Biology Reference
In-Depth Information
a fabricator of a particular X. Following Rosen (1991) we call f a 'metabolic'
mapping; r will be called the 'construction' mapping;
this is therefore a
metabolism construction system.
It should be clear that without the ability of constructor r to make itself
with the help of its own blueprint, the factory cannot become self-fabricating.
Although it seems to be generally accepted that constructors can in principle do
this, what if it turns out not to be so? It is now time to turn to life-as-we-know-it
and ask how organisms manage to fabricate themselves. Does what we know
about metabolism and protein synthesis match the organisation sketched in Fig. 8
or do organisms do it differently?
6. SELF-FABRICATION IN LIVING SYSTEMS
For the purpose of matching known biochemistry to our abstract representation
of a self-fabricating factory, consider the diagram in Fig. 9. It has been pointed
out many times in the literature that ribosomes are really the only known exam-
ples of Von Neumann constructors. They fit the description perfectly: on its own
a ribosome can do nothing, but in conjunction with the information embedded in
a messenger RNA molecule that has been transcribed from DNA it can (with the
help of a plethora of auxiliary enzymes, cofactors and an energy source, GTP)
string amino acids together in the specified sequence. However, and this seems to
have been universally ignored, the genetic blueprint for a ribosome is made up of
Nutrients
Metabolic
enzymes
{Metabolic
enzymes,
Ribosomes
X
m-RNA
Building
blocks
Folding/
self-assembly
Polypeptides
Ribosomes,
Transcription
enzymes}
Transcription
enzymes
DNA
Figure 9
A summary of the biochemistry relevant to self-fabrication.
As explained in the text, the assembly of supramolecular complexes (the shaded box) must be a
spontaneous, unassisted process if the diagram is to depict a self-fabricating organisation. For the
sake of clarity the diagram has been kept simple; for example, ribosomal RNA and its synthesis
by transcription enzymes has been omitted. The chaperones that assist in the folding of some
polypeptides are also absent on the diagram, but are discussed in the text.
