3.5 Cloning Vectors for E. coli are of three main types
1. Plasmids--circular DNA vectors that are self-replicating (separate from the host DNA). Plasmids are usually 5-40 kilobase pairs (Kbp) in size, and DNA fragments up to 15 Kbp can be inserted into the plasmids.
2. Bacteriophages--include modified bacteriophage lambda, which has 48.5 Kbp. About one-third of the genome is not essential for DNA replication and may be replaced with up to 23 Kbp of inserted DNA. The total length of the DNA must be of the correct size to be packaged into virus particles within the infected cells
3. Cosmids--a combination of plasmid and bacteriophage. Small (5-7 Kbp) circular DNA containing an origin for DNA replication (ori), markers and restriction sites plus a sequence from lambda needed for packaging the DNA (cos site). Cosmids may be used to clone large DNA molecules of up to 45 Kbp.