  It had been proposed that overstretching double stranded DNA from the 5-5
ends will produce a new fiber form of dsDNA that is narrower than the S form
that has been suggested to result when the dsDNA is overstretched from the 3-3
ends. We present the first comparison of the structures that result when dsDNA
is overstretched from the 5-5, 3-3, and 3-5 ends and show that the stability of
the overstretched form depends on the ends to which the force is applied. The
stability also depends strongly on the ionic environment as well as the
presence of reagents that bind to ssDNA.
