  We unzip DNA molecules using optical tweezers and determine the sizes of the
cooperatively unzipping and zipping regions separating consecutive metastable
intermediates along the unzipping pathway. Sizes are found to be distributed
following a power law, ranging from one base pair up to more than a hundred
base pairs. We find that a large fraction of unzipping regions smaller than 10
bp are seldom detected because of the high compliance of the released single
stranded DNA. We show how the compliance of a single nucleotide sets a limit
value around 0.1 N/m for the stiffness of any local force probe aiming to
discriminate one base pair at a time in DNA unzipping experiments.
