Based on the available crystal structure a model is presented for the polymerization activity and switching transition between polymerase and exonuclease sites of a DNA polymerase molecular motor. Using the model, the fast polymerization rate for correctly base-paired DNA and much reduced polymerization rate after an incorporation of a mismatched base can be well explained. The dependences of the polymerization rate and exonuclease rate on mechanical tension acting on the DNA template are studied. The switching rates between the two sites are analyzed. All the results show good quantitative agreement with the available experimental results.