Difference between Cosmid and Lambda vectors

The DNA that carries the desired genes to the host cell is called gene cloning vector. It is also known as cloning vector, vector, cloning vehicle or carrier DNA. Plasmids, viral DNAs and cosmids are used as gene cloning vectors.

 Cosmid is a derived vector. Cosmid was first constructed by Collins and Hohn in 1978. Cosmid is an artificial plasmid containing 'cos'  sites of lambda (λ) DNA. It is formed by joining ends of a linearlized plasmid DNA with ‘cos’ sites of a lambda  (λ) DNA.  It has an origin of replication, selectable markers and gene cloning sites of the plasmid DNA. They lack structural and regulatory genes of λ DNA. Hence, there is no lysis and integration  of cosmid DNA in the host cell.

Example: Col EI cosmid, pHC 79, pJB8, pWE cosmid etc.

Lambda Phage DNA can carry DNA fragments and integrate into chromosomal DNA of host cell. Therefore, it is used as a vector for gene cloning. The constructed lambda DNA vectors are of two types:

• Insertation vectors: Target DNA is inserted into the vector.
• Replacement vectors: Target DNA replaces a piece of lambda cloning vector

Lambda Phage DNA vs Cosmid

Cosmid Vectors	Lambda vectors
Insert size is 45-50 kb	Insert size is 20-25 kb
Less clones required for complete library.	More clones required for complete library.
Cosmid  libraries are difficult to make	Libraries are easier to make.
Cosmid libraries are more difficult for screening and less efficient.	Libraries are simpler for screening and more efficient too.
Colony hybridisation technique is used.	Plaque hybridisation technique is used.
Yield of recombinants per micro grams of starting starting DNA is lower with cosmid vector.	Yield of recombinants per mg of starting DNA is higher with lambda vectors.
With cosmid vectors less steps are required to walk along the genome.	With lamda vectors more steps are required to walk along the genome.
Long eukaryotic genes can be included in a single DNA fragment that is cloned.	Long eukaryotic gene may not be included in a single DNA fragment that is cloned.
Cosmid libraries are maintained in form of  transformed bacteria. If each bacterium population carries the recombinant cosmid, 3,50,000 transformants are to be generated.	Lambda stock consisting library  of recombinant  clones in total of DNA. It is concentrated by CsCl density gradient. Maintenance of phage socks is tedious.