Cell-cycle analysis is a method in
cell biology that employs
flow cytometry to distinguish cells in different phases of the
cell cycle. Before analysis, the cells are and treated with a
fluorescent dye that stains
DNA quantitatively, usually
propidium iodide (PI). The fluorescence intensity of the stained cells at certain wavelengths will therefore correlate with the amount of DNA they contain. As the DNA content of cells duplicates during the
S phase of the cell cycle, the relative amount of cells in the
G0 phase and
G1 phase (before S phase), in the S phase, and in the
G2 phase and
M phase (after S phase) can be determined, as the fluorescence of cells in the G
2/M phase will be twice as high as that of cells in the G
0/G
1 phase.