Vectors

• A vector, as related to molecular biology, is a DNA molecule (often plasmid or virus) that is used as a vehicle to carry a particular DNA segment into a host cell as part of a cloning or recombinant DNA technique.

Properties of a good vector

• Able to replicate autonomously i.e. independent of the host chromosome replication.
• Should be easy to isolate and purify.
• Should be easily introduced into host cells.
• Should have suitable marker genes that allow easy detection or selection of transformed host cells.
• Should contain unique target sites for as many restriction enzymes as possible.
• Should contain suitable control elements like operator, promoter, ribosome binding site, etc. Such vectors are called expression vector.

Types of vectors

a. Plasmid Vector

• A plasmid is a naturally occurring extra chromosomal double stranded DNA, circular DNA.
• It replicates autonomously within bacterial cell.
• Plasmid carries an origin of replication.
• Plasmid vectors are the simplest cloning vectors.
• It is most widely used for gene cloning.
• Common plasmid vectors are PBR322, PUC, Pi and Ri plasmid.

b. Cosmids Vector

• It is a type of hybrid plasmid.
• It contains lambda phage cos sequence.
• Cosmids = cos sites + plasmid.
• Genomic size of cosmids is about 30 to 52 kb.
• If they have suitable origin of replication than they can replicate as Plasmid within the host cells, E.g.- SV40 Ori, ColE1 ori.
• It also contains selectable marker such as Ampicillin resistance gene.
• Collins and Hohn in 1978 was first to describe cosmids.

c. Phage Vector or Bacteriophage Vector

• Bacteriophages are viruses that attacks bacteria.
• The Phages are simple in structure.
• It consists of DNA molecules having several gene for replication which is surrounded by Capsid.

On the basis of structure bacteria phases are of two types:

I. Head and Tail Phages- E.g.: lambda phage.

ii. Filamentous phage- E.g.: M13 phage.

Lambda Phage Vectors

• Its genome size is about 48,502 bp.
• It contains origin of replication, genes for head and tail protein and enzymes for DNA replication
• It has more than one recognition sequence for almost all the restriction enzymes.
• It should be larger than 38 kb and smaller than 52 kb to packaged into phage particles.

M13 Phage Vectors

• M13 vectors are used to obtain single- stranded copies of cloned DNA.
• It is 6407 nucleotides long.
• It is circular and 6.4kb in size.
• M13 vector only cause infection in F+ and F’ cells.
• It is used to produce several copies of M13 mp series of vectors.
• Example- M13mp8, M13mp9 etc.

d. Shuttle Vector

• Shuttle vectors are created to replicates in cell of different type of species.
• They contain two origin of replication, in which one is particular for each host species, also those genes required for their replication and not provided by the host cell.
• This type of vectors are developed by recombinant techniques.