Content-Type: message/rfc822 Date: Wed, 20 Sep 2006 13:32:16 -0400 From: "Nicholas Grundy" To: "Vladimir Nabokov Forum" Subject: Re: [NABOKV-L] Victor Fet on symmetry in biology Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="=__Part77532E30.1__=" --=__Part77532E30.1__= Content-Type: multipart/alternative; boundary="=__Part77532E30.2__=" --=__Part77532E30.2__= Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit (This is basically a note about developmental biology as it applies to left-right symmetry breaking - so it may not be relevant to the list, although arguably chirality is... I leave the decision over whether to run it entirely to you!) Victor Fet is spot-on about external symmetry and internal asymmetry, and discoveries in the last few years have revealed just how left and right are defined in the human and other creatures. In short, the question isn't "why is symmetry broken?" - there are several theories about why asymmetric hearts and viscera are more efficient (they involve fluid dynamics and organ packing in the abdomen) - the question is "how is it broken in the same way every time?". People have their hearts on the left hand side almost without fail, and the mirror-image picture (/situs inversus/) is vanishingly rare - but defining left as distinct from right is extremely difficult. The development of the other two body axes, the superior/inferior (or rostral/caudal), and the front/back (or anterior/posterior) , is understood - but even with those in place, how can you get left reliably? You get an idea of it if you try, using *only* instructions featuring 'front', 'back', 'up', and 'down', to define your left hand. Until recently the existence of a chiral F-molecule was postulated, which the developing organism could align with its superior-inferior and anterior-posterior axes and have left defined by the third bar (make a fist, point your index finger forward, your thumb up, and your middle finger towards your other hand, and you get the idea!). What actually happens is that there are little hair-like structures (cilia) which rotate clockwise (and, because of the intrinsic chirality of proteins, only clockwise) in a pit on the surface of the organism. In and of itself, of course, that doesn't create a flow in either direction - but then the organism simply angles the cilia 45 degrees back, so that the rotation from 9 through 12 to 3 (as on a clockface) hits the floor of the pit and creates no flow, and the rotation from 3 through 6 to 9 creates a leftward flow. Not strictly relevant - so I half apologise - but it's got a certain magic to it, IMHO. Nick. PS: Chris McManus' 'Left hand, Right hand' is very good on this and on all sorts of other left-right distinctions in culture, society, and development. -- "Forgotten is forgiven." F.Scott Fitzgerald ||Skype nick_grundy|ICQ 32475468|AIM rhuiri|MSN nmg@talk21.com|Yahoo grundyn|| Search the archive: http://listserv.ucsb.edu/archives/nabokv-l.html Contact the Editors: mailto:nabokv-l@utk.edu,nabokv-l@holycross.edu Visit Zembla: http://www.libraries.psu.edu/nabokov/zembla.htm View Nabokv-L policies: http://web.utk.edu/~sblackwe/EDNote.htm Search the archive: http://listserv.ucsb.edu/archives/nabokv-l.html Contact the Editors: mailto:nabokv-l@utk.edu,nabokv-l@holycross.edu Visit Zembla: http://www.libraries.psu.edu/nabokov/zembla.htm View Nabokv-L policies: http://web.utk.edu/~sblackwe/EDNote.htm --=__Part77532E30.2__= Content-Type: text/html; charset=US-ASCII Content-Transfer-Encoding: 7bit Content-Description: HTML (This is basically a note about developmental biology as it applies to left-right symmetry breaking - so it may not be relevant to the list, although arguably chirality is... I leave the decision over whether to run it entirely to you!)

Victor Fet is spot-on about external symmetry and internal asymmetry, and discoveries in the last few years have revealed just how left and right are defined in the human and other creatures. In short, the question isn't "why is symmetry broken?" - there are several theories about why asymmetric hearts and viscera are more efficient (they involve fluid dynamics and organ packing in the abdomen) - the question is "how is it broken in the same way every time?". People have their hearts on the left hand side almost without fail, and the mirror-image picture (situs inversus) is vanishingly rare - but defining left as distinct from right is extremely difficult.

The development of the other two body axes, the superior/inferior (or rostral/caudal), and the front/back (or anterior/posterior) , is understood - but even with those in place, how can you get left reliably? You get an idea of it if you try, using only instructions featuring 'front', 'back', 'up', and 'down', to define your left hand. Until recently the existence of a chiral F-molecule was postulated, which the developing organism could align with its superior-inferior and anterior-posterior axes and have left defined by the third bar (make a fist, point your index finger forward, your thumb up, and your middle finger towards your other hand, and you get the idea!).

What actually happens is that there are little hair-like structures (cilia) which rotate clockwise (and, because of the intrinsic chirality of proteins, only clockwise) in a pit on the surface of the organism. In and of itself, of course, that doesn't create a flow in either direction - but then the organism simply angles the cilia 45 degrees back, so that the rotation from 9 through 12 to 3 (as on a clockface) hits the floor of the pit and creates no flow, and the rotation from 3 through 6 to 9 creates a leftward flow.

Not strictly relevant - so I half apologise - but it's got a certain magic to it, IMHO.

Nick.

PS: Chris McManus' 'Left hand, Right hand' is very good on this and on all sorts of other left-right distinctions in culture, society, and development.

-- 
“Forgotten is forgiven.” F.Scott Fitzgerald
||Skype nick_grundy|ICQ 32475468|AIM rhuiri|MSN nmg@talk21.com|Yahoo grundyn||

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