Plasmid vs Cosmid
We all know about chromosomal DNA and its structure. However, there is another DNA molecule called Plasmid that is known to microbiologists and those who study genetics. This is a molecule that can replicate itself without chromosomal DNA and can exist without it. Normally found in bacteria, they are also seen in eukaryotic organisms. They are different from bacterial DNA in the sense that they are circular and double stranded. Cosmids are special type of plasmids that can be packaged in vitro into λ phage coats. A cosmid can also replicate itself like a plasmid and used as gene cloning vector. However, there are differences between the two that will be talked about in this article.
Plasmids are vectors that are capable of replication when they find a suitable host. They were first discovered by American scientist Joshua Lederberg in 1952. Plasmids are considered similar to viruses in the sense that they are not a form of life. However, unlike viruses they are DNA without encoding genes that are necessary for transfer to a new host. They are made heavy use of in genetics and molecular biology and even used commercially. In creating rat genetic diseases in labs, plasmids are commonly used. They are also used for gene therapy.
Many different types of plasmids can exist in a single cell. Scientists have proved the presence of different plasmids in E. coli.
Cosmids are a type of plasmids that were discovered by scientists Collins and Hohn in 1978. in fact, they are hybrid plasmids that contain cos sequences. They are able to carry DNA sequences to the tune of 37-52 kb which is considerably higher than plasmids that can carry only 1-20kb DNA. They are different in the sense that they can be packaged in phage capsids. Plasmids become useless as they are prone to recombination after a certain amount of DNA has been inserted into them. This is why phage transduction through cos sites is used to prevent this problem.
Plasmid and Cosmid
• Plasmids can accept DNA inserts up to only 15 kb while cosmids are better as they can carry more DNA material (up to 50kb)
• Because of this ability, cosmids are considered more efficient for cloning larger pieces of foreign DNA. They are preferred vectors while constructing libraries of genome fragments.