A comet assay is a technique used to determine the amount of double-strand breaks in DNA (DNA damage) in cells. The nucleus of an individual cell is placed on a microscope slide coated with an agarose gel. An electric
current is applied to the gel that causes DNA to move (electrophoresis), and the DNA is stained with a
fluorescent dye. When viewed using a microscope, undamaged DNA from the nucleus appears as a round shape
(the head), and the fragments of damaged DNA extend out from the head (the tail). The length of the tail
corresponds to the amount of the damage in the DNA (see Figure 1).
(a) To explain the movement of DNA fragments in the comet assay, identify one property of DNA and provide
reasoning to support how the property contributes to the movement during the comet assay technique.
(b) In a different experiment, cells are treated with a chemical mutagen that causes only nucleotide substitutions
in DNA. Predict the likely results of a comet assay for this treatment.