To replicate, the DNA double-helix first "unzips" by allowing enzymes known as helicases to break the hydrogen bonds which hold the strands of base pairs together. After being divide into a leading strand and lagging strand (forming a Y-shape), an RNA molecule called a primer bonds to the end of both the leading and lagging strand. DNA polymerase now binds to the strand and gradually goes along it, adding complementary base pairs to complete the strand. Meanwhile, RNA primer binds chunks of DNA to the lagging strand. Once all the base pairs are completed, the primer is stripped away and an enzyme called ligase helps complete the bond between the base pairs. The two strands now now wind up into two different helixes.
DNA can be damaged by natural mutation, radiation, heat, and also through mechanical seperation.