What are Cosmids?

Cosmids are hybrid plasmids that contain the cos sequence of a Lambda phage (bacteriophage). Structurally these are DNA hybrids of plasmids and a lambda phage.

Plasmids refer to small circular DNA in the cytoplasm of bacteria or protozoans. These genetic structures in a cell can replicate independently of the chromosomes. Typically, plasmids are used in the laboratory manipulation of genes.

In other words, a vector that contains the two cos (cohesive) ends of phage lambda (λ) and one or more selectable markers such as an antibiotic resistance gene of a plasmid is termed the cosmid.

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Collins, in the year 1978 discovered the first cosmid vector. Cosmid vectors are developed by combining the features of the plasmid vector and the bacteriophage vector. Origin of replication, multiple cloning sites, and selectable markers are obtained from the plasmid and only the cohesive site or cos site region is taken from lambda phage. These are fused together to obtain the cosmid vector.

The cos site in cosmid is the only requirement for DNA to be packaged into a phage particle. Cosmids were developed in light of this observation.

Advantage of Cosmids over Lambda Phage

Independent phage head portion can accept between 12 to 20 kb of insert DNA. However, cosmids exploit specific properties of phage lambda (λ) to enable large, 40-50 kb, DNA fragments to be cloned at high efficiency. Cosmids and cosmid recombinants replicate as plasmids.

During the cloning process using cosmid, only about 200 base pairs of lambda phage sequences are cloned into the cosmid vector.This consists of cosN, cosB, and cosQ.

A cosmid vector may have one or two cos sites. Cosmid vectors are used in the construction of genomic libraries. The cloning of a foreign DNA in a cosmid vector involves the following steps:
  • Ligation of the foreign DNA between two cos sites;
  • making a concatemeric DNA;
  • in vitro packaging to introduce the DNA into the phage head to form the matured phage particle; and
  • introduction of the cloned DNA into E. coli by transduction.

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  1. A cosmid is a DNA fragment that is a cosmid vector.

    It is a DNA vector used in genetic engineering. The cosmid vector is a linearised plasmid DNA isolated from a circular plasmid. It is capable of self-replication in E. coli, a single-stranded circular molecule of double-stranded DNA. The cosmid vector contains a single replication origin, a bacterial promoter, and a selectable marker gene, and is designed to incorporate foreign DNA.

  2. A cosmid is a circular piece of DNA that is made up of a smaller circle of DNA and a larger circle of DNA. A cosmid is used to make a library of DNA cloned into a larger circle of DNA. The smaller circle of DNA is known as the \”cos site\”.

    The DNA that is cloned into the cosmid is called the insert. The cosmid is treated with restriction enzymes and a ligase. The enzymes and ligase allow the DNA to be cut at the cos site and then ligated to the insert. This is why the insert is also called a DNA insert. The insert can be any kind of piece of DNA that is cloned into the cosmid, it does not have to be DNA that is artificially made.

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