  Weak $B^-\rightarrow D^0, \pi^0$ and $D^-\rightarrow {K}^0, \pi^0$ transition
form factors are described in both the space- and time-like momentum transfer
regions, within a constituent-quark model. Neutrino-meson scattering and
semileptonic weak decays are formulated within the point form of relativistic
quantum mechanics to end up with relativistic invariant process amplitudes from
which meson transition currents and form factors are extracted in an
unambiguous way. For space-like momentum transfers, form factors depend on the
frame in which the $W M M^\prime$ vertex is considered. Such a frame dependence
is expected from a pure valence-quark picture, since a complete, frame
independent description of form factors is supposed to include non-valence
contributions. The most important of such contributions are the $Z$-graphs,
which are, however, suppressed in the infinite-momentum frame ($q^2<0$). On the
other hand, they can play a significant role in the Breit frame ($q^2<0$) and
in the direct decay calculation ($q^2>0$), as a comparison with the
infinite-momentum-frame form factors (analytically continued to $q^2>0$)
reveals. Numerical results for the analytically continued
infinite-momentum-frame form factors agree very well with lattice data in the
time-like momentum transfer region and the experimental value for the slope of
the $F^+_{B\rightarrow D}$ transition form factor at zero recoil is reproduced
satisfactorily. These predictions satisfy heavy-quark-symmetry constraints and
their $q^2$ dependence is well approximated by a pole fit, reminiscent of a
vector-meson-dominance-like decay mechanism. We discuss how such a decay
mechanism can be accommodated within an extension of our constituent-quark
model, by allowing for a non-valence component in the meson wave functions. We
also address the question of wrong cluster properties inherent in the
Bakamjian-Thomas formulation.
