Give reasons why :
(a) DNA cannot pass into a host cell through the cell membrane.
(b) Proteases are added during isolation of DNA for genetic engineering.
(c ) Single cloning site is preferred in a vector.

Explanation

(a) DNA cannot pass into a host cell through the cell membrane as the DNA molecule is polar (negatively charged) and plasma membrane is non-polar (hydrophobic) in nature.
(b) Proteases are added druing isolation of DNA for genetic engineering to hydrolyse the proteins present in cell.
(c ) Cloning site : To link the foreign DNA, the vector needs to have every few, preferably single recognition sites for hte commonly used restriction enzymes. Presence of more than one recognition sites within the vector will generate several fragments, which will complicate gene cloning.