COSMIDS.pptx

COSMIDS.pptx

1. COSMIDS BP221518 A.Sathiyaraji M.Sc Applied Microbiology Sacred Heart College-TPT

2. The first cosmid vector was described by Collins in 1978. Cosmid vectors are developed by combining the features of the plasmid vector and the bacteriophage vector. Origin of replication, multiple cloning site and selectable marker 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. COSMIDS

3. Cosmid vectors are hybrid vectors composed of plasmid and phage λ vectors, capable of incorporating up to 42 kb of DNA. Cosmid vectors are prepared by the insertion of the cos region of the phage vector into the plasmid vectors. Cosmid vectors are large-sized vectors with sizes ranging from 400 base pairs to 30 kb. These can carry DNA sequences having sizes ranging from 28 to 46 kb. Cosmid vectors are created in order to incorporate large-sized DNA molecules that cannot be carried by plasmids . COSMIDS

4. Since these are hybrid vectors, these can replicate within the host cell like plasmids or remain packaged like a phage. Cosmid vectors do not have many phage characteristics except the signal sequences that promote phage-head stuffing. The hybrid structure of cosmid enables the phage heads to be incorporated within all donor DNA for transfer. The use and production of cosmid vectors have increased over the years as the packaged system is highly efficient and selective for the recovery of larger hybrids. One of the examples of the cosmid vectors prepared and used in practice are cosmid pHC79 which is a cos -containing derivative of the vector pBR322.

5. Structure of Cosmids

6. Independent phage head portion can accept between 12 to 20 kb of insert DNA. However, cosmids exploit certain 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 sequence are cloned into the cosmid vector .   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 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 . Advantage of Cosmid over Lambda Phage

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