This is a type of toxin which is generally released in the environment to reduce competition for nutrition. An example of R plasmid is RP4 which is commonly present in Pseudomonas sp.Ĭol Plasmid: Col plasmids are responsible for the production of colicin and commonly found in some strains of E. In clinical microbiology, R plasmid plays an important role as it is the main reason behind the development of antibiotic resistance in bacterial pathogens. R plasmid or Resistance: This type of plasmids contains one of the multiple antibiotic resistance genes.
The five groups are the following:į plasmid or Fertility: F plasmid contains specifically the tra gene and facilitates the transfer of plasmids by conjugation. the ligation of physically unlinked fragments.The naturally occurring plasmids are classified under five major groups on the basis of the genes present in the plasmids. This is usually done by partial restriction followed by either size fractionation or dephosphorylation (using calf-intestine phosphatase) to avoid chromosome scrambling, i.e. Target DNA: the genomic DNA to be cloned has to be cut into the appropriate size range of restriction fragments. Phage libraries are also stored and screened more easily than cosmid libraries. This compensates for their slightly lower loading capacity. These differences are reflected in the different infection frequencies seen in favour of lambda-replacement vectors. These extracts will recognize and package the recombinant molecules in vitro, generating either mature phage particles (lambda-based vectors) or recombinant plasmids contained in phage shells (cosmids). coli cI857 lysogens (red- gam- Sam and Dam (head assembly) and Eam (tail assembly) respectively). The necessary packaging extracts are derived from E. coli host via a technique called in vitro packaging. Also the clone density is much lower with around 10 5 – 10 6 CFU per µg of ligated DNA.Īfter the construction of recombinant lambda or cosmid libraries the total DNA is transferred into an appropriate E. Cosmids, therefore, always form colonies and not plaques. Selection against wild type cosmid DNA is simply done via size exclusion. The cloning procedure involves the generation of two vector arms which are then joined to the foreign DNA. The loading capacity of cosmids varies depending on the size of the vector itself but usually lies around 40–45 kb. Depending on the particular aim of the experiment, broad host range cosmids, shuttle cosmids or 'mammalian' cosmids (linked to SV40 oriV and mammalian selection markers) are available. Scheme of DNA cloning in a cosmid vector.Ĭosmids are predominantly plasmids with a bacterial oriV, an antibiotic selection marker and a cloning site, but they carry one, or more recently two, cos sites derived from bacteriophage lambda. Absence of inverted repeats was noted in the first Hohn & Collins publication cited above see also ). This instability can largely be counteracted by using a host bacterium with specific mutations affecting DNA recombination (N.B. Since there is a requirement for in vitro packaging whereby at least 38 kb of DNA is required between the cos sites, the vector without insert DNA will not be packaged (plasmids instability is increased if the novel inserted DNA contains many direct repeats or palindromic (inverted repeats) DNA. The hybrid cosmid DNA in the capsids can then be transferred into bacterial cells by transduction. Unlike plasmids, they can also be packaged in vitro into phage capsids, a step which requires cohesive ends, also known as cos sites also used in cloning with a lambda phage as a vector, however nearly all the lambda genes have been deleted with the exception of the cos sequence. Those cells which did not take up the cosmid would be unable to grow. They frequently also contain a gene for selection such as antibiotic resistance, so that the transformed cells can be identified by plating on a medium containing the antibiotic. They can replicate as plasmids if they have a suitable origin of replication (ori): for example SV40 ori in mammalian cells, ColE1 ori for double-stranded DNA replication, or f1 ori for single-stranded DNA replication in prokaryotes. Ĭosmids can contain 37 to 52 (normally 45) kb of DNA, limits based on the normal bacteriophage packaging size. They were first described by Collins and Hohn in 1978. Cosmids can be used to build genomic libraries. They are often used as a cloning vector in genetic engineering. ( Learn how and when to remove this template message)Ī cosmid is a type of hybrid plasmid that contains a Lambda phage cos sequence. JSTOR ( April 2014) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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