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B Shepherd, University of Washington;
GB Schaalje, KL O'Neill, Brigham Young University
The single cell gel electrophoresis (comet) assay is a popular method
for analyzing nuclear DNA damage. In this technique, a measurement
called the "tail moment" quantifies DNA damage for an
individual cell. The distribution of tail moments among a group
of cells on a slide (experimental unit) often follows a skewed bimodal
distribution, perhaps because cells are at different stages of the
cell cycle when exposed to treatments. To better examine DNA damage,
the distribution of tail moments on a slide can be modeled using
a mixture of two gamma distributions. Maximum likelihood, modified
to accommodate left censored data, can be used to estimate the 5
parameters of the gamma mixture distribution for each slide. A weighted
analysis of variance on the parameter estimates for the gamma mixtures
can be performed to determine differences in DNA damage between
treatments. These methods were applied to an experiment on the effect
of thimadine kinase in DNA damage and repair. Analysis based on
a mixture of gamma distributions was found to be more statistically
valid and more scientifically informative than an analysis on the
log-transformed tail moments.
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