The Boltzmann factor, DNA melting, and Brownian ratchets: Topics in an   introductory physics sequence for biology and premedical students
  Three, interrelated biologically-relevant examples of biased random walks are presented: (1) A model for DNA melting, modelled as DNA unzipping, which provides a way to illustrate the role of the Boltzmann factor in a venue well-known to biology and pre-medical students; (2) the activity of helicase motor proteins in unzipping double-stranded DNA, for example, at the replication fork, which is an example of a Brownian ratchet; (3) force generation by actin polymerization, which is another Brownian ratchet, and for which the force and actin-concentration dependence of the velocity of actin polymerization is determined. 
S. G. J. Mochrie
q-bio.BM cond-mat.soft cond-mat.stat-mech physics.bio-ph