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.