complexity, and she seemed to sort of flounder along. Eventually I got to the part where I had a 20x15 table of frequencies to compare to background frequencies and she sighed with relief, drew a deep beath, and started muttering about regressions.

[mmcirvin.livejournal.com]

I just had to look up the word "epitope" to follow that, but otherwise I could...

In the process I discovered that "racemic" comes from a term meaning "from grape juice".

[mmcirvin.livejournal.com]

...I have a layman's interest in most fields of science, but I've always been acutely conscious of the fact that, for a variety of reasons, the last actual biology course I ever took was a high-school introductory course taught by a man who had primarily been hired as an athletic coach and was fairly ignorant (his other major task was doing the morning PA announcements; I recall partiallyclips could do a really good impression of him).