Rownd, Robert H. Department of Molecular Biology, Northwestern University Medical School, Chicago, Illinois.
- Plasmid incompatibility
- R plasmids
- Insertion sequences and transposable elements
- Rearrangements and evolution
- Genetic engineering
- Links to Primary Literature
- Additional Readings
A circular extrachromosomal genetic element that is ubiquitous in prokaryotes and has also been identified in a number of eukaryotes. In general, bacterial plasmids can be classified into two groups on the basis of the number of genes and functions they carry. The larger plasmids are deoxyribonucleic acid (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 that 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 (see table). 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.
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