  In cells, helicase translocation along nucleic acid is essential for many
biological processes. However, so far, the mechanism of this translocation is
not fully understood. Recent studies show that helicase might translocate
through two processes, active process and passive process, with different
translocation rate. In this study, a model including such two processes is
presented. In which, each of these two processes consists of two sub-processes,
chemical sub-process in which needed translocation factors are attached, and
mechanochemical sub-process in which helicase makes a forward translocation
step. Helicase can switch stochastically between these two processes with
external force dependent rates. By this model, ribosome translocation along
message RNA is detailed discussed. We found that, with the increase of external
force, the mean translocation rate of ribosome increases from one lower limit
to one upper limit, and both of these two limits increase with concentrations
of the translocation factors. Under high external force, ribosome translocates
mainly through active process. At saturating concentrations of translocation
factors, the translocation is governed by mechanochemical sub-processes.
