ABSTRACT
https://www.niso.org/standards/z39-96/ns/oasis-exchange/table"> Structure of plasmids A plasmid is defined as an entire molecule of DNA that is replicated independently of the host chromosome. When over 100 kbp or more, they are known as megaplasmids. Removal of a plasmid is known as curing. There are more than 300 different types of naturally occurring plasmids for Escherichia coli alone, some of which have been adapted for applications in recombinant DNA technology. Replication of plasmids The plasmids of E. coli are complete, closed, circular (ccc) double-stranded DNA molecules. Plasmids are replicated by the host cell machinery. Episomal plasmids integrate themselves into the chromosome and are replicated along with it. The number of plasmids per cell is known as the copy number, and this can vary between very low (stringent plasmids) with only one or two copies per cell, and very high (relaxed plasmids) with hundreds of copies per cell. As maintenance of a plasmid in a cell is energetically expensive, the cell will try to delete the plasmid. Thus to maintain a plasmid in a strain, some form of selective pressure is applied to ensure that as many cells as possible keep the plasmid. Plasmid incompatibility In common with the bacteriophages and other viruses, some plasmids have evolved mechanisms to repel reinfection by different types of plasmid. When this occurs, the two plasmids are said to be incompatible. Plasmids can be grouped according to their compability. Function of plasmids The best known of the E. coli plasmids is the F’ plasmid. The genes that govern transfer are known as tra genes and code for proteins with a variety of functions including enablement of the movement of DNA and the rolling circle replication associated with it. Other plasmids carry genes that confer heavy metal resistance, entire metabolic pathways, toxins, virulence factors or antibiotics. There is transfer of plasmids between members of the same species, but promiscuous plasmids maybe transferred between cells of different species. Vectors for recombinant DNA technology The plasmids used in molecular biology have been engineered to remove most of the unfavorable aspects of the wild typ e. Most are small (<5 kbp) so that they are easy to manipulate in vitro and are less likely to recombine with the host chromosome. The tra genes are absent so that the recombinant DNA will be contained in the designated host cell. >Recombinant DNA Recombinant strains of E. coli have been developed that can harbor foreign DNA but cannot survive outside he laboratory. Adapted E. coli plasmid cloning vectors maintain foreign DNA in an easily accessible form, while xpression vectors are plasmids used to generate large amounts of recombinant protein. Shuttle vectors allow the sy creation of specific arrangements of genes or gene fragments in E. coli before the construct is transferred toanother organism. Part of the attraction of E. coli is the ease with which genomic and plasmid DNA can be isolad. Plasmids can be isolated from lysed E. coli by techniques such as cesium chloride density centrifugation or bythe use of the many commercially available spin columns. Related topics
(F8) Transfer of DNA between cells
(F10) Bacteriophages
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