  We present a study on the role of defects on the stability of short DNA
molecules. We consider short DNA molecules (16 base pairs) and investigate the
thermal as well as mechanical denaturation of these molecules in the presence
of defects that occurs anywhere in the molecule. For the investigation, we
consider four different kinds of chains. Not only the ratio of AT to GC
different in these molecules but also the distributions of AT and GC along the
molecule are different. With suitable modifications in the statistical model to
show the defect in a pair, we investigate the denaturation of short DNA
molecules in thermal as well as constant force ensemble. In the force ensemble,
we pulled the DNA molecule from each end (keeping other end free) and observed
some interesting features of opening of the molecule in the presence of defects
in the molecule. We calculate the probability of opening of the DNA molecule in
the constant force ensemble to explain the opening of base pairs and hence the
denaturation of molecules in the presence of defects.
