Extrachromosomal genomes

Plasmids are also encountered in eukaryotes, although less commonly than in bacteria. Examples are the 2- x circle in yeast, the senescence (aging) plasmids in the fungus Neurospora and the plasmids associated with male sterility in corn. In addition, there are many other extrachromosomal genomes such as those from mitochondria, chloroplasts and viruses. In addition, colony hybridization type experiments can be carried out on single genes or on transcripts and may resemble in situ hybridization, except for the transfer to membranes. 

The existence of an extrachromosomal genome localized in mitochondria as well as machinery for transcription and translation in these cell organelles is now well established. There has been considerable speculation on the origin of mitochondria and the idea of a symbiosis between eukaryotic cells and prokaryotic organisms is favored by many scientists (e.g., Nass, 1971, Fridovich, 1974, but cf. Raff and Mahler, 1972). 

Nakai, H., Yant, S. R., Storm, T. A., Fuess, S., Meuse, L. and Kay, M. A. (2001). Extrachromosomal recombinant adeno-associated virus vector genomes are primarily responsible for stable liver transduction in vivo. J. Virol. 75, 6969-6976. 

If the host cells are bacteria, commonly nsed vectors are bacteriophage (viruses that infect bacteria), plasmids (extrachromosomal pieces of circnlar DNA that are taken up by bacteria), or cosmids (plasmids that contain DNA seqnences from the lambda phage). When eukaryotic cells are used as the host, the vectors are often retroviruses, adenoviruses, free DNA, or DNA coated with a lipid layer (liposomes). The foreign DNA sometimes integrates into the host cell genome or it exists as eip-somes (extrachromosomal fragments of DNA) (See section III.D. of this chapter.)... 

Casjens, S., Palmer, N van Vugt, R., Huang, W.M., Stevenson, B., Rosa, P Lathigra, R., Sutton, G Peterson, J., Dodson, R.J., et al. (2000) A bacterial genome in flux the twelve linear and nine circular extrachromosomal DNAs in an infectious isolate of the Lyme disease spirochete Borrelia burgdorferi. Mol. Microbiol. 35, 490-516. 

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