Re: Mitotic spindle and magnetic poles.

On Tue, 25 Nov 2003 20:08:46 +0000, Tom Anderson
<[EMAIL PROTECTED]> wrote:

>On Tue, 25 Nov 2003, r norman wrote:
>
>> On 24 Nov 2003 22:06:58 -0800, [EMAIL PROTECTED] (Robert Clark)
>> wrote:
>>
>> >Has there been any investigation of the possibility that the origin of
>> >the mitotic spindle really is due to electromagnetism?
>
>Not as far as i know.
>
>> There is no reason to believe magnetism is a mechanism just because
>> the pictures look similar.  Look at the electric field of a dipole
>> http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dipole.html to see
>> the same thing.
>
>That's right, and there are probably other examples.
>
>> The similarty in shape is caused by processes which share some
>> superficial similarities but major differences
>
>Ah, now i'd say that here we have processes which share some major
>similarities but have superficial differences :). The two systems are
>entirely different at the level of specifics - proteins in a cell versus,
>er, vibrations in the ether or whatever it is now. However, i have to
>wonder, in an ignorant, D'Arcy-Thomsonesque way (if only!), if the two are
>actually governed by very similar general organising principles, whose
>similarity leads to the similarity of the forms.
>
>The magnetic thing is about small elements (the iron filings) polarising
>and linking up to form a polar filament under the influence of an
>organising field, which is established as a tension between two poles, and
>orienting themselves at every point so that they lie in the direction of
>the greatest gradient. The cellular example is, to me, tantalisingly
>similar - obviously, the spindle is homopolar (the two poles are of the
>same type, rather than north-south opposites), but the microtubules are
>indeed polar chains of polar elements.
>
>Now, i'm not saying that tubulin monomers behave anything like iron
>filings - they grow from a point, rather than spontaneously aggregating -
>but the idea of a field of some quantity (eg activity of a kinase or
>G-protein), defined by the interaction of the centromeres and governing
>the form of the spindle, is an interesting one. Since the spindle is
>homopolar, the true poles of the field would probably have to be, on the
>one hand, the centromeres, and, on the other, the metaphase plate.
>Although that just begs the question of how the position of the metaphase
>plate is defined!
>
>Anyway, it's all a very interesting, and poorly understood, complex of
>problems - just the sort of thing we like!
>

The pattern of field lines in a magnetic dipole has absolutely nothing
to do with the fact that a physical magnet is often formed from small
particles polarising and aligning.  Current flowing in a circle
produces the same dipole and there are n small particles aligned.  A
pair of magnetic monopoles (they could exist, theoretically) would do
the same thing.  An electric dipole shows the same behavior.  The fact
is that there is a tendency to radiate out from the poles and there is
a tendency to run from pole to pole.  That is the similarity.