There are enzymes within the cells that detect errors or alterations in DNA molecules and begin a repair of those errors. First, enzymes known as restriction endonucleases, specialized in cutting DNA molecules (also used in genetic engineering), cut the affected piece of DNA. Then polymerase enzymes build correct sequences of nucleotides correspondent to the affected piece taking as template the DNA chain complementary to the affected chain. Finally the new correct sequence is bound in the DNA under repair by specific enzymes.
