Cell growth briefly at M phase when the nucleus and then the cell divide in two. The process of cell division, called cell cycle, has four major parts called phases. The first part, called G phase is marked by synthesis of various enzymes that are required for DNA replication. The second part of the cell cycle is the S phase, where DNA replication produces two identical sets of chromosomes. The third part is the G phase in which a significant protein synthesis occurs, mainly involving the production of microtubules that are required during the process of division, called mitosis. The fourth phase,
DNA re-replication and Cdc6 expression. Loss of p53 regulation in cells can also be observed frequently in cell lines that overexpress Cdt1 or Cdc6. For the special case of cell cycle-regulated DNA replication in which DNA synthesis is uncoupled from cell cycle progression refer to endoreduplication. Endoreduplication is an important and widespread mechanism in many cell types. It does not adhere to many of the cell cycle checkpoints and damage controls in regularly dividing cells, but it does not result in uncontrolled re-replication. Endoreduplication is a controlled process and occurs to perform a specific cell function. In some cells, it has been