When the DNA solution, after partial denaturation, is neutralized, segments of
single-stranded DNA can be wrenched out of partially unwound molecules. For
instance, if we assume that the unwinding precedes regularly from the free ends of
a molecule, it may be that the branching points are subject to particularly intense
stresses and in these points the DNA will most probably break. In this case we should
observe in the CsCl density gradient a band in the position of denatured DNA,
even if no molecule has effectively reached strand separation. If, moreover, the
remaining unwound double-stranded segments would renature quickly, we should
observe only another band in the position of the native DNA, and the all-or-none
effect would be explained.