G2

Meaning: G2 is the second gap phase of interphase, between S phase and mitosis. DNA replication is finished, so each chromosome consists of two sister chromatids. The cell keeps growing and makes proteins and organelles needed for division, such as microtubule subunits. The G2/M checkpoint confirms that DNA is fully replicated and undamaged; if it is not, the cycle pauses for repair. Entry into mitosis depends on active cyclin B–Cdk1 (also called MPF, maturation-promoting factor).

On the MCAT: Flow cytometry graphs plot cell count against DNA content. A peak at 2n is G1, a peak at 4n is G2 plus M, and cells in between are in S. Passages may add DNA damage or a Cdk inhibitor and ask which peak grows. A block at the G2/M checkpoint piles cells up at 4n. Questions may also ask which phase a cell is in from its DNA amount.

Common confusion: G2 cells have twice the DNA of G1 cells, but the chromosome number is unchanged, because sister chromatids joined at a centromere count as one chromosome. Also, G2 is not G0. G0 is a non-dividing resting state, usually entered from G1, while a G2 cell has already replicated its DNA and is normally headed into mitosis.