G2 phase

G₂ phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated. G₂ phase ends with the onset of prophase, the first phase of mitosis in which the cell’s chromatin condenses into chromosomes.

G₂ phase is a period of rapid cell growth and protein synthesis during which the cell prepares itself for mitosis. Curiously, G₂ phase is not a necessary part of the cell cycle, as some cell types (particularly young Xenopus embryos^[1] and some cancers^[2]) proceed directly from DNA replication to mitosis. Though much is known about the genetic network which regulates G2 phase and subsequent entry into mitosis, there is still much to be discovered concerning its significance and regulation, particularly in regards to cancer. One hypothesis is that the growth in G₂ phase is regulated as a method of cell size control. Fission yeast (Schizosaccharomyces pombe) has been previously shown to employ such a mechanism, via Cdr2-mediated spatial regulation of Wee1 activity.^[3] Though Wee1 is a fairly conserved negative regulator of mitotic entry, no general mechanism of cell size control in G2 has yet been elucidated.

Biochemically, the end of G₂ phase occurs when a threshold level of active cyclin B1/CDK1 complex, also known as Maturation promoting factor (MPF) has been reached.^[4] The activity of this complex is tightly regulated during G₂. In particular, the G₂ checkpoint arrests cells in G₂ in response to DNA damage through inhibitory regulation of CDK1.