Which one of the following is indispensable in genetic engineering?

$A$ : $Pst I$ generates single-stranded overhanging stretches in $DNA$ after digestion that facilitate ligation.
$R$ : These short extensions can form hydrogen bonds with their complementary counterparts,which are then sealed by $DNA$ ligase to form phosphodiester bonds.

The most important feature in a plasmid to be used as a vector is

The stickiness of the ends facilitates the action of which enzyme?

Eco-$RI$ cuts the $DNA$ between . . . . . . only when the sequence $GAATTC$ is present in the $DNA$.

Give scientific reasons: Viruses can be used in the manufacturing of proteins.