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Phases of
the Cell Cycle
Cell cycle
progression is typically divided into phases G1, S, G2
(collectively called interphase) and M (mitosis) shown
schematically above (A). DNA replication occurs exclusively
during S phase, such that G2‑phase cells have twice the cellular
DNA content compared to the G1 cells (B). After completion of
mitosis (M) the cell divides (undergoes cytokinesis) generating
two daughter cells (G1) each having half the DNA content of the
G2 cell. G1, S and G2/M cells can therefore be distinguished
based on their DNA content. When DNA content is measured in a
large population of cells and the data plotted in a DNA content
frequency histogram, G1 and G2/M phase cells create peaks at DNA
index (DI) = 1.0 and 2.0, respectively. S‑phase cells are
distributed in between the peaks (C).
Accuracy
of DNA Content Measurement
The accuracy
of DNA content measurement is reflected by variation in
fluorescence intensity between individual cells with identical
DNA content, such as G0/G1 cells. This variation can be assessed
the value of coefficient of variation (CV) of the mean value of
DNA content of the G0/G1 cell population. The CV of the
DNA‑associated mean fluorescence of G1 cells is thus considered
an index of the accuracy of the DNA measurements. High accuracy
is required in particular in assessing DNA ploidy to distinguish
between DNA diploid and aneuploid cells, which may differ
minimally in DNA content. Accurate DNA content measurement is
also critical in analysis of cell cycle distributions. LSC
provides DNA content measurement on a par with flow cytometry
and well within this optimal range for appropriately prepared
samples.
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