Rownd, Robert H. Department of Molecular Biology, Northwestern University Medical School, Chicago, Illinois.
Last reviewed:January 2021
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- Plasmid incompatibility
- R plasmids
- Insertion sequences and transposable elements
- Rearrangements and evolution
- Genetic engineering
- Related Primary Literature
- Additional Reading
A circular extrachromosomal genetic element that is ubiquitous in prokaryotes and has also been identified in a number of eukaryotes. Plasmids are small, circular molecules of deoxyribonucleic acid (DNA) that differ from chromosomal DNA and are able to replicate autonomously. Plasmids occur in prokaryotes (being particularly well studied in bacteria; Fig. 1) and some eukaryotes (for example, certain fungi, including yeasts). In general, bacterial plasmids can be classified into two groups on the basis of the number of genes and functions that they carry. The larger plasmids are DNA molecules of around 100 kilobase (kb) pairs, which is sufficient to code for approximately 100 genes. There is usually a small number of copies of these plasmids per host chromosome, so their replication must be precisely coordinated with the cell division cycle. Their replication genes are clustered within a small segment of the genome. Larger plasmids usually can mediate their own transfer from cell to cell by bacterial conjugation. The genes mediating transmissibility are clustered in a segment of approximately 25 kb. The other regions (around 70 kb) of these larger plasmids can accommodate as many as 70 additional genes, which determine a variety of properties (for example, genetic transfer, antibiotic resistance, sugar fermentation, resistance to agents that damage DNA, and toxin production). The plasmids in the second group are smaller in size, about 6–10 kb. These plasmids may harbor 6–10 genes and are usually present in multiple copies (10–20 per chromosome). Although the smaller plasmids do not carry transfer genes, many of them can be mobilized to recipient cells if a larger transmissible plasmid is present in the same host cell. See also: Bacteria; Bacterial genetics; Deoxyribonucleic acid (DNA); Gene; Genetics; Prokaryote
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