DNA replication underlies the process of inheritance at all levels (cellular, organismal, population). DNA replication occurs as a prelude to cell division ( S phase of cell cycle in eukaryotes). DNA in all organisms is the end point in a continuous series of replications going back to the origin of life, almost 4 billion yrs ago. DNA replication is based on the complementarity of DNA molecules and on the ability of proteins to form specific interactions with specific sequences of DNA.