G2 phase
/ JEE-TOO fayz /
Imagine you have just finished photocopying a huge, important document. Before you staple the copies and hand them out, you take a moment to flip through, fix any smudged pages, and gather the staplers and folders you will need. A cell does the same thing after copying its DNA. G2 is the final preparation stage before division, where the cell tidies up, double-checks its freshly copied DNA, and assembles the tools it will need to split apart.
G2 (the second 'gap' phase) is the last part of interphase, coming after S phase and just before mitosis. By now the cell has two copies of every chromosome (sister chromatids). During G2 it continues to grow, produces the proteins needed for division (including components of the mitotic spindle), and runs a critical quality-control inspection at the G2/M checkpoint. There it verifies that all the DNA was copied completely and correctly, and that any damage has been repaired, before it lets the cell tip over into the dramatic events of mitosis.
G2 matters because it is the last chance to catch a problem while it is still cheap to fix. If the checkpoint finds incompletely copied or damaged DNA, it halts the cell here and buys time for repair — entering mitosis with broken chromosomes can shatter them and scatter genes wrongly between the two daughter cells. A subtle point often missed is that G2 is not just waiting: it is an active stage of growth and assembly, building the molecular machinery that the next, much faster phase will spend all at once.
If a cell's DNA is hit by radiation, the G2/M checkpoint can hold the cell in G2 for hours — radiotherapy partly works by trapping cancer cells here when they cannot repair the damage, so they never make it into division.
The G2/M checkpoint can trap a cell with damaged DNA before it divides.
G2 ends and mitosis begins when an enzyme complex (CDK1 bound to cyclin B, historically called maturation-promoting factor) is suddenly switched on — a near-instant chemical 'go' signal, not a gradual drift.