(i) Describe the characteristics a cloning vector must possess. (ii) Why DNA cannot pass through the cell membrane ? Explain. How is a bacterial cell made competent to take up recombinant DNA from the medium?

Explanation

A cloning vector must have the following characteristics :
(a) ori or origin of replication
(b) Selectable marker, which helps is identifying and eliminating non-transformants and selectively permitting the growth of the transformants.
(c) Cloning site or recognition site for the restriction enzyme to recognise.
(ii) DNA is a hydrophilic molecule, therefore it cannot pass through the cell membrane. The bacterial cells can be made competent by treating them with a specific concentration of a divalent ion like calcium. The cells are then incubated on ice followed by a heat shock by placing them briefly at `42^@` C and then putting back in ice.