  We introduce and numerically study a model designed to mimic the bulk
behavior of a system composed of single-stranded DNA dendrimers.
Complementarity of the base sequences of different strands results in the
formation of strong cooperative intermolecular links. We find that in an
extremely narrow temperature range the system forms a large-scale, low-density
disordered network via a thermo-reversible gel transition. By controlling the
strand length, the gel transition temperature can be made arbitrarily close to
the percolation transition, in contrast with recent model systems of physical
gelation. This study helps the understanding of self-assembly in this class of
new biomaterials and provides an excellent bridge between physical and chemical
gels.
