In bacteria and partly in yeast cells, mostly ring-shaped extrachromosomal DNA molecules that are present in addition to chromosomal DNA (linear plasmids have been discovered in bacteria of the genera Streptomyces and Borrelia, among others). Plasmids comprise 1-2% of the total genome.
Plasmids usually contain only a few genes. They are replicated as an independent genetic unit because they each have at least 1 origin of replication. They may be single or multiple in number. Plasmids of different sizes may also be present.