During the S phase, which occurs between the G1 and G2 phases of the cell cycle, DNA replication and much of the DNA repair activity occurs in cells. The S phase is responsible for the synthesis or replication of DNA, and the genetic material of a cell is doubled before it enters mitosis or meiosis, allowing there to be enough DNA to be split into daughter cells. The most important event occurring in S phase is the replication of DNA, which produces double the amount of DNA, providing the basis for the chromosome sets of the daughter cells. The S phase is regulated by regulatory pre-replication complexes, which signal where DNA synthesis should begin. During S-phase, the cell continuously scrutinizes its genome for abnormalities, and detection of DNA damage induces activation of three canonical S-phase "checkpoint pathways" that delay or arrest further cell cycle progression.