Biology Reference
In-Depth Information
organisms the mother cell after division and the daughter cell are effectively
the same; division yields two daughters and the mother ceases to exist. In other
organisms such as
Saccharomyces cerevisiae
, division is asymmetric, and the
mother differs from the daughter, yet appreciable mixing has occurred. Impor-
tantly also, the DNA, mRNA and proteins of the young daughter cell have been
synthesized by the DNA polymerase, RNA polymerase and ribosomes of the
mother cell. Consequently, rather than that each cell is an entirely new physical-
chemical phenomena, all cells are in fact continuous with each other. If there
were a process of excessively slow relaxation in a cell, the same process would
be in the same state in all daughter cells. That this not in part is reflected by
observations of epigenetic phenomena.
The extent to which this possible hysteresis is actually important is unclear
at the moment. For molecules of low molecular weight and complexity, it is
unimportant because relaxation to an equilibrium structure is fast enough. For
macromolecules and for the regulatory state of networks it might be important.
This issue simply has not been looked at sufficiently yet. In some cases of
regulation, such as for instance with the
lac
operon in
E. coli
, the regulatory state
is effectively inheritable through this type of mechanisms, which has the effect
of zonation of its colonies. In its ultimate form the point of hysteresis appears
obvious. All amino acids in proteins have the L-stereoisomeric constellation. The
mirror world with all R amino acids should be energetically equally probable.
Yet new cells with all their proteins in the R form do not arise, because the
enzymes that make their amino acids make the L form.
The conclusion is that the feature that it is too difficult to calculate structures
of proteins on the basis of physical-chemical principles may not even be too
relevant. It is quite possible that most of the structures that exist in living
cells are determined by more than the straightforward physical chemistry of
those molecules themselves. They may also depend on pre-existing structures
of other molecules with which they interacted during synthesis. The fact then
that biochemistry and molecular biology do not start from underlying physical-
chemical principles but with their own elementary objects such as enzymes and
genes, may be an asset rather than a disadvantage. The corollary is that also the
irreducibility of biochemistry and molecular biology to physics is much more
fundamental than technical. Any molecule-based biology may therewith be a
science that is fundamentally different from physics.
Evolution has not selected structures with maximum entropy (Schrödinger,
1944), minimum free energy (Nicolis & Prigogine, 1977) or maximum thermo-
dynamic efficiency (Westerhoff & van Dam, 1987), and in fact much of the
functioning of biological replication may have been structured so as to prevent
relaxation to such a state. Also here simple physical-chemical considerations
do not suffice to understand biology. As also proposed by Schrödinger (1944),
biology warrants its own explanatory principles.
