  DNA's shape mostly lies on its total energy $F$. Its corresponding
equilibrium shape equations can be got by classical variation method: letting
the first energy variation $\delta^{(1)}F=0$. Here we not only provide the
first variation $\delta^{(1)}F$ but also give the second variation
$\delta^{(2)}F$ in planar case. Moreover, the general shape equations of DNA
are obtained and a mistake in [Zhang, \emph{et al}. {\it Phys. Rev. E} {\bf 70}
051902 (2004)] is pointed out.
