  We develop a propagating front analysis, in terms of a local probability of
zipping, for the helicase activity of opening up a double stranded DNA (dsDNA).
In a fixed-distance ensemble (conjugate to the fixed-force ensemble) the front
separates the zipped and unzipped phases of a dsDNA and a drive acts locally
around the front. Bounds from variational analysis and numerical estimates for
the speed of a helicase are obtained. Different types of helicase behaviours
can be distinguished by the nature of the drive.
