Biology Reference
In-Depth Information
CHAPTER
3
The Power and Limitations
of Self-Assembly
INTRODUCTION TO SELF-ASSEMBLY
Self-assembly is one of the simplest possible mechanisms for morphogenesis. Its defining
feature is that the components that assemble together contain, in their shapes and binding
characteristics, sufficient information to determine the structure they produce; there is no
need for any external regulation and no need for any special prior spatial arrangements or
special timing. For some highly specialized biological systems, such as viruses, self-assembly
can be the sole mechanism of morphogenesis although most organisms are more complex.
Nevertheless, it is important that researchers into morphogenesis are familiar with the char-
acteristics of self-assembling systems for two main reasons. First, even complex morpho-
genetic systems use self-assembly many times in the construction of sub-assemblies; and
second, an understanding of the power and limitations of simple self-assembly will, by
providing a contrast, assist understanding of the more subtle and complex mechanisms
covered later in this topic.
Self-assembly of macromolecular complexes d whether by (non-covalent) 'polymeriza-
tion' of a series of identical monomers or by coalescence of a structure composed of several
different constituents d is driven by the ability of subunits to bind to each other in a specific
way. As in all chemistry, this binding is determined by the difference in free energy between
the system (consisting of the subunits, the surrounding water, and so on) in the separated
state, and assembled state. Generally, the interactions between subunits depend on a multi-
tude of individually weak, non-covalent bonds such as dipolar forces, hydrogen bonds and
van der Waals attraction. Stable associations form when the free energy change caused by
bonding is substantially larger than the energies of thermal collisions that would tend to
knock a complex apart. The use of these weak bonds, rather than covalent ones, has conse-
quences for the abilities of self-assembling structures to perform quality control functions,
as will be discussed below.
Reliable self-assembly requires that the coming together of components in the right way
is not too improbable. This partly reflects the energy requirements of the process (partic-
ularly if the transition of a subunit from an unbound state to a bound one involves the
crossing of an energy barrier), and partly reflects the proportion of phase space that is
 
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