Which of the following is a cloning vector?
(a) DNA of Salmonella typhimurium
(b) Ti plasmid
(c) Amp' and Tet' loci
(d) Ori minus PBR322
(b) The DNA used as a carrier for transferring a fragment of foreign DNA into a suitable host is called vehicle DNA or cloning vector or gene carrier. The Ti plasmid (tumour inducing plasmid) is present in Agrobacterium tumefaciens, a Gram negative soil bacterium that infects a wide range of plants and causes tumorous growth specially at the root /stem junction (crown gall). The Ti plasmid comprises the gene responsible for the tumorous growth, gets incorporated into the genome of infected plant cells. This property is of interest for genetic engineering as Ti plasmid can be used as DNA vector by replacing the tumour inducing genes with the gene of interest and a marker gene to enable selection of transformed cells. The Ti plasmid, is widely used in plant genetic engineering as a vector, novel plant genes being spliced into the plasmid sequence by gene manipulation and thus carried into the host plant cells. This provides the opportunity to develop new and better species.