REAL GENES, REAL INTELLIGENCE 121We now know that the molecular networks of cells do that through
changes in the reactions between molecular components. As with the
Bénard cells mentioned above, networks self- organize in response to the
disequilibria set up by the signals. It “depends on proteins that are as-
sembled from a toolkit of modular domains, each of which confers a
specifi c activity or function.”^15 Th e structure may then be played out
in vari ous ways: cell growth, cell division, cell movement (for a number
of purposes), cell diff erentiation to form this or that tissue, production of
a hormone for transfer to other cells, and so on. Response pathways can
involve hundreds or thousands of components, calling up ge ne tic re-
sources as required (see below). So crucial are these networks to normal
function that approximately 20 percent of the human genome codes for
proteins involved in signal transduction.
Some responses, such as those to ste roid hormones, involve only two
or three steps, from initial ligand reception to ge ne tic transcription and
synthesis of product. More typically, though, extensive cascades are con-
ducted through wider networks. Th ese need many intersections as control
points where they can “listen” to so many other cell states or events. Box-
and- arrow diagrams, like that in fi gure 4.5, can give a rough impression
of the interrelations. But they cannot capture the more abstract nature
of the statistical structures. Th ese have only been approximated through
the use of computer models.
An example of this coordination of internal and external structure
is that of epidermal growth factor (EGF), the subject of thousands of
research papers since its (Nobel Prize– winning) discovery in 1962. EGF
is a peptide produced in the brain and circulating in body fl uids, where it
binds to EGF receptors on many cells. Th is binding initiates, not a single
ste reo typed response, but one or another of a wide range of possibilities:
cell growth, cell division, diff erentiation, migration, and so on, depending
on external and internal contexts.
It is impor tant to note how the pathways— the ripples of chemical
reactions— through signaling networks are being continuously recon-
fi gured in response to changing environmental structure. Th at is, the net-
works “learn,” or alter biases of response, in ways that always makes
current responding contingent on past experience. Th ey are actively cre-
ating new, adaptive, variations in the pro cess.
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