Bacterial chromosome came first, plasmids or viruses? Here we have a true chicken and egg situation. Almost certainly some present day plasmids are derived from viruses and equally certainly some present day viruses are derived from plasmids. However, the ultimate origins of both kinds of element remain obscure.
Most plasmids are circular, made of DNA, and much smaller than chromosomes.
Some plasmids are present in one or two copies per cell whereas others occur in m ultiple copies.
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