Answer :

(a) Recombinant DNA/Desired DNA is inserted into the coding sequence of an enzyme β-galactosidase , this results into inactivation of the gene for the synthesis of this enzymes, presence of chromogenic substrate gives blue coloured colonies if the plasmid in bacteria does not have the insert( Non recombinants) , but presence of insert (Recombinants) leads to the growth of bacterial colonies with no color.

<b>or</b>

(b)A single stranded DNA or RNA tagged with a radioactive molecule (probe), is allowed to hybridize with its complementary DNA in a clone of cells, followed by detection using autoradiography, the clone having the mutated gene will hence not appear on the photographic film.

   //

Polymerase Chain Reaction/PCR is used to detect a disease even before any clinical symptoms appears, involves denaturation, annealing, to amplify DNA of the pathogen using pathogen specific primers.