[Free Registration/Login may be required to access all resource tools.] DNA must replicate (copy) itself so that each resulting cell after mitosis and cell division has the same DNA as the parent cell. DNA replication occurs during the S phase of the cell cycle, before mitosis and cell division. The base pairing rules are crucial for the process of replication. DNA replication occurs when DNA is copied to form an identical molecule of DNA. Learn more about dna structure and replication in this learning module produced by CK-12.