Biology Reference
In-Depth Information
a set of individual blueprints for the myriad of protein and ribonucleic acid com-
ponents that make up a ribosome; there is no contiguous genetic blueprint for a
complete ribosome. Therefore, a ribosome never directly makes a ribosome, only
the protein bits from which it is made up (the ribosomal RNAs are of course made
by ribosomally synthesised enzymes). Note that the problem of whether a Von
Neumann constructor can fabricate itself directly therefore does not arise in the
cell. Nevertheless, we still need to explain how the ribosomal components assem-
ble into a fully functional entity. The fabrication of all ribosomes entails two pro-
cesses: the construction of the parts (here the polypeptide chains and ribosomal
RNA), and their subsequent assembly into a fully functional entity. In fact, there
is another process wedged in between, namely that of the folding of newly syn-
thesised polypeptide chains into a functional, three-dimensional conformation.
Above I argued, following Rosen, that for a system to be self-fabricating
it must be closed to efficient causation. The existence of the noncovalent,
supramolecular processes of folding and assembly therefore forces us to search
for their efficient causes inside the system, which immediately confronts us with
the 'insufficient number of machines' dilemma described above. Adding extra
blueprints does not solve the problem; each addition implies a new polypeptide
that has to be accounted for in terms of internal efficient cause for folding and
possible association with other proteins. The recently discovered existence of
chaperones that assist the folding of some polypeptides also cannot fully solve
that part of the supramolecular problem: chaperones are themselves proteins
that need to fold in order to become active. There may be chaperones that assist
the folding of other chaperones, but somewhere along the line there must then
be chaperones that either fold spontaneously or assist in their own folding (or
there must be a group that form a closed autocatalytic system). However, as far
as we know chaperones fold spontaneously on their own. Similarly, with regard
to assembly we are reasonably certain that supramolecular complexes such
as ribosomes, spliceosomes, proteasomes, multimeric and oligomeric enzymes
self-assemble spontaneously - the efficient and formal causes of self-assembly
are embedded in the properties of the subunits of these complexes and in the
properties of the environment. It is possible to dissociate these complexes
in
vitro
and then have them reassemble themselves spontaneously. There appears
to be no need for a physical agent to assist in the assembly process. It therefore
turns out that, at least for life as we know it, unassisted self-assembly is the
process that makes self-fabrication, and therefore life, possible. It is interesting
to note that not one of the myriad of definitions of life listed in Barbieri
(2003) and Popa (2004), nor the two regularly quoted sets of criteria for
life - the Seven Pillars (de Duve, 1991) or PICERAS (Koshland Jr, 2002) -
mention self-assembly as a necessary condition for life. In fact, I conjecture
that if we discover life elsewhere in the universe, we shall recognise it by two
properties: being autonomously self-fabricating by having learnt how to harness
