Biotechnology: Principles and Processes — BIOLOGY STD 12 Science — Question

Features of Cloning Vector There are certain features that are required to facilitate cloning into a vector. These are:

1. Origin of replication (ori): This is a sequence from where replication starts and any piece of DNA, when linked to this sequence can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked DNA.
   
2. Selectable marker: It helps in identifying or selecting transformants and eliminating non-transformants by selectively permitting the growth of the transformants. 'Transformation is a procedure, through which a piece of DNA is introduced into the host bacterium. Normally, the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline, kanamycin, etc., are considered as useful selectable markers for E. coli. The normal E. coli cells do not carry resistance against any of these antibiotics.
   
3. Cloning (recognition) sites: These are generally required to link the foreign or alien DNA with the vector. For this, the vector requires very few or single recognition sites for commonly used restriction enzymes. If more than one recognition sites is present within the vector, it will generate several fragments that will lead to more complication in gene cloning.
   

Vectors for cloning genes in plants and animals: In plants, Agrobacterium tumefaciens (a pathogen of several dicot plants) is able to deliver a piece of DNA known as 'T-DNA' (transfer-DNA) to transform normal plant cells into tumour cells to produce chemicals required by pathogen. This is done as the tumour inducing (Ti) plasmid of Agrobacterium tumefaciens has been modified as a cloning vector which is no longer pathogenic to plant but is still able to deliver genes of interest. Similarly, retrovirus, adenovirus, papilloma virus are also used as a cloning vectors in animals because of their ability to transform normal cells into cancerous cells.