After isolation of the desired gene(s), a vector is required which can incorporate this gene and along with it enter in the host cell & replicate it’s DNA in it. This vector is called cloning vector. Plasmid, Bacteriophages and Cosmids are main cloning vectors used in recombinant DNA technology. 

Plasmids: 

• These are extrachromosomal components in the bacterial cell.
• The DNA is a circular and double-stranded molecule. 
• They contain an origin of replication site and can replicate independently of a bacterial chromosome. 
• They have specific restriction sites where the desired gene can be incorporated. 
• They have marker sites. 
• The plasmid may contain 3 to one thousand genes. 

Bacteriophage: 

• The viruses which infect bacterially and cause lysis of bacterial cell are called bacteriophages. Example: λ (Lambda) phage and M₁₃ phage etc. 
• Bacteriophages are a better vector as compared to plasmids. 
• Large DNA segments (24 Kbp) can be cloned in the bacteriophages.
• Each bacteriophage produces a plaque in the culture. Hence their identification is easy. 

Cosmid:

• This is a hybrid of plasmid and λ (Lambda) phage.
• These can replicate in the host cell just like a plasmid.
• Due to the presence of ‘Cos’ site, these are packed like phage particles.
• Cosmids can be used to clone the DNA segment of up to 45 Kbp.