DNA is the genetic material found in chromosomes in the nucleus of cells. It has a double helix structure like a twisted ladder, with sugar and phosphate molecules forming the sides and nitrogen base pairs forming the rungs. There are four nitrogen bases - adenine, thymine, guanine and cytosine - that always pair up in the same way: A pairs with T and G pairs with C. DNA replication is the process where the DNA double helix unwinds and each half acts as a template for a new partner strand, resulting in two identical DNA molecules.