Cell cycle analysis quantifies the proportion of cells in each phase of the cell division cycle. The most common methods measure DNA content by flow cytometry.

DNA Content Analysis by Flow Cytometry

Cells are fixed and permeabilized, then stained with a stoichiometric DNA dye. The fluorescence intensity is proportional to DNA content:

• G0/G1: diploid (2N) DNA content.
• S phase: DNA content between 2N and 4N (DNA synthesis ongoing).
• G2/M: tetraploid (4N) DNA content.

The key assumption is that the dye binds DNA in direct proportion to mass. Propidium iodide (PI) is the most common DNA stain for fixed cells, but it also binds RNA. An RNase A or RNase T1 treatment step is therefore essential.

Propidium iodide protocol:

1. Harvest 1×10⁶ cells and wash with PBS.
2. Fix in 70% ethanol at −20 °C for at least 2 hours (can be stored for weeks).
3. Wash with PBS to remove ethanol (vortex during wash to prevent clumping).
4. Resuspend in PI/RNase staining buffer (50 µg/mL PI, 100 µg/mL RNase A in PBS, 0.1% Triton X-100).
5. Incubate at 37 °C for 30 minutes in the dark.
6. Analyze by flow cytometry (linear scale, 488 nm excitation, >575 nm emission).
7. Collect at least 10,000 events at a slow flow rate (low events/second) for best CV.

Data Analysis

The DNA content histogram shows peaks at 2N and 4N with a S-phase plateau between them. Modeling software (FlowJo, ModFit, FCS Express) fits a mathematical model to the data:

• The G0/G1 and G2/M peaks are modeled as Gaussian distributions (CV = 2–5% for high-quality data).
• The S-phase is modeled as a broad rectangle or polynomial with an increasing slope from 2N to 4N.
• Debris and aggregates are modeled by a debris curve (exponential) and aggregate peaks (at 6N, 8N, etc.).

A CV >8% on the G0/G1 peak indicates poor quality data, cell clumps, incomplete RNase treatment, or too-high flow rate.

BrdU Incorporation

Bromodeoxyuridine (BrdU) is a thymidine analog incorporated into DNA during S phase. After labeling (30–60 minutes), cells are fixed, permeabilized, and stained with an anti-BrdU antibody. The DNA is counterstained with 7-AAD or DAPI. Flow cytometry with dual-parameter analysis (BrdU vs. DNA content) resolves:

• G0/G1: low BrdU, 2N DNA.
• S phase: high BrdU, intermediate DNA.
• G2/M: low BrdU, 4N DNA.
• G0: very low BrdU, 2N DNA (prolonged G0 can be distinguished from G1 by Ki-67 staining).

BrdU staining requires partial DNA denaturation (HCl or DNase I) to expose the incorporated BrdU to the antibody, which degrades DNA integrity and can broaden the DNA content histogram. EdU, a BrdU analog detected by copper-catalyzed click chemistry, avoids the denaturation step and preserves high-quality DNA content analysis.

Mitotic Index

Cells in mitosis (M phase) can be identified by staining for phosphorylated histone H3 at Ser10 (p-H3S10). This modification is specific to mitotic cells. Combined with DNA content analysis, p-H3S10 staining resolves mitotic cells from the broader G2/M population.

Common Pitfalls

• Doublets: two G1 cells passing through the laser together register as a single G2/M event. Use a doublet discrimination gate (FL2-A vs. FL2-W) to exclude them.
• Hypodiploid (sub-G1) population: cells with <2N DNA content are apoptotic or necrotic. If >10% of events are sub-G1, the culture conditions may be inducing cell death.
• Fixation: methanol fixation gives better DNA content resolution than ethanol but is not compatible with all cell surface markers.