What is Cosmids?

Cosmids is a hybrid DNA formed by the joining of a plasmid and lambda phage DNA carrying a “cos site” in brief cosmid is a plasmid carrying the cos site of a l phage DNA. The cosmid is not naturally found in living cells. It is a “constructed vector”.

Example: Col E1 cosmid is a typical cosmid used in genetic engineering.

Construction of Cosmids:

col E1 cosmid is constructed from col E1 plasmid and l-phage DNA. The plasmid is cut with a restriction endonuclease enzyme, which removes a portion of DNA from the plasmid. The same restriction enzyme is used to cut the l-phage DNA to get a DNA fragment containing “Cos site”.

These two DNA fragments are mixed together in the presence of the enzyme DNA ligase, which link together the two DNA fragments end to end. The resulting recombinant plasmid is called “Col E1 cosmid”.

Characteristic features:

• The cosmid is a plasmid containing cos site.
• It is a circular double stranded DNA.
• H contains complementary single strand regions, the complementary single strand region is abbreviated as “Cos site”.
• The cos site consists of two complementary single strands held  together by complementary base pairing both these two strands.
• At the cos site, 3’-end of each of the DNA strands does not establish covalent bond with 5’-end of the same chain that is a definite nick is present in each of the two strands.
• The nicks are restrained in the cosmid for a number of generations.
• The cosmid DNA does not code for the synthesis of viral proteins.
• The cosmid does not participate in the multiplication of phage particles.
• The cosmid DNA packed with in the protein coat of bacteriophage. Thus the transformed virus particle is formed.

Advantages:

• The cosmids transfers a somewhat larger foreign gene into the bacterial cell.
• The cosmid pickup even long sized genes. Hence it is used in the genome of the organisms.
• The cosmids are also used in the study of some non-sense sequences found in the genome of the organisms.

Disadvantages:

• Each cosmid requires two cos sites for the successful packing of recombinant cosmid within the protein coat of bacteriophage. In the recombinant cosmids the packaging enzyme fails to pack the DNA into the protein coat of bacteriophage.
• The transfer of gene from the transformed cells is difficult the cosmids need additional work for this gene transfer.
• The package fails when the distance of separation exceeds 54,000 bps (or) when it is less than 38,000 bps.
• Main use is gene cloning for successful result.