PlasmidsExtrachromosomal elements are genetic species outside the chromosome. Plasmids, transposons, and viruses are all forms of extrachromosomal DNA.Plasmids are circular pieces of DNA and very small when compared to the chromosome. Plasmids contain between two thousand and two hundred thousand base pairs, whereas chromosomes typically contain four million base pairs. Most are one-thousandth the size of the host genome.Plasmids possess their own origin of replication, and exhibit autonomous replication. Anything being capable of autonomous replication due to an independent origin of replication is known as a replicon. This includes chromosomes and plasmids. All plasmids are replicons.Most plasmids hold between two and thirty genes, making them small compared to a chromosome such as e. coli, which contains five thousand genes. Generally, plasmids are not essential for bacterial host survival unless under special conditions. For example, the survival of plague bacillus inside a macrophage, the plasmids in this
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