Biology Reference
In-Depth Information
To see whether a mechanistic explanation is successful, two additional steps
have to be completed. The first is a question of sufficiency. Quantitative models
will yield definite predictions - if they are within the specified range of tolerance,
they will count as sufficient:
(5) We have to know whether the constructed behavior is within the specified
degree of tolerance (this is the sufficiency test).
The second question is one of adequacy for the explanatory problem at hand.
A model will incorporate a variety of variables and parameters (e.g., the affinities
of enzymes for its substrates, the amount of enzymes). We need to know, first
of all, that the model postulates only parts and processes present in the complex
system whose behavior is to be explained (e.g., a cell). It is possible to have a
sufficient model, which predicts cell behavior, with fictitious parts; for example,
a model that assumed the presence of an enzyme that
E. coli
cannot synthesize
would be inadequate even if the model was empirically sufficient. An adequate
model also requires that the parameter values incorporated in the model are
realistic, that is, that they at least approximate the values in the cell. For example,
if a model incorporates enzyme concentrations greater than those present in the
cell, then that too would be an inadequate model.
(6) We have to check whether the mechanism is the actual one (this is the
realism test).
3. THE DEVELOPMENT OF THE
LAC
OPERON AS A
MECHANISTIC MODEL
In the early 1960s, Jacob and Monod published a series of ground-breaking
papers focused on genetic regulation.
4
Though there were several alternatives to
the Jacob and Monod model offered in the years which followed, their account
of the
lac
operon is still widely accepted, at least in outline. The revolutionary
aspect of the
lac
operon, within genetics, was that it introduced a distinctive class
of genetic regulatory elements, governing the synthesis of enzymes and other
products relevant to cell regulation. By roughly 1959, Jacob and Monod had
established negative control as an important biological mechanism. In reflect-
ing on the comparison between the bacterial model and phage, both of which
exhibited the crucial regulatory phenomena, Jacob says this in his Nobel lecture:
4
For a straightforward overview, see Beckwith (1967); and for a more comprehensive treatment, Beckwith &
Zipser (1970).
