... double-stranded extrachromosomal DNA molecule (about 2,000 to 10,000 base pairs) that contains genes, which play an important role in the bacterium. This can often be a gene that encodes a protein, which makes the bacteria resistant to an antibiotic. Plasmids are present in multicopy within bacteria, and they are replicated independently from their own origin of replication. Plasmids probably came about as a result of bacteria evolving in close proximity to other heterotrophs. Bacteria often grow in the same environment as molds and fungi and compete with them for food (complex organic material). As a result, molds and fungi have evolved to make toxins that kill bacteria. These are widely used in medicine as antibiotics. Bacteria are thought to have evolved the ability to make proteins that inactivate the toxins. In nature, some plasmids contain genes, which are involved in the process of plasmid transfer between two bacteria by the process ...

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