Cosmid Packaging and Infection of E. coli
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Cosmids are cloning vectors that were developed to enable large fragments of DNA to be cloned and maintained (
1
–
3
). Cosmid vectors allow the cloning of fragments up to 45 kilobases (kb) and are commonly used in genomic library construction. The distinguishing feature of a cosmid is the presence of bacteriophage λ
cos
sites, which enable the vector, and cloned genomic DNA, to be packaged into bacteriophage heads (
1
,
4
). Cosmids also contain several other components, including a drug-resistance marker that is necessary for the selection of cosmid-containing bacteria, a plasmid origin of replication, usually ColE1, which regulates cosmid replication and copy number within the
Escherichia coli
host (
1
,
5
), and restriction sites that facilitate cloning of the desired DNA fragments and insert verification by restriction digestion (
see
Fig. 1 ).
Fig. 1.
Representation of a general cosmid vector.
