Ideal Characteristics of Gene Cloning Vector - Host cell & Insert capacity

What is a cloning vector?

Any DNA molecule that has the ability to replicate inside the host to which the desired gene is integrated for cloning- include plasmids, bacteriophages, cosmids, BAC, yeast vectors, shuttle vectors etc.
Cloning vectors are used for obtaining millions of copies of the cloned DNA fragments. The cloned genes in these vectors are not expected to express themselves at the transcription or translation level.

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Cloning vectors (Cloning vehicles) are used for creating genomic libraries, preparing probes, genetic engineering, or other basic studies.

The most important requirement for recombinant DNA technology is the cloning vector and expression vectors. Expression vectors are designed for expressing the protein encoded by the inserted gene.

Vectors may be plasmids, cosmids, phagemids, transposons, a virus, YAC or BAC.

Following are the desirable features for a cloning vector:

pBR 322

• Should contain an origin of replication.
• Should have unique restriction sites as polylinkers or multiple cloning site (MCS).
• Should have one or more marker genes for identification ad isolation of subpopulation of bacteria containing vector.
• Should have a relaxed mode of replication.
• The vector must be able to replicate in host cells.

Different Types of Vectors

No	Cloning Vector	Host	Insert Size (kb)
1	Plasmid	E.coli	~10 kb
2	Bacteriophage	E.coli	9- 22 kb
3	P1 phage	E.coli	70-100 kb
4	Cosmids	E.coli	33- 47 kb
5	BAC	E.coli	75-350 kb
6	YAC	Yeast	1000-3000 kb
7	Human Artificial Chromosomes (HACs)	Cultured Human Cells	>2000kb

• BAC (Bacterial Artificial Chromosome): A vector used to clone DNA fragments of 100-300 kb insert size in E.coli cells. Based on the naturally occurring f factor plasmid found in the bacterium E.coli. 
• YAC (Yeast Artificial Chromosome): A vector of hundreds to kilobases long used for cloning of DNA fragment. 

pBR 322:

• First artificial cloning vector (1977) constructed by Boliver & Rodriguez from E.coli plasmid.
• It is a 4362 kb long widely used cloning vector.
• pBR 322 plasmids that have been engineered in the laboratory from natural plasmids. So that it features which are useful for molecular cloning experiments.

Cosmids:

• Cosmids are the novel cloning vectors which possess properties of both plasmid and phage.
• First developed in 1978 by Barbara & John Collins.
• Cosmid can be defined as the hybrid vectors derived from plasmids that contain cos site of phage which is essential for packaging of nucleic acid into protein coat plus essential features of plasmids and several unique restriction sites for the insertion of DNA to be cloned.

Reference: Bajpai B. High Capacity Vectors. Advances in Biotechnology. 2013 Oct 22:1–10. doi: 10.1007/978-81-322-1554-7_1. PMCID: PMC7120981.