In many cases, extrachromosomal DNA is contained in cytoplasmic organelles, such as mitochondria (animals), and in chloroplasts and plastids (plants). Extrachromosomal DNA can also be found in the plasmids of bacteria, or in intracellular parasites such as viruses.
Certain types of cells (such as cancer cells and cells maintained in artificial culture) are known to acquire extrachromosomal DNA which provides them a selective advantage for growth under specific conditions. One example being the extrachromosomal double minutes which have been shown to contain oncogenes and provide growth enhancement in certain cancers. Extrachromosomal DNA sometimes contains addiction modules, a system that mediates cell death.