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From: plimpton@batcomputer.tn.cornell.edu (Steve Plimpton)
Newsgroups: sci.bio
Subject: Re: Human Asymmetry
Message-ID: <633@batcomputer.tn.cornell.edu>
Date: Wed, 8-Apr-87 09:47:47 EST
Article-I.D.: batcompu.633
Posted: Wed Apr  8 09:47:47 1987
Date-Received: Mon, 13-Apr-87 04:08:49 EST
Reply-To: plimpton@batcomputer.UUCP (Steve Plimpton)
Distribution: sci
Organization: Theory Center, Cornell University, Ithaca NY
Lines: 67


	Thanks to all who responded to my query on why all humans have the
same left/right asymmetry.  In a nutshell, your answer is that all of us
(excepting those with Kartegner's Syndrome who also often have reversed organs)
have cilia/microtubules/cytoplasm in our cells that are/is asymmetric the
same way.
	This was news to me, so I am enlightened - but still have a couple
of follow-up questions.  My confusion likely comes from looking at this as a
geometry problem and not a biological one, but here goes ... 


1.	It only makes sense, when looking at an object, to label one side the
left and the other right, if it is already asymmetric top vs. bottom and front
vs. back.  Otherwise you can rotate it to reverse left and right and the object
looks the same.  So when

eddy@boulder.Colorado.EDU (Sean Eddy) in <865@sigi.Colorado.EDU> writes
(and others similarly) 

> ... it appears it is either the asymmetric beat wave
> form of these subcellular organelles or some asymmetric aspect of
> microtubules themselves that determines organismic asymmetry.
> ... a likely guess is that ciliary beating early in the developing embryo 
> establishes a handedness for the embryo. 

does this mean that one could take a human egg and by examining its
cilia/microtubule structure or motion (assuming you could look at it in
sufficient microscopic detail), one could assign a unique top/bottom,
front/back, left/right (TBFBLR) to it?  And that *all* human eggs are the same
in that respect?  So that in essence every egg is a human in miniature with the
same TBFBLR orientation as each other and adults? 

	If this isn't so, then I'm still having trouble visualizing, when the
embryo consists of a few cells (20 or 20,000 - I don't know) and there is a
definable top/bottom and front/back, but not yet a left and right, how a cell
on the left is in any different a chemical/biological environment than one on
the right and hence senses it's the one that should become a heart.  It doesn't
(to me) seem enough to say (for example) all the cilia are beating towards the
left at that point therefore that's where the heart will end up, because if the
top or front had formed the opposite way (which it seems they could have if
there wasn't a TBFBLR to start with), the cilia would be beating to the right. 
Any help here? 


2.	In article <1003@aecom.UUCP>, werner@aecom.UUCP (Craig Werner) writes

>	The molecule in question here is Dynein, the component of
> microtubules that provides the molecular motor.  It interacts with
> polymerized tubulin to form the skeleton of cilia.  Since tubulin
> has a direction, dynein binds only one way, and hence all cilia
> have a forward beat and a reverse beat.
>	It turns out that this ciliary movement ensures that the heart will
> always be on the left side, and the liver on the right.

I have no idea what tubulin is, but is its handedness a direct result of the
handedness of amino acids and sugars?  And if so, does this mean that in the
organic soup bubbling on earth a few years ago, if the first (tubulin,
amino acids, self-replicating gizmos, whatever) had been the other handed,
that nature as we know it would be reversed?  I.e. that we'd all have
right-side hearts, 90% of us be left-handed, want to sleep on the other side
of the bed, da Vinci would have painted Peter sitting on the other side of 
Christ in "The Last Supper", and so on ad infinitum? 

Thanks again ...

Steve Plimpton			ArpaNet:  plimpton@cheme.tn.cornell.edu
UseNet:  {cmcl2,shasta,uw-beaver,rochester}!cornell!batcomputer!plimpton