Relatively constant in duration, the G2 phase is characterized by a very large nucleus (DNA content doubled, increase in fluids) and polarization of the cell. Once the cell division plane is established, mitosis is triggered by M phase promoting factor, which is found in the cytoplasm. Functionally, the G2 phase consitutes a safety buffer before the cell enters mitosis (M phase). At the G2 checkpoint, the cell ensures that the DNA replication was complete and error-free, that the environment is favorable for cell division, and that the cell has grown large enough. Like the G1 checkpoint, the G2 checkpoint occurs at the end of a gap phase. Stop proteins prevent mitosis until the cell has fulfilled all of the requirements for cell division.