Differentiate between cosmids and phagemids.
Cosmids are hybrid vectors that replicate like a plasmid but can be packaged in vitro into lambda phage coats. They contain replication functions, restriction sites, and selective markers from plasmid DNA, combined with a λ DNA segment having cos sites. Cosmids can accommodate DNA inserts as large as 45 kbp.
Phagemids are true hybrid vectors between phage and plasmid. They are linear duplex DNAs whose ends are lambda phage DNA containing all genes required for lytic infection, while the middle region is a linearised plasmid. Both lambda phage and plasmid replication functions remain intact in phagemids.
The key difference is that cosmids have only cos sites from lambda (about 250 bp), while phagemids contain complete lambda genes for lytic infection and can replicate like a phage inside E. coli cells.
Explanation
The textbook clearly distinguishes between these two hybrid vectors. Cosmids (Section 2.3.3) are essentially plasmids with added cos sites from lambda phage, allowing them to be packaged into phage coats while replicating as plasmids. They are designed to carry large inserts (up to 45 kbp). Phagemids (Section 2.3.4) are more complex - they are linear DNA molecules with lambda phage DNA at both ends (containing all lytic infection genes) and a plasmid region in the middle. This structural difference is fundamental: cosmids are plasmid-based with minimal lambda sequences, while phagemids have complete lambda genes enabling phage-like replication.