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 . . . . . . .

$PCR$ (Polymerase Chain Reaction) technique is useful in:

Identify the diagram given below.

Which of the following statements are correct with respect to $DNA$ separation,isolation and visualization?
$A$. The cutting of $DNA$ is done by molecular scissors.
$B$. The $DNA$ fragments separate according to their size in an agarose gel,upon electrophoresis.
$C$. The separated $DNA$ fragments can be seen without staining when exposed to $UV$ light.
$D$. The separated $DNA$ fragments,when stained with ethidium bromide,can be seen in visible light.
Choose the correct answer from the options given below:

In the presence of a chromogenic substrate,recombinant bacteria give:

Which of the following is a difficulty in getting prokaryotic cells to express eukaryotic genes?