Definition: A very common vector for inserting a new gene into cells is the plasmid. Plasmids are small circular pieces of DNA that many bacteria produce, much smaller than their genomes (the circular DNA that carries the bulk of genes they need to survive). Plasmids can be isolated from bacteria, using what are now routine gene cloning techniques. The plasmid is cut with restriction enzymes, and the new gene inserted through a process called ligation, which reattaches the ends of the circular DNA strand. Plasmids often have genes on them for antibiotic resistance, which can be used to select GM cells after transformation on specialized media, but other marker genes can also be used, such as GFP or firefly luciferase. The presence of the vector can also be detected by agarose gel electrophoresis or by probing.
In order for the gene tranfer to be successful, the new gene must be flanked on the plamid by a promoter region and a transcription terminator sequence. The most useful plasmids have strong promoters or those that are inducible, meaning expression of the gene can be controlled in some way, by adding an inducing agent to the cell culture media. Besides having a means of detecting the plasmid in cells, it's important to be able to tell if the inserted gene is being expressed, by protein purification methods, and if the gene product is viable (eg. by measuring the specific activity).
Also Known As: Vectors
Examples:
Transformation of the cells was performed using a plasmid carrying the new gene.