  It is well known that during homology recognition and strand exchange the
double stranded DNA (dsDNA) in DNA/RecA filaments is highly extended, but the
functional role of the extension has been unclear. We present an analytical
model that calculates the distribution of tension in the extended dsDNA during
strand exchange. The model suggests that the binding of additional dsDNA base
pairs to the DNA/RecA filament alters the tension in dsDNA that was already
bound to the filament, resulting in a non-linear increase in the mechanical
energy as a function of the number of bound base pairs. This collective
mechanical response may promote homology stringency and underlie unexplained
experimental results.
