DNA Effects Tests. Genotoxins can induce DNA damage by any of several mechanisms. Among the methods for evaluating such effects is the mouse bone marrow micronucleus test. This in vivo assay detects damage to the chromosomes or the mitotic apparatus of immature red blood cells found in bone marrow. During cell division, undamaged chromosomes give rise to normal daughter nuclei, but if the chromosomes are broken or the mitotic apparatus of the cell is damaged, chromosome fragments may be incorporated in secondary nuclei instead of into the main nucleus.
Secondary nuclei are much smaller than the main nucleus and are referred to as micronuclei.
Murine erythrocytes following exposure to a known genotoxic agent. The micronucleus is distinguished by the small, differentially stained segment of nuclear material. The sister chromatid exchange test is also used to evaluate DNA effects. In this assay, metabolically active and inactive mammalian cells are exposed to the test extract and the differentially stained chromatids are then examined to see if segments of DNA have been reciprocated or exchanged between the sister chromatids. Evidence of such exchanges appears as striations or a banding effect.