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Plasmids are typically circular DNA molecules that range from one to many hundreds of kilobase pairs (kbps) in lenght. By definition they are not essential -- if they were essential of they would be considered chromosomes. Plasmids contain the genetic elements required for their propagation in the cell. They could almost be considered molecular parasites (or symbionts). |
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There are low and high copy plasmids. As you might expect, they differ in the number of copies present per cell. There can be as many as 20 or more copies of a high copy plasmid in a cell, and as few as 1 copy of a low copy plasmid. In addition to the genetic elements required for their own propagation, plasmids can carry genes that enhance the survival of their host bacterium.
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| Plasmids can even carry genes
that will kill the host bacteria if the plasmid is lost!
Plasmids can carry the genes responsible for antibiotic resistance and can be picked up by cells through a process known as transformation. This enables genetic traits to be transferred between unrelated organisms -- a process known as horizontal gene transfer. (The fact that you share genes with your parents and your children is an example of vertical gene transfer). The flexibility of plasmids has made them an essential tool in modern biological research. Scientists have re-engineered plasmids to force bacterial cells to make specific proteins. |
| The evolutionary relatedness of all organisms is reflected in the fact that genes from organisms as distinct as potatos and whales can direct the synthesis of the 'correct' proteins bacterial and archeal cells (an amazing fact, if you think about it!). |
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