How does a flow cytometer determine which part of the cell cycle a cell is in?

A flow cytometer determines which part of the cell cycle a cell is in by measuring the DNA content of individual cells within a given population. This can be done from the  methods of: sample preparation, light detection, cell suspension, and data analysis. In a sample preparation, cells of interest are treated with specific dyes (e.g. propidium iodine) or fluorescent markers to stain their DNA. The dyes emit fluorescence that is proportional to the amount of DNA inside the cell. In cell suspension, cells are suspended in a fluid to create a single-cell stream. A laser is then used on the cells, which excites the fluorescent dyes bound to the DNA. The light then becomes detected and measured by detectors. The flow cytometer then can generate a histogram that represents the distribution of DNA content within the cell population. Each peak on the histogram represents a specific phase of the cell cycle. A distinct peak at diploid (2n) corresponds to cells in the G1 phase. A peak with twice the DNA content (4n) corresponds to cells in the G2/M phase. The S phase is visualized by an increase in DNA content between G1 and G2 peaks.