Fosmids contain several functional elements as discussed below,

•  OriT (Origin of Transfer): The sequence which marks the starting point of conjugative transfer.

•  OriV (Origin of Replication): The sequence starting with which the plasmid-DNA will be replicated in the recipient cell.

•  tra -region (transfer genes): Genes coding the F-Pilus and DNA transfer process.

•  IS (Insertion Elements): so-called "selfish genes" (sequence fragments which can integrate copies of themselves at different locations).

Fig 1-4.4: Schematic diagram of fosmids derived from pCC1FOS- Ceu I

1-4.5 Phagemid

Although M13 vectors are very useful for the production of single-stranded recombinant genes, they have certain disadvantages. There is a limit to the size of DNA fragment that can be cloned in an M13vector, with 1.5 kb being the ideal capacity, although fragments up to 3 kb have occasionally been cloned. To overcome this limitation, phagemid vectors were developed by combining a part of the M13 genome with plasmid DNA.

•  Phagemids are cloning vectors developed as a hybrid of the filamentous phage M13 and plasmids to produce a vector that can get packed as a phage particle but also can propagate as a plasmid.

•  They contain an origin of replication ( ori ) for double stranded replication inside E. coli host, as well as an “ f1 ori” to enable single stranded replication and packaging into phage particles. Many commonly used plasmids contain an f1 ori and are thus phagemids.

•  Phagemid generally encode no or only one of the capsid proteins of virus. Other structural and functional proteins necessary for phage lifecycle are provided by the helper phage.

•  The components present in a phagemid vector are:

• Origin of replication (ori) of a plasmid.
  
• Intergenic region (IG region) which contains the packaging signal for the phage particle and also has replication origin inside phage.
  
• A gene encoding phage coat protein.
  
• A selection marker.
  
• Restriction enzyme recognition sites.