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The cell cycle is the sequence of events by which a biological cell prepares for and completes division, including DNA replication and distribution of chromosomes. In Saccharomyces cerevisiae, changing bud appearance provides an accessible readout; see the Burgess, Powers and Mell account and Hartwell, Culotti and Reid's primary study cited there.

For a typical cycling diploid eukaryotic biological cell, the order is G1 phase, S phase, G2 phase, then M phase. DNA replication doubles nuclear DNA amount during S phase; mitosis distributes the copies, and cytokinesis separates the daughters. The useful distinction is between copying a library and allocating its copies: DNA replication changes how much material exists, whereas division changes which biological cell contains it.

Gene expression continues to supply RNA and proteins as the biological cell grows; it is different from duplicating the genome. In the yeast cross-species complementation experiment, restored growth requires successful repeated cell cycles, but growth alone does not locate a defect within a phase. Bud appearance in Saccharomyces cerevisiae provides another readout, while a flow cytometric DNA-content assay asks how much nuclear DNA is present and DNA pulse labeling asks whether new DNA was synthesized during an exposure.

Cell cycle analysis assigns evidence to stages of the cell cycle. A flow cytometric DNA-content assay measures accumulated nuclear DNA, whereas DNA pulse labeling detects synthesis over an exposure window. Agreement between these readouts is useful because an amount and a synthesis event answer different questions.

Flow cytometry records signals from suspended particles passing an illuminated interrogation region. For a flow cytometric DNA-content assay, a laser excites a DNA-binding dye and a detector records its fluorescence. An event is a recorded passage, not a guarantee that exactly one intact biological cell was measured.