A genetic duplication which involves almost one-third of the chromosome has been described in the bacterium Salmonella typhimurium. This duplication occurs at a spontaneous frequency of approximately 10-4 per cell plated on medium selective for the duplication. Following exposure of cells to ultraviolet light, the frequency of occurrence of the duplication is increased. The ultraviolet light-induced damage which stimulates duplication formation is subject to photorepair. The spontaneous and induced merodiploids are genetically unstable and produce haploids at high frequency. In addition to inducing the duplication in haploid strains, ultraviolet light exposure accelerates loss of the duplication in merodiploid strains.

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