sentences:  Hopfield suggested a simple way to achieve smaller error rates using a molecular ratchet which takes many irreversible steps, each testing to see if the sequences match.At each step, energy is expended and specificity (the ratio of correct substrate to incorrect substrate at that point in the pathway) increases.The requirement for energy in each step of the ratchet is due to the need for the steps to be irreversible; for specificity to increase, entry of substrate and analogue must occur largely through the entry pathway, and exit largely through the exit pathway.