Because DNA is a double helix with two chains that are complementary, either side may act as a template for its replication. DNA is the only molecule that can copy itself. Its subunits are called nucleotides. These contain a sugar--deoxyribose, a phosphate group and one of four bases--adenine, guanine, cytosine or thymine. These bases pair up with their complement on the adjacent chain. The rules are A pairs with T and C pairs with G. During replication, the hydrogen bonds between complementary bases are overcome and either chain can be replicated. Free nucleotides attach to exposed bases. For example, if the DNA code reads: A, T, C, G, then a complementary chain will be produced with the basesT, A, G, C. Because of this property, life can go on. DNA can be replicated for daughter cells produced during mitosis and also, for gamete production during meiosis.
DNA molecules have the unique ability to self-replicate. This ability is important in cell division, which occurs during growing, development, and replacement of damaged cells. As DNA contains genetic information, it is necessary for DNA to replicate before division, otherwise the cells would not function correctly.
DNA is also a very stable molecule, it is hard to break apart without the help of enzymes, etc. This is important because DNA has to store the information to code for every single protein produced by the organism. If DNA were to fall apart, the cells would not function.