  We discuss theoretically the force F between two colloidal particles, each of
them carrying one single strand DNA. The two strands are complementary only on
a finite sequence of consecutive base pairs. We define an adjustment length (a
few base pairs): in the adjustment regions near both ends of the paired region,
the tension is still mainly on one single strand. But in the central part, the
two backbones are equally loaded. This leads to a rupture force, increasing
linearly with length, and saturating for large lengths.
