Genetically engineered vector containing phage and plasmid genetic elements which accepts inserts of larger length (average of 45 kb) than phage (15 kb) or plasmid (1–10 kb) derived vectors. The replication origin and antibiotic resistance gene come from plasmids. The cos sites required for packaging come from phages. Cosmids are packaged into capsids and are able to infect host cells, thus efficiently injecting the vector DNA which is then amplified and maintained as a giant plasmid in the host cell.
