the ratchet-like motion can be reobtained by decreasing the spring constant to 100 pN/A ̊ (the pulling speed is still 10 A ̊ /ns). However, if further decreasing the spring constant to 20 pN/A ̊, only two steps can be detected (see arrows in Fig. 3), with the first one corresponding to the simultaneous release of the first two nucleotides and the second for the passage of all the remaining nucleotides. When the pulling speed is chosen to be 100 A ̊ /ns, one cannot achieve the ratchet-like motion even when the spring constant was set to 100 pN/A ̊ . In addition, for a nanopore larger than