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in plants and animals.
• In plants, the Tumour inducing (Ti) plasmid of Agrobacterium tumefaciens is used as a
cloning vector.
-» Agrobacterium tumefaciens is a pathogen of several dicot plants.
-» It delivers a piece of DNA known as T-DNA in the Ti plasmid which 1 transforms normal
plant cells into tumour cells to produce chemicals
required by pathogens.
• Retrovirus, adenovirus, papillomavirus are also now used as cloning vectors in animals
because of their ability to transform normal cells into cancerous cells.
9. Competent host organism (for transformation with recombinant DNA) is required
because DNA being a hydrophilic molecule, cannot pass through cell membranes, Hence, the
bacteria should be made competent to accept the DNA molecules,
(i) Competency is the ability of a cell to take up foreign DNA.
(ii) Methods to make a cell competent are as follow.
(a) Chemical method In this method, the cell is treated with a specific concentration of | a
divalent cation such as calcium to increase pore size in cell wall.
The cells are then incubated with recombinant DNA on ice, followed by placing them briefly
at 42°C and then putting it back on ice. This is called heat shock treatment.
• This enables the bacteria to take up the recombinant DNA.
(b) Physical methods In this method, a recombinant DNA is directly injected into the nucleus
of an animal cell by microinjection method.
• In plants, cells are bombarded with high velocity microparticles of gold or tungsten coated
with DNA called as biolistics or gene gun method.
(c) Disarmed pathogen vectors when allowed to infect the cell, transfer the recombinant
DNA into the host.