Subject
Symmetry and biology (but not linked to VN)
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Date
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(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.
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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.
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