Biology Reference
In-Depth Information
1936). The first proteins that were studied between 1950 and 1960
indeed complied with this principle (Kendrew
et al.
, 1958). From
these individual observations it was extrapolated that, in general,
biological molecules recognise each other owing to their shape and
their electrical charges, as do locks and keys. Each molecule pos-
sesses specific non-covalent linkage sites corresponding to the most
stable interactions with other molecules. A group of molecules
brought together would spontaneously form the most stable, ordered
structure possible, i.e. the one that maximises the number of spe-
cific linkages (Fig. 6). Such a process, called self-assembly, was put
forward in the first instance to explain the genesis of viral struc-
tures (Caspar and Klug, 1962), and was then generalised to all cell
structures (Bouck and Brown, 1976; Inoué, 1982).
As Jacques Monod said, in a process of stereospecific self-
assembly, “
As in a crystal, the structure of the assembled molecules
itself constitutes the source of 'information' for the construction of
the whole. These epigenetic processes therefore consist essentially in
this: the overall scheme of a complex multimolecular edifice is con-
tained '
in posse
' in the structure of its constituent parts, but only
comes into actual existence through their assembly
” (CN pp. 86-87).
In as far as the three-dimensional structure of proteins depends
on their linear sequence in amino acids, which depends in turn on
the nucleotide sequence of the DNA, according to this theory, onto-
genesis really is the transformation of the genetic information into a
material process, complying with the principle of order from order.
However, there is a major point to note. For the spontaneous
self-assembly of a structure to be reproducible, the possible inter-
actions between molecules have to be unequivocal or very limited
in number, so as to avoid generating too many possible combina-
tions which would prevent the genesis of a unique ordered structure
(Kupiec, 1999). Caspar and Klug's models of viral structures (1962)
help to illustrate this problem (Fig. 6B). A viral particle is pro-
duced by the self-assembly of identical proteins, complying with a
repeated basic motif. Each molecule, diagrammatically represented
by an irregular polygon, can only combine with another molecule
via identical binding sites, symbolised by the homologous sides of
