The lambda bacteriophage is a bacterial virus that infects E. coli. It consists of a head (capsid) and a tail. The head contains the phage genome, which is a linear double stranded DNA of 48,490 base pairs. Each end has a 12 bases single stranded segment called cos sequences (cohesive ends), which are complementary to each other. Inside the host cell, these cos sequences pair up and are ligated to form a circular genome of 48,502 base pairs. The genome contains a replication origin site, genes for viral head and tail proteins, and enzymes involved in lytic and lysogenic cycles.

The lambda phage has been widely used as a cloning vector. Bacteriophage vectors are more efficient than plasmids for cloning large inserts, and screening bacteriophage plaques is easier than screening bacterial colonies. The middle region of the lambda genome (about one-third) is dispensable for successful lytic infection. This non-essential region is replaced with an insert segment. The recombinant DNA size must be between 38 to 52 kbp for efficient packaging into the λ head.

Lambda vectors fall into two classes: 'insertion vectors' with a single restriction enzyme site for insert DNA, and 'replacement vectors' with a pair of cloning sites flanking a non-essential segment.

Cosmids are hybrid vectors combining plasmid features with lambda cos sites, allowing plasmid replication with phage packaging capability.