Because of DNA's composition (all genetic information is stored in two complimentary yet independent strands), it is easily replicated. One polymer of a DNA molecule is the opposite of the other, like a photo negative. If you have one side, you could recreate the other. It's this duality that makes it surprisingly simple to make a copy of a DNA molecule.

DNA molecules are synthesized, then, through a process called replication. Replication begins with the unwinding of the double helix by an enzyme called helicase. The unwinding can start anywhere along the strand, and once begun, enzymes create two "replication forks" that continue to unzip the helix in both directions. After the DNA has started to unwind and straighten out, another enzyme called DNA polymerase goes to work. Its job is to match up the exposed nitrogenous bases with new nucleotides, which it finds in the surrounding nuclear fluid. These nucleotides bond to the two seperated polymers according to the normal Watson-Crick pairing rules. When the entire DNA molecule has been separated and re-matched in this manner the result is two perfect copies of the original.

Much is still unknown about DNA replication. What signals the process to start? What, exactly, keeps some cells from replicating? What goes wrong in cancerous cells that makes them replicate uncontrollably?

Replication in Prokaryotes
In prokaryotes (bacteria) most or all of an organism's genetic information is stored in one long, circular DNA ring instead of multiple chromosomes of DNA strings with unconnected ends. These rings are replicated in a very similar manner to eukaryotic DNA, the only real difference being that here only two replication forks are used. While eukaryotes had many replication sites in action at once, prokaryotes replicate so quickly that only two, one working in each direction around the ring, are needed. Bacterial DNA is replicated at a rate of about one million base pairs per minute, while other organisms only range from 500 to 5000 pairs per minute.