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Plasmid

Plasmids are circular DNA molecules that are separate from the chromosomal DNA (Fig. 1). They usually occur in bacteria, sometimes in eukaryotic organisms (e.g., the 2-micron-ring in Saccharomyces cerevisiae). Their size varies from 1 to 250 kbp or thousand base pairs. There are from one copy, for large plasmids, to fifty copies of the same plasmid present in a single cell.

Figure 1 : Schematic drawing of a bacterium with plasmids enclosed. (1) Chromosomal DNA. (2) Plasmids.

Plasmids usually contain one or two genes that confer a selective advantage on the bacterium harboring them, e.g., the ability to build an antibiotic resistance. Every plasmid contains at least one DNA sequence that serves as an origin of replication[?] or ori (a starting point for DNA replication), which enables the plasmid DNA to be duplicated independently from the chromosomal DNA (Fig. 2).

Figure 2 : Schematic drawing of a plasmid with antibiotic resistances (1&2) and an ori(3).

Episomes are plasmids that can integrate themselves into the chromosomal DNA of the host organism (Fig. 3). For this reason, they can stay intact for a long time, be duplicated with every cell division of the host, and become a basic part of its genetic makeup.

Figure 3 : Comparison of non-integrating plasmids (top) and episomes (bottom).
  1. Chromosomal DNA.
  2. Plasmids.
  3. Cell division.
  4. Chromosomal DNA with integrated plasmids.

There are two basic groups of plasmids, conjugative and non-conjugative. Conjugative plasmids contain a so-called tra-gene, which can initiate conjugation, the sexual exchange of plasmids, with another bacterium (Fig. 4). Non-conjugative plasmids are incapable of initiating conjugation, and therefore, their movement to another bacterium, but they can be transferred together with conjugative plasmids, during conjugation.

Figure 4 : Schematic drawing of bacterial conjugation.
  1. Chromosomal DNA.
  2. Plasmids.
  3. Pilus.

Several different types of plasmids can coexist in a single cell, e.g., up to seven in E. coli. Two plasmids can be incompatible, resulting in the destruction of one of them. Therefore, plasmids can be assigned into incompatability groups, depending on their ability to coexist in a single cell.

An obvious way of classifying plasmids is by function. There are five main classes:

  • Fertility-(F-)plasmids, which contain only tra-genes. Their only function is to initiate conjugation.
  • Resistance-(R-)plasmids, which contain genes that can build a resistance against antibiotics or poisons.
  • Col-plasmids, which contain genes that code for (determine the production of) colicines[?], proteins that can kill other bacteria.
  • Degrative plasmids, which enable the digestion of unusual substances, e.g., toluole[?] or salicylic acid.
  • Virulence plasmids, which turn the bacterium into a pathogen.

Plasmids that exist only as a single copy in each bacterium are, upon cell division, in danger of being lost in one of the segregating bacteria. To ensure that the cell has an "interest" in keeping a copy of the plasmid in each dividing cell, some plasmids include an addiction system. They produce both a long-lived poison and its short-lived antidote. The cell that keeps a copy of the plasmid will survive, while the cell without the plasmid will die because it is running out of antidote shortly.

Plasmids serve as important tools in genetics and biochemical labs, where they are commonly used to multiply or express particular genes. There are many plasmids that are commercially available for such uses. Initially, the gene to be replicated is inserted in a plasmid. But, these plasmids contain, in addition to the inserted gene, one or more genes with antibiotic resistance. The plasmids are next inserted into bacteria, which are then grown on specific antibiotic(s). As a result, only the bacteria with antibiotic resistance can survive, the very same bacteria containing the genes to be replicated. The antibiotic(s) will, however, kill those bacteria that did not receive a plasmid, because they have no antibiotic resistance genes. In this way the antibiotic(s) acts as a filter selecting out only the modified bacteria. This is a cheap and easy way of mass-producing a gene or the protein it codes for--for example, insulin or even antibiotics.

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