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Un '''plásmido''' es un fragmento intracelular separado de ADN<ref name=Alberts>{{cita libro|autor=Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walters, Peter|título=Molecular Biology of the Cell|edición=5ª|editorial=Garland Science|año=2008|ubicación=New York and London|página=21|isbn=0-8153-4105-9}}</ref> que puede replicarse independientemente del ADN cromosómico dentro de una célula. plasmids are found primarily in bacteria, but may be present occasionally in [[archaea]] and [[eukaryotes]]. Plasmids are generally circular DNA molecules.<ref name=watson>{{cite book|author=Watson, Jamed D.; Baker, Tania A.; Bell, Stephen P.; Gann, Alexandrer; Levine, Michael. Losick, Richard|title=Molecular Biology of the Gene|edition=5th|publisher=Pearson, Benjamin Cummings/CSHL Press|year=2004|location=San Francisco, CA|page=111;131|isbn=0-8053-4642-2}}</ref> Unlike the chromosomal DNA of bacteria, plasmids are typically not essential for bacterial growth.<ref name=watson /> Plasmids provide an important additional dimension towards the flexibility of the body's response to changes in their environment, whether those changes are hostile or potentially favorable.<ref name=Dale>{{cite book|author=Dale, Jeremy W.; Park, Simon F|title=Molecular Genetics of Bacteria|page=137-164|year=2004|location=West Sussex, England|publisher=John Wiley & Sons|edition=4th|isbn=0-470-85084-1}}</ref> A plasmid which is able to integrate into the chromosome of the bacterium is called episome.<ref name=Junqueira>{{cite book|author=Junqueira, Luiz C.; Carneiro, José|title=Biologia Celular e Molecular|trans_title=Cell and Molecular Biology|publisher=Guanabara Koogan|edition=8th|year=2005|location=Rio de Janeiro|page=269|isbn=978-85-277-1045-9}}</ref>