  The polymerization dynamics of double-stranded polymers, such as actin
filaments, is investigated theoretically using simple chemical kinetic models
that explicitly take into account some microscopic details of the polymer
structure and the lateral interactions between the protofilaments. By
considering all possible molecular configurations, the exact analytical
expressions for the growth velocity and dispersion for two-stranded polymers
are obtained in the case of the growing at only one end, and for the growth
from both polymer ends. Exact theoretical calculations are compared with the
predictions of approximate multi-layer models that consider only a finite
number of the most relevant polymer configurations. Our theoretical approach is
applied to analyze the experimental data on the growth and fluctuations
dynamics of individual single actin filaments.
