FEATURES  TO CLONE  A VECTOR :

(i) Origin of replication (ori)

• 1. This is a DNA sequence that is responsible for initiating replication.
• 2. Ori also controls the copy numbers of the linked DNA.

(ii) Selectable marker

• It helps to select the host cells which contain the vector (transformants) and eliminate the non – transformants.
• Genes encoding resistance to antibiotics like amplicillin, chloramphenicol, tetracycline or kanamycin, are useful selectable markers for E. coli.
• The normal E. coli cells do not carry resistance against any of these antibiotics.

(iii) Cloning sites

• To link the alien DNA, the vectors require very few (mostly single) recognition sites for the restriction enzymes.
• More than one recognition sites within the vector, can complicate the gene cloning as it will generate several fragments.

(iv) Vectors for cloning genes in plants and animals

• 1. In plants, the tumour inducing plasmid (Ti) of Agrobacterium tumefaciens is used and A. tumefaciens is a pathogen of several dicot plants.
• 2. It delivers a piece of DNA known as ‘T-DNA’ in the Ti plasmid which transforms normal plant cells into tumor cells to produce chemicals against pathogens.
• 3. Retrovirus, adenovirus, papillomavirus are also now used as cloning vectors in animals because of their ability to transform normal cells into cancerous cells