COSMIDS

These are the vectors which can accommodate DNA segments up to 45 kbp . 

These are actually plasmid particles to which specific DNA sequences, namely those for the cos sites are inserted .

Since the cos sites enable the DNA to be packed in the phage particle so we can say that cosmids allow the packaging of DNA in phage in vitro thus permitting their purification .

Like plasmids these cosmids these cosmids also perpetuate in bacteria .

The advantage of cosmids for cloning is that its efficiency is high enough to produce a complete genome library of 10 to the power 6 - 10 to the power 7 clones from a mere 1gm of insert DNA . 

The disadvantage is its inability to accept more then 40-50 kbp of DNA.