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20-September-2008 09:29:09 - Cosmid 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 kbp of DNA, while normal plasmids are able to carry only 1-20 kbp. 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. However, 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 Because of the fixed size of the phage head, terminase can only package cosmids that are between 75% and 105% of the length of the normal phage. Thus the practical upper limit of the insert size is around 40kb, since there will also need to be origins of replication, selection genes and multiple cloning sites. To package even more DNA into a vector, bacterial artificial chromosomes or yeast artificial chromosomes can be used. References Bruce A. Voyles 2002 The biology of viruses 2nd ed. ISBN 0-07-237031-9 Stryer, Lubert 1995 Biochemistry 4th ed. ISBN 0-7167-2009-4 Eurekah Biosciences Collection: Viruses, @NCBI External links MeSH Cosmid v d e Types of nucleic acids Constituents Nucleobases | Nucleosides | Nucleotides | Deoxynucleotides Ribonucleic acids RNA | mRNA pre-mRNA/hnRNA | tRNA | rRNA | aRNA | gRNA | miRNA | ncRNA | piRNA | shRNA | siRNA | snRNA | snoRNA | stRNA | ta-siRNA | tmRNA Deoxyribonucleic acids DNA | cDNA | gDNA | msDNA | mtDNA Nucleic acid analogues GNA | LNA | PNA | TNA | morpholino Cloning vectors phagemid | plasmid | lambda phage | cosmid | P1 phage | fosmid | BAC | YAC | HAC Major families of biochemicals Saccharides | Carbohydrates | Glycosides | | Amino acids | Peptides | Proteins | Glycoproteins | | Lipids | Terpenes | Steroids | Carotenoids Alkaloids | Nucleobases | Nucleic acids | | Enzyme cofactors | Flavonoids | Polyketides | Tetrapyrroles This biotechnology article is a stub. This genetics article is a stub. Retrieved from http://en..org/wiki/Cosmid Categories: Molecular biology | Laboratory techniques | Biotechnology stubs | Genetics stubs Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Català Deutsch Español Français Italiano 日本語 Polski Tiếng Việt 中文 This page was last modified on 11 June 2008, at 20:01
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