Cloning vectors are the DNA molecules that can carry a foreign DNA segment into the the host cell.The vectors used in recombinant DNA technology can be plasmids, cosmids, bacteriophages etc.
Plasmids: Self replicating, circular, extra chromosomal DNA present in bacteria. Plasmids have only one or two copies per cell.
Bacteriophages: Virus infecting bacteria. Bacteriophages have high number per cell, so their copy number is also high in genome.
Cosmids: Hybrid vectors derived from plasmids which contain cos site of lambda phage.
Feature required to facilitate cloning into vector: Origin of replication, Selectable marker, unique restriction sites. The purpose of cloning vector is often to make numerous copies of the inserted gene.
Expression Vectors: The cloning vector containing suitable expression signals to have maximum gene expression, is called expression vector. The following expression signals are introduced into gene cloned vectors to get maximum expression:
Insertion of a strong promoter.
Insertion of a strong termination codon.
Adjustment of distance between promoter and cloned gene.
Insertion of transcription termination sequence.
Insertion of a strong translation initiation sequence.
