  The Fermi surface (FS) nesting properties of URu$_2$Si$_2$ are analyzed with
particular focus on their implication for the mysterious hidden order phase. We
show that there exist two Fermi surfaces that exhibit a strong nesting at the
antiferromagnetic wavevector, $\boldsymbol{Q}_0$=(0,\,0,\,1). The corresponding
energy dispersions fulfill the relation $\epsilon_{1}(\boldsymbol{k})$=$-
\epsilon_{2} (\boldsymbol{k}\pm \boldsymbol{Q}_0)$ at eight FS hotspot lines.
The spin-orbital characters of the involved $5f$ states are {\it distinct}
($j_z$=$\pm$5/2 {\it vs.} $\pm$3/2) and hence the degenerate Dirac crossings
are symmetry protected in the nonmagnetic normal state. Dynamical symmetry
breaking through an Ising-like spin and orbital excitation mode with $\Delta
j_z$=$\pm$1 induces a hybridization of the two states, causing substantial FS
gapping. Concomitant spin and orbital currents in the uranium planes give rise
to a rotational symmetry breaking.
