Phagemid

A phagemid or phasmid in molecular biology, a plasmid carrying an origin of replication for single-stranded replication of phage f1.

Properties

Phagemids were developed in the early 1980s and used as a vector.

A phagemid can be normally propagated as a plasmid - as it shall bear an appropriate origin of replication - and then is double-stranded ago. Alternatively, one can multiply the result of the phagemid f1 ori with the aid of a helper phage. For this purpose, the bacterial culture must be infected with a helper phage, because only then in the host cell, the viral components necessary for replication and packaging of single-stranded phagemid DNA are formed in a phage particle. It is often used M13 phage or phage f1. The helper phage are usually chosen so that their own DNA are less effectively incorporated into new phage particles than the phagemid. Consequently, new phages contain mainly the phagemid and not helper phage DNA. In the phage DNA is due to the selected f1 ori before single-stranded.

Many of today used plasmids contain an f1 ori and are therefore phagemids. Such as a plasmid, a phagemid can be used to clone DNA fragments.