what happens in the s phase of the cell cycle

The S phase of the cell cycle occurs during interphase, before mitosis or meiosis, and is responsible for the synthesis or replication of DNA. During this phase, the cell synthesizes a complete copy of the DNA in its nucleus. DNA replication and much of the DNA repair activity occurs in cells during the S phase, which occurs between the G1 and G2 phases of the cell cycle. Replication increases the DNA content of the cell from 2n to 4n, so cells in S have DNA contents ranging from 2n to 4n. The S phase is one of the three stages of interphase, which is the period of the cell cycle when the cell grows and makes a copy of its DNA.