Properties of Plasmids

• Circular DNA elements, always double-stranded DNA, Supercoiled
• Can occur in as few as 1 copy per cell (single copy plasmids) to as many as several dozen (multicopy plasmids).
  
  
  
• Variable sizes; small plasmids about 0.1% size of host chromosome, large plasmids can be as much as 10% the size of host chromosome. Smaller plasmids have few genes (30 or less). Size ranges from 1000 bp (1 kbp) to 1000 kbp.
• Ubiquitous; almost all cells isolated in nature carry plasmids, often more than one kind. (In E. coli alone, more than 300 different plasmids isolated.)
• View EM of plasmid DNA
• Have a replicon (origin for DNA replication), number of copies per cell regulated. Large plasmids typically only 1-5 copies/cell (stringent control); small plasmids ~10-50 copies/cell (relaxed control)
• Many plasmids are incompatible; if one is present, cell cannot support another plasmid of same compatibility group.
• Not essential to cell under all circumstances; can be "cured" by agents that impair DNA replication ----> cured cell lacking plasmid. Can be spontaneously lost over time unless some selection makes plasmid valuable to cell.
• Extend range of environments in which a cell can live (e.g., by degrading antibiotics, or providing enzymes for digestion of novel catabolites).