  We study theoretically the unzipping of a double stranded DNA from a
condensed globule state by an external force. At constant force, we find that
the double stranded DNA unzips an at critical force Fc and the number of
unzipped monomers M goes as M~(Fc-F)^{-3}, for both the homogeneous and
heterogeneous double stranded DNA sequence. This is different from the case of
unzipping from an extended coil state in which the number of unzipped monomers
M goes as M~(Fc-F)^{-chi}, where the exponent chi is either 1 or 2 depending on
whether the double stranded DNA sequence is homogeneous or heterogeneous
respectively. In the case of unzipping at constant extension, we find that for
a double stranded DNA with a very large number N of base pairs, the force
remains almost constant as a function of the extension, before the unraveling
transition, at which the force drops abruptly to zero. Right at the unraveling
transition, the number of base pairs remaining in the condensed globule state
is still very large and goes as N^{3/4}, in agreement with theoretical
predictions of the unraveling transition of polymers stretched by an external
force.
