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.

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Difference between plasmid and cosmid