Navigation Links
Plasmid


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

Plasmids are (typically) circular double-stranded DNA molecules that are separate from the chromosomal DNA (Fig. 1). They usually occur in bacteria, sometimes in eukaryotic organisms (e.g., the 2-micrometre-ring in Saccharomyces cerevisiae). Their size varies from 1 to over 400 kilo base pairs (kbp). There are from one copy, for large plasmids, to hundreds of copies of the same plasmid present in a single cell.

Contents

Antibiotic resistance

Plasmid often contain genes or gene-cassettes that confer a selective advantage to 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

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

Types of plasmid

One way of grouping plasmids is by their ability to transfer to other bacteria. Conjugative plasmids contain so-called tra-genes, which perform the complex process of conjugation, the sexual transfer of plasmids to another bacterium (Fig. 4). Non-conjugative plasmids are incapable of initiating conjugation, hence they can only be transferred with the assistance of conjugative plasmids, by 'accident'. An intermediate class of plasmids are mobilisable, and carry only a subset of the genes required for transfer. These plasmids can 'parasitise' another plasmid, transferring at high frequency in the presence of a conjugative plasmid.

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

It is possible for several different types of plasmids to coexist in a single cell, e.g., seven different plasmids have been found in E. coli. On the other hand, related plasmids are often 'incompatible', resulting in the loss of one of them from the cell line. Therefore, plasmids can be assigned into incompatibility groups, depending on their ability to coexist in a single cell. These incompatibility groupings are due to the regulation of vital plasmid functions.


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

  • Fertility-(F)plasmids, which contain tra-genes. They are capable of conjugation.
  • Resistance-(R)plasmids, which contain genes that can build a resistance against antibiotics or poisons. Historically known as R-factors, before the nature of plasmids was understood.
  • 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 can belong to more than one of these functional groups.


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. Such single-copy plasmids have systems which attempt to actively distribute a copy to both daughter cells.

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. This is an example of plasmids as selfish DNA.

Applications of plasmids

Plasmids serve as important tools in genetics and biochemistry labs, where they are commonly used to multiply (make many copies of) 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 . These plasmids contain, in addition to the inserted gene, one or more genes capable of providing antibiotic resistance to the bacteria that harbors them. The plasmids are next inserted into bacteria by a process called transformation, which are then grown on specific antibiotic(s). Bacteria which took up one or more copies of the plasmid then express (make protein) the gene that confers antibiotic resistance. This is typically a protein which can break down any antibiotics that would otherwise kill the cell. 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. Now these bacteria can be grown in large amounts, harvested and lysed to isolate the plasmid of interest.

Another major use of plasmids is to make large amounts of proteins. In this case you grow the bacteria containing a plasmid harboring the gene of interest. Just as the bacteria produces proteins to confer its antibiotic resistance, it can also be induced to produce large amounts of proteins from the inserted gene. This is a cheap and easy way of mass-producing a gene or the protein it then codes for--for example, insulin or even antibiotics.

See also


'"/>


(Date:10/22/2014)... discovery by Northwestern Medicine® scientists helps explain how bipolar ... to new drug therapies to treat the mental illness. ... -- the same method recognized with the 2014 ... into brain tissue from mice with bipolar-like behaviors. In ... discovered tiny "nanodomain" structures with concentrated levels of ANK3 ...
(Date:10/18/2014)... 2014 Psychological stress and stress-related psychiatric disorders are ... but the molecular mechanisms underlying this relation ... to the development of targeted preventive strategies ... devastating diseases. This work is presented at ... Berlin., Now an international group of researchers ...
(Date:10/17/2014)... Copenhagen have shown for the first time how bacteria ... patients, giving them the opportunity to get tremendous insights ... The study also discovered the bacterial growth in chronic ... or slowed down by the immune cells. The researchers ... helped "suffocate" the bacteria, forcing the bacteria to switch ...
Breaking Biology News(10 mins):Bipolar disorder discovery at the nano level 2Researchers find why depression and aging linked to increased disease risk 2Scientists opens black box on bacterial growth in cystic fibrosis lung infection 2
... biochemists have developed an approach that allows them to ... in a protein. The scientists were then able ... protein based on structural information, a novel outcome that ... Academy of Sciences. , Professor of biochemistry John Markley, ...
... are so crucial to survival that the brains of all ... long enough to allow the animal to eat, drink -- ... from researchers at the University of Chicago, published early online ... that by activating "OFF" cells and shutting down "ON" cells ...
... classic "Flowers for Algernon," but adds a happy ending, UCLA ... reverse the attention deficits linked to the leading genetic cause ... Biology reports the findings, which were studied in mice bred ... , The results proved so hopeful, that the Food and ...
Cached Biology News:Scientist uses form to explain function of key building blocks of life 2Brainstem blocks pain to protect key behaviors 2Brainstem blocks pain to protect key behaviors 3Recreating 'Flowers for Algernon' with a happy ending 2Recreating 'Flowers for Algernon' with a happy ending 3
Other biology definitionOther Tags