  Atomic Force Microscopy analysis is employed to study the geometrical and
topological properties of $3000$kbp DNA molecules fixed in mica substrates with
$MgCl_{2}$. We found that the aggregates on the substrate surface for certain
salt concentrations form a honeycomb stable structure with the addition of
salt. The honeycomb, and the transition to other structures, were characterized
by the \emph{Betti numbers}, which is a topological invariant property and by
\emph{Hausdorff-Besicovitch fractal dimension}.
