Identify the diagram given below.

In gel electrophoresis,the separated $DNA$ fragments are visualized after staining the $DNA$ with $...A...$ followed by exposure to $...B...$. Here $A$ and $B$ refer to:

Study the given figure carefully and select the correct statements regarding this.
$I.$ It represents typical agarose gel electrophoresis showing differential migration of $DNA$ fragments.
$II.$ Lane $1$ contains undigested $DNA$ fragments.
$III.$ Lanes $2$ to $4$ contain digested $DNA$ fragments.
$IV.$ Smallest $DNA$ bands are present at $(A)$ position and largest $DNA$ bands are present at $(B)$ position.

$DNA$ fragments can be separated by using which of the following techniques?

Polymerase chain reaction is most useful in

r-$DNA$ is inserted within the coding sequence of an enzyme,$\beta$-galactosidase. This results in the inactivation of the gene for the synthesis of this enzyme,which is referred to as . . . . . . .