How does a cell know when to stop dividing?

Mapson mapson at mapson.com
Mon Jan 25 13:23:54 EST 1999

On Tue, 19 Jan 1999 23:11:38 -0500, Matt Morrison wrote:

>I am an undergrad in biochemistry, so don't take this as gospel truth, but
>the way I understand it, we don't fully understand it yet. Somehow the final
>size and shape of an organism is hard-coded into its DNA, which is expressed
>through proteins and other chemicals in the cells, which then direct each
>other to grow or to stop growing. 

Interesting- so in a sense, cell walls might actually have "features"
to them that indicate directions and magnitude (or cessation) of
growth? This is an interesting idea- sort of like a interlocking
jigsaw notion with a chemical flavor.

If I understand the idea right, the problem is that this would require
unimaginable precision to achieve a consistent overall form within a
species. IOW, makes one wonder why normally people don't end up with a
leg on the shoulder and a head on their back- because the
environmental noise would surely steer things widely. maybe there is
some sort of averaging inherent in the noise, however.

For instance, in a macro crystalline structure like, say, a 6 inch
quartz crystal, there is always a lot of noise expressed in the final
form... but the apex still has 6 visible points, and a sort of
"abstraction of a pure form" is still in large part maintained.

But a quartz crystal doesn't have veins or muscles in it, or quartz
material that altered slightly in some regions to form organs! So the
biological system is seemingly many orders of magnitude more complex-
and at the same time, even more precise in achieving an intended form.

>Back to your question. I don't think there's a way to shape the point's
>growth without some sort of pre-determined routine. Unless you have some
>rules for which the point can grow along the lines of 'the game of life' I
>don't really know if you can do it. Am I correct in understanding that you
>can't define a routine or something that says 'you eventually want to look
>like a torus' or something like that?

That is right- that would be playing "god", having an external formula
to exert on some matter. Instead, the matter somehow has to "have it
in itself" to create it's own form- and this requires, it would seem,
some sort of feedback mechanisms such that a given cell has "some idea
of where it is" in the overall form. Or at least within a region that
"has an overall idea", to break it down a bit.

Thanks for replying.

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