Phage A is an effective cloning vector

have the following components: (1) a plasmid origin of replication (ori); (2) one or more unique restriction sites; (3) one or more selectable markers; and (4) cos sites to allow the packaging of DNA into phage heads.

Foreign DNA is inserted into cosmids in the same way that DNA is introduced into plasmids: the cosmid and foreign DNA are both cut by a restriction enzyme that produces complementary (sticky) ends, and they are joined by DNA ligase. Recombinant cosmids are incorporated into the coats, and the phage particles are used to infect bacterial cells, where the cosmid replicates as a plasmid. Table 18.3 compares the properties of plasmids, phage X vectors, and cosmids.

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