Biomedical Engineering Reference
In-Depth Information
Figure 1.13.
ActA coated polystyrene disks in motility media can have clouds
(typically in the non-motile nucleation stage), perimeter tails, single-face tails or
double-face tails. Figure is reprinted from [25]. (Copyright (2004), with permission
from Elsevier).
mesoscopic stress distribution in the actin gel, and the solid-on-solid friction
that the gel exerts on the bacterium surface. One should keep in mind, that
only the polymerization process misses (so to speak) half of the problem. In
particular, recognizing the importance of the nonlinear friction allows us to
understand the saltatory mutant as a system working in a “stick-slip” regime
familiar in solid friction. In general, the system gets into the saltatory regime
at a well-defined threshold called a Hopf bifurcation. In its vicinity, measur-
able quantities such as velocity and comet density modulations are sinusodal
functions (of time and space, respectively). Away from the bifurcation, vari-
ations are more abrupt, like in Figure 1.2. It is important to understand that
the role of mutation is simply to drive the system in a region of phase space
where the behavior is saltatory, but that many other external perturbations
could have the same result without changing the genome. This point is well-
illustrated in bio-mimetic experiments, where a simple bead diameter change
drives the behavior from regular to saltatory. This example shows that the
relationship between genotype and phenotype will not be easy to unravel. One
will need to fully understand all the complex dynamical diagrams governing
biological systems before being able to fully exploit the formidable genetic
data that we are getting now.
It is important to stress that the mesoscopic approach is not antagonistic
to microscopic ones, but rather complementary. Microscopic theories can be
used as input for writing boundary conditions in mesoscopic theories. They
