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The concept of chromosomes with a ring structure was born during the early studies of bacterial sexuality, and the discovery of fertility factors- episomes or plasmids-provided much later the key tools for gene cloning and biotechnology. But the plasmid-mediated transfer of antibiotic and other resistances, as well as pathogenicity, has served bacteria well in their own adaptive evolution.
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Kohiyama et al. (2003) studied this question.
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