  The growth dynamics of rigid biopolymers, consisting of $N$ parallel
protofilaments, is investigated theoretically using simple approximate models.
In our approach, the structure of a polymer's growing end and lateral
interactions between protofilaments are explicitly taken into account, and it
is argued that only few conformations are important for biopolymer's growth. As
a result, exact analytic expressions for growth velocity and dispersion are
obtained for {\it any} number of protofilaments and arbitrary geometry of the
growing end of the biopolymer. Our theoretical predictions are compared with a
full description of biopolymer growth dynamics for the simplest N=2 model. It
is found that the results from the approximate theory are approaching the exact
ones for large lateral interactions between the protofilaments. Our theory is
also applied to analyze the experimental data on the growth of microtubules.

Statistical Mechanics;Soft Condensed Matter;Biomolecules