Difference Between Cosmid and Phagemid

The key difference between Cosmid and Phagemid is on the type of sequences it contains. A Cosmid contains a cos site and a plasmid. Therefore, it is a hybrid vector while a Phagemid is a plasmid that contains an F1 origin of replication of the F1 phage.

Cosmids and Phagemids are used for cloning purposes, especially to clone larger fragments of DNA. These are widely used especially in cloning genes responsible for the production of various proteins. Cosmids and Phagemids are either replicated alone as plasmids or are packaged into viral particles and then replicated.

CONTENTS

1. Overview and Key Difference
2. What is a Cosmid
3. What is a Phagemid
4. Similarities Between Cosmid and Phagemid
5. Side by Side Comparison – Cosmid vs Phagemid in Tabular Form
6. Summary

What is a Cosmid?

A Cosmid, referred as a hybrid plasmid, comprises cos sites extracted from Lambda phage particles and a plasmid. These cos sites are long fragments of DNA with 200 basepairs. Hence, they have cohesive or sticky ends that allow the plasmid to fit into the viral DNA. Therefore, the cos sites are vital for packaging of the DNA.

There are three cos sites;

• cosN site – involved in nicking the DNA strand by terminase activity
• cosB site – engaged in holding the terminase.
• cosQ site – involved in preventing the degradation of the DNA by DNases.

Figure 01: Cosmid

Also, Cosmids can either replicate single-stranded DNA or double-stranded DNA using a suitable origin of replication. Cosmids usually contain antibiotic resistance genes as markers for selection of transformants. Thus, the use of a cosmid as a vector can facilitate the cloning, and the restriction enzyme digestion of the vector can then extract these fragments.

What is a Phagemid (Phasmid)?

Phagemid, also termed as Phasmid, is a type of hybrid vector too. Phagemid contains a special origin of replication termed as the f1 origin of replication. The f1 origin of replication extracts from an f1 phage.

Phagemid can replicate both single-stranded and double-stranded DNA. Replication can either take place as a plasmid while undergoing independent replication or can get packaged into phage particles and finally infect bacterial host E coli. When infecting the E coli cells, the f1 phage requires the presence of a pilus. Hence, the sex pili are important during in vitro packaging of Phagemids.

What are the Similarities Between Cosmid and Phagemid?

• Both Cosmid and Phagemid are cloning vectors used in recombinant DNA technology.
• Cosmid and Phagemid can replicate single-stranded and double-stranded DNA.
• Both can undergo independent replication similar to plasmids.
• Cosmid and Phagemid can undergo in vitro packaging and infect bacterial cells.
• Both Cosmid and Phagemid require a suitable origin of replication for cloning.

What is the Difference Between Cosmid and Phagemid?

Summary – Cosmid vs Phagemid

Cosmid and Phagemid are cloning vectors used in recombinant DNA technology. Cosmids are hybrid vectors that contain special sticky ends known as cos sites. in-vitro packaging require these cos sites. While phagemids are plasmids that contain an f1 origin of replication extracted from f1 phage. Both cosmid and phagemid can undergo independent replication or in vitro packaging to bacterial cells. These are the important differences between cosmid and phagemid.

Reference:

1.Qi, H, et al. “Phagemid Vectors for Phage Display: Properties, Characteristics and Construction.” Journal of Molecular Biology., U.S. National Library of Medicine, 30 Mar. 2012. Available here 
2.Collins, J, and B Hohn. “Cosmids: a Type of Plasmid Gene-Cloning Vector That Is Packageable in Vitro in Bacteriophage Lambda Heads.” Proceedings of the National Academy of Sciences of the United States of America, U.S. National Library of Medicine, Sept. 1978. Available here  

Image Courtesy:

1.’Cosmid (English)’By Zlir’a (CC0) via Commons Wikimedia

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