Proofreading and RepairEveryone makes mistakes, and so does DNA polymerase. When this enzyme puts the wrong nucleotide into the growing chain, the result is a mutation which changes the genetic make up of an organism. Therefore, mistakes cannot be tolerated.
Fortunately, DNA polymerase has another ability besides synthesizing DNA strands: it also proofreads its work. If an error is detected, the enzyme cuts out the incorrect nucleotide and replaces it with the correct one.
In addition to random errors during replication, DNA can be mutated by other factors, such as UV light and cancer causing agents (carcinogens). If these mutations are not fixed, the function of the cell will be altered. This may cause the cell to die, or to grow in an unregulated fashion (cancer). Many repair mechanisms exist in the cell to detect and correct mutations, and more often than not they work just fine.