Abstract
When populations of single Chinesee hamster ovary (CHO) cells were exposed to the mutagen ethyl methane sulfonate (EMS), allowed to grow into colonies, and stained for glucose-6-phosphate dehydrogenase (G6PD) activity, two types of unstained colonies were observed at a frequency of about on per thousand stained colonies. These negative-staining colonies consisted of (1) colonies uniformly deficient in staining activity (pure); and (2) colonies containing both stained and unstained sectors (mosaic) in various relative sizes and patterns. Unstained cells isolated from mosaic colonies were genetically stable and had significantly reduced or absent G6PD activity. Random cell aggregation or chromosome segregation from tetraploid cells is not a significant cause of the sectoring phenomenon. Also, mosaic colonies are not principally caused by mutation at one of two replicated G6PD genes and their subsequent segregation during division. The simplest explanation for this phenomenon is that EMS induces a mutational change in one of the two DNA strands and DNA replication then produces normal and mutant double-stranded DNAs which segregate into wild-type and G6PD-deficient cell types, producing a mosaic colony.
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