= Flow cytometric DNA-content assay
Under controlled staining and detector settings, integrated <fluorescence> can be approximately proportional to nuclear <DNA> amount. A <histogram> of accepted single-cell events then separates a $2C$ region, an intermediate region and a $4C$ region. These are ordinarily assigned to <G0 phase>/<G1 phase>, <S phase>, and <G2 phase>/<M phase> respectively in the stated <diploid> model.
\Image[/dna-content-assay-illustrative-peaks.png]
{title=Illustrative <DNA>-content assay peaks: equal amount need not mean equal <cell cycle> stage}
{description=Original synthetic curves, not measured data or a fit to an experiment. The horizontal axis is a proportional <fluorescence> scale expressed in $C$ units for a reference <diploid> population. Component shapes and weights are arbitrary. The $2C$ peak combines <G0 phase> and <G1 phase>; the $4C$ peak combines <G2 phase> and <M phase>. The plotted curve cannot itself distinguish these paired states.}
A Cornell protocol fixes a single-cell suspension by adding cold 70% <ethanol> dropwise while mixing, reducing clumping. After washing, its staining solution contains propidium iodide at $50\,\mathrm{\mu g/ml}$; <ribonuclease A> removes interfering <RNA>. It specifies excitation at $488\,\mathrm{nm}$ and emission around $600\,\mathrm{nm}$. The resulting readout is <fluorescence> per event, not a direct count of <chromosomes>. <DNA Cell Cycle Analysis with PI>[DNA Cell Cycle Analysis with PI, p. 1: dye properties, staining steps 1–7 and reagent recipe]{ref}
Useful controls include an unstained sample for background and a reference population with known nuclear <DNA> amount, processed with identical settings. Two attached $2C$ <biological cells> can mimic one $4C$ event. Comparing signal integral with pulse height or width helps exclude such doublets; a gate selecting intact single particles also limits debris. These are proposed assay checks, not measurements underlying the illustration.
A $4C$ peak cannot distinguish <G2 phase> from <mitosis>; a $2C$ peak cannot distinguish <G1 phase> from <G0 phase>. Mixed <ploidy>, uneven staining and aggregates can invalidate phase assignments. Peak areas describe the accepted sample at collection, and do not by themselves establish phase durations, division rates or a particular arrest mechanism. A treatment comparison needs matched handling and another readout appropriate to the claimed mechanism.
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