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For the physics phenomenon, see plasmoid. For the computer user interface, see KDE Plasma Workspaces.

A plasmid is a small DNA molecule that is physically separate from, and can replicate independently of, chromosomal DNA within a cell. Most commonly found as small circular, double-stranded DNA molecules in bacteria, plasmids are sometimes present in archaea and eukaryotic organisms. In nature, plasmids carry genes that may benefit survival of the organism (e.g. antibiotic resistance), and can frequently be transmitted from one bacterium to another (even of another species) via horizontal gene transfer. While the chromosomes are big and contain all the essential information for living (an adequate analogy is the hard-drive of a computer), plasmids usually are very small and contain additional information (in this analogy, plasmids are the USB flash drives). Artificial plasmids are widely used as vectors in molecular cloning, serving to drive the replication of recombinant DNA sequences within host organisms.

The term plasmid was first introduced by the American molecular biologist Joshua Lederberg in 1952.[1] Plasmid sizes vary from 1 to over 1,000 kbp.[2] The number of identical plasmids in a single cell can range anywhere from one to thousands under some circumstances. Plasmids can be found in all three major domains: Archaea, Bacteria, and Eukarya.[3]

Plasmids are considered replicons, capable of replicating autonomously within a suitable host. However, plasmids, like viruses, are not considered by some to be a form of life.[4] Plasmids can be transferred between bacterial hosts through a process known as bacterial conjugation. Because conjugation is a mechanism of horizontal gene transfer, plasmids can be considered part of the mobilome. Unlike viruses (which encase their genetic material in a protective protein coat called a capsid), plasmids are "naked" DNA and do not encode genes necessary to encase the genetic material for transfer to a new host. However, some classes of plasmids encode the conjugative "sex" pilus necessary for their own transfer.

Plasmid host-to-host transfer can also require changes in incipient host gene expression allowing the intentional uptake of the genetic element by transformation. Microbial transformation with plasmid DNA is neither parasitic nor mutualistic in nature, because each implies the presence of an independent species living in a detrimental or commensal state with the host organism. Rather, plasmids provide a mechanism for horizontal gene transfer within a population of microbes and typically provide a selective advantage under a given environmental state. Plasmids may carry genes that provide resistance to naturally occurring antibiotics in a competitive environmental niche, or the proteins produced may act as toxins under similar circumstances. Plasmids can also provide bacteria with the ability to fix nitrogen or to degrade recalcitrant organic compounds that provide an advantage when nutrients are scarce.[3]


Further information: Vector (molecular biology)

Plasmids used in genetic engineering are called vectors. Plasmids serve as important tools in genetics and biotechnology labs, where they are commonly used to multiply (make many copies of) or express particular genes.[5] Many plasmids are commercially available for such uses. The gene to be replicated is inserted into copies of a plasmid containing genes that make cells resistant to particular antibiotics and a multiple cloning site (MCS, or polylinker), which is a short region containing several commonly used restriction sites allowing the easy insertion of DNA fragments at this location. Next, the plasmids are inserted into bacteria by a process called transformation. Then, the bacteria are exposed to the particular antibiotics. Only bacteria that take up copies of the plasmid survive, since the plasmid makes them resistant. In particular, the protecting genes are expressed (used to make a protein) and the expressed protein breaks down the antibiotics. In this way, the antibiotics act as a filter to select only the modified bacteria. Now these bacteria can be grown in large amounts, harvested, and lysed (often using the alkaline lysis method) to isolate the plasmid of interest.


A plasmid can contain inserts of up to 30-40 kbp. To clone longer lengths of DNA, lambda phage with lysogeny genes deleted, cosmids, bacterial artificial chromosomes, or yeast artificial chromosomes are used.


Disease models[edit]


Gene therapy[edit]





Overview of bacterial conjugation
Electron micrograph of a bacterial DNA plasmid (chromosome fragment).


It is possible for plasmids of different types to coexist in a single cell. Several different plasmids have been found in E. coli. However, related plasmids are often incompatible, in the sense that only one of them survives in the cell line, due to the regulation of vital plasmid functions. Thus, plasmids can be assigned into incompatibility groups.


  • Virulence plasmids, which turn the bacterium into a pathogen.

Plasmids can belong to more than one of these functional groups.


Plasmid maintenance[edit]

Main article: addiction module



Yeast plasmids[edit]

Yeast are organisms that naturally harbour plasmids. Notable plasmids are 2 µm plasmids - small circular plasmids often used for genetic engineering of yeast, and linear pGKL plasmids from kluyveromyces lactis, that are responsible for killer phenotypes.[9]



Plasmid DNA extraction[edit]


There are several methods to isolate plasmid DNA from bacteria, the archetypes of which are the miniprep and the maxiprep/bulkprep.[5] The former can be used to quickly find out whether the plasmid is correct in any of several bacterial clones. The yield is a small amount of impure plasmid DNA, which is sufficient for analysis by restriction digest and for some cloning techniques.





  • Nicked open-circular DNA has one strand cut.




Simulation of plasmids[edit]

The use of plasmids as a technique in molecular biology is supported by bioinformatics software. These programs record the DNA sequence of plasmid vectors, help to predict cut sites of restriction enzymes, and to plan manipulations. Examples of software packages that handle plasmid maps are LabGenius,[10] pDraw32, Geneious, Lasergene, GeneConstructionKit, ApE, Clone Manager, VectorFriends, and Vector NTI. These software help conduct entire experiments in silico before doing wet experiments [1].

See also[edit]


  2. ^ Thomas, Christopher M; Summers, David (2008). Bacterial Plasmids. doi:10.1002/9780470015902.a0000468.pub2. 
  4. ^ Sinkovics, J; Harvath J; Horak A. (1998). "The Origin and evolution of viruses (a review)". Acta microbiologica et immunologica Hungarica (St. Joseph's Hospital, Department of Medicine, University of South Florida College of Medicine, Tampa, FL, USA.: Akademiai Kiado) 45 (3–4): 349–90. ISSN 1217-8950. PMID 9873943. 
  6. ^ Kandavelou K; Chandrasegaran S (2008). "Plasmids for Gene Therapy". Plasmids: Current Research and Future Trends. Caister Academic Press. ISBN 978-1-904455-35-6. 
  9. ^ Gunge, N; Murata, K; Sakaguchi, K (July 1982). "Transformation of Saccharomyces cerevisiae with linear DNA killer plasmids from Kluyveromyces lactis". Journal of bacteriology 151 (1): 462–4. PMC 220260. PMID 7045080. 
  10. ^ Cormier, Catherine. "PSI: Biology in the Spotlight". Retrieved 5 November 2012. 

Further reading[edit]

  • Albert G. Moat, John W. Foster, Michael P. Spector (2002). Microbial Physiology. Wiley-Liss. ISBN 0-471-39483-1. 


External links[edit]