Spontaneous damage to DNA occurs at a rate of one event per billion nucleotide pairs per minute. Assuming this rate in a human cell, DNA is damaged every twenty-four hours at 10,000 different sites in the body. DNA has a number of quality control mechanisms. DNA polymerase (the enzyme that catalyzes DNA replication) has a proofreading function that immediately corrects 99 percent of these errors during replication. Those errors that pass through are corrected by a mismatch repair system. When a mismatch base (such as A-G) is detected, the incorrect strand is cut and the mismatch is removed. The gap is then filled in with the correct base, and the DNA is resealed.