Click on the image to see a larger version. Selected References These references are in PubMed. This may not be the complete list of references from this article. Recircularization and autonomous replication of a sheared R-factor DNA segment in Escherichia coli transformants. DNA nucleotide sequence restricted by the RI endonuclease. Recognition sequence of a restriction enzyme.
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See Article History Plasmid, in microbiology, an extrachromosomal genetic element that occurs in many bacterial strains. Plasmids are circular deoxyribonucleic acid DNA molecules that replicate independently of the bacterial chromosome. They are not essential for the bacterium but may confer a selective advantage. One class of plasmids, colicinogenic or Col factors, determines the production of proteins called colicins, which have antibiotic activity and can kill other bacteria.
Another class of plasmids, R factors, confers upon bacteria resistance to antibiotics. Some Col factors and R factors can transfer themselves from one cell to another and thus are capable of spreading rapidly through a bacterial population. A plasmid that is attached to the cell membrane or integrated into the bacterial chromosome is called an episome q.
Plasmids are extremely valuable tools in the fields of molecular biology and genetics, specifically in the area of genetic engineering q. They play a critical role in such procedures as gene cloning, recombinant protein production e. In such procedures, a plasmid is cut at a specific site or sites using enzymes called restriction endonucleases.
A foreign DNA element such as the gene for insulin is then spliced into the plasmid. The resulting circular structure, a recombinant DNA molecule, is then introduced into bacterial cells a process called transformation. The autonomous replication of the plasmid within the bacterial cells makes it possible to produce large numbers of copies of the recombinant DNA molecule for experimental manipulation or commercial purposes such as the production of large amounts of insulin.
Plasmids are well suited to genetic engineering in other ways. Their antibiotic resistance genes, for example, prove useful in identifying those bacterial cells that have taken up the recombinant DNA molecule in a high background of untransformed cells transformation frequencies are only about 1 out of every , cells. Learn More in these related Britannica articles:.
Purificación del plásmido
New York: W. Freeman ; Search term Section 7. In the case of DNA , this is feasible for relatively short molecules such as the genomes of small viruses. But genomes of even the simplest cells are much too large to directly analyze in detail at the molecular level. The problem is compounded for complex organisms. Cleavage of human DNA with restriction enzymes that produce about one cut for every base pairs yields some 2 million fragments, far too many to separate from each other directly.
Do you like this video? Plasmids are powerful, but excessive use of them leads to physical and mental addiction and instability. This was a major factor in the eventual downfall of Rapture society. This led to tensions growing in the city.
History[ edit ] The term plasmid was introduced in by the American molecular biologist Joshua Lederberg to refer to "any extrachromosomal hereditary determinant. In order for plasmids to replicate independently within a cell, they must possess a stretch of DNA that can act as an origin of replication. The self-replicating unit, in this case, the plasmid, is called a replicon. A typical bacterial replicon may consist of a number of elements, such as the gene for plasmid-specific replication initiation protein Rep , repeating units called iterons , DnaA boxes, and an adjacent AT-rich region.