A cosmid, first described by Collins and Hohn in 1978, is a type of hybrid plasmid (often used as a cloning vector) that contains cos sequences, DNA sequences originally from the Lambda phage. Cosmids can be used to build genomic libraries.

Cosmids are able to contain 37 to 52 kb of DNA, while normal plasmids are able to carry only 1–20 kb. They can replicate as plasmids if they have a suitable origin of replication: for example SV40 ori in mammalian cells, ColE1 ori for double-stranded DNA replication or f1 ori for single-stranded DNA replication in prokaryotes. They frequently also contain a gene for selection such as antibiotic resistance, so that the transfected cells can be identified by plating on a medium containing the antibiotic. Those cells which did not take up the cosmid would be unable to grow.

Unlike plasmids, they can also be packaged in phage capsids, which allows the foreign genes to be transferred into or between cells by transduction. Plasmids become unstable after a certain amount of DNA has been inserted into them, because their increased size is more conducive to recombination. To circumvent this, phage transduction is used instead. This is made possible by the cohesive ends, also known as cos sites. In this way, they are similar to using the lambda phage as a vector, but only that all the lambda genes have been deleted with the exception of the cos sequence.

Cos sequences are ~200 base pairs long and essential for packaging. They contain a cosN site where DNA is nicked at each strand, 12bp apart, by terminase. This causes linearization of the circular cosmid with two "cohesive" or "sticky ends" of 12bp. (The DNA must be linear to fit into a phage head.) The cosB site holds the terminase while it is nicking and separating the strands. The cosQ site of next cosmid (as rolling circle replication often results in linear concatemers) is held by the terminase after the previous cosmid has been packaged, to prevent degradation by cellular DNases.Picture

Cosmid features and uses

Cosmids 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. 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. The loading capacity of cosmids varies depending on the size of the vector itself but usually lies around 40–45 kb. The cloning procedure involves the generation of two vector arms which are then joined to the foreign DNA. Selection against wildtype cosmid DNA is simply done via size exclusion. Cosmids, however, always form colonies and not plaques. Also the clone density is much lower with around 105 - 106 CFU per µg of ligated DNA.

After the construction of recombinant lambda or cosmid libraries the total DNA is transferred into an appropriate E.coli host via a technique called in vitro packaging. The necessary packaging extracts are derived from E.coli cI857 lysogens (red- gam- Sam and Dam (head assembly) and Eam (tail assembly) respectively). 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). These differences are reflected in the different infection frequencies seen in favour of lambda-replacement vectors. This compensates for their slightly lower loading capacity. Phage library are also stored and screened easier than cosmid (colonies!) libraries.

Target DNA: the genomic DNA to be cloned has to be cut into the appropriate size range of restriction fragments. This is usually done by partial restriction followed by either size fractionation or dephosphorylation (using calf-intestine phosphatase) to avoid chromosome scrambling, i.e. the ligation of physically unlinked fragments.


Eurekah Biosciences Collection: Viruses, @NCBI

External links

A cosmid

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Cosmid — DNA from a bacterial virus into which is spliced a small fragment of a genome to be amplified and sequenced. A cosmid is an artificially constructed structure. It is used in cloning (copying) pieces of DNA. (On a technical level, a cosmid… …   Medical dictionary

  • cosmid — cos·mid (kŏzʹmĭd) n. Genetics A hybrid vector that has been spliced with plasmid DNA for cloning large genes or gene fragments.   [Blend of cos (sequence), sequence of DNA allowing the cosmid to form as a circle (short for cohesive ends), and… …   Universalium

  • cosmid — kosmidė statusas T sritis augalininkystė apibrėžtis Plazmidė, skirta klonuoti ir dauginti dideliems DNR fragmentams. atitikmenys: angl. cosmid rus. космида …   Žemės ūkio augalų selekcijos ir sėklininkystės terminų žodynas

  • Cosmid — Als Cosmide bezeichnet man Plasmide, die sogenannte cos sites (cohesive site) enthalten. Dabei handelt es sich um DNA Sequenzen, die aus dem λ Phagen stammen. Diese Abschnitte liegen an den Enden des linearen Phagenchromosoms und sind jeweils von …   Deutsch Wikipedia

  • cosmid — (koz mid) A plasmid vector with lambda phage cos sites that can be packaged in a phage capsid; it is useful for cloning large DNA fragments …   Dictionary of microbiology

  • cosmid — A type of bacteriophage lambda vector. Often used for construction of genomic libraries, because of their ability to carry relatively long pieces of DNA insert, compared with plasmids …   Dictionary of molecular biology

  • cosmid — noun ˈkɑzmɪd A type of plasmid (often used as a cloning vector) constructed by the insertion of cos sequences, which are DNA sequences of the lambda phage …   Wiktionary

  • Cosmid — Cos|mid 〈n.; Gen.: s, Pl.: e; Biochemie〉 künstlich hergestellter Klonierungsträger, der das zu klonierende DNA Segment enthält u. in Plasmide von Bakterienwirten eingebaut werden kann [Etym.: <cos Gen + Plasmid] …   Lexikalische Deutsches Wörterbuch

  • cosmid — A synthetic plasmid which incorporates the cos ends, and one or more selectable markers such as an antibiotic resistance gene. Cosmids were designed as vectors able to incorporate DNA fragments up to 40 50 kb in size …   Glossary of Biotechnology

  • cosmid — noun (genetics) a large vector that is made from a bacteriophage and used to clone genes or gene fragments • Topics: ↑genetics, ↑genetic science • Hypernyms: ↑vector …   Useful english dictionary